Final Exam Study Guide

Think of the gastric gland as a small factory with five workers. Parietal cells sit in the body and fundus. They make hydrochloric acid, using the H+/K+ ATPase proton pump, and they also make intrinsic factor. One cell, two products. That is why a single autoimmune attack causes both achlorhydria, meaning no acid, and B12 deficiency. Chief cells release pepsinogen, and acid itself cuts it into active pepsin. Enterochromaffin-like (ECL) cells release histamine, which is the final amplifier of acid secretion. G cells in the antrum release gastrin into the blood. D cells release somatostatin, the off-switch for the whole system. Acid secretion has three inputs: acetylcholine from the vagus nerve, gastrin from G cells, and histamine from ECL cells. Histamine amplifies the other two, so H2 blockers dull all three paths indirectly. Proton pump inhibitors shut the pump itself, so they work more completely.

The feedback loop is the most testable piece of physiology here. Acid arriving in the antrum tells D cells to release somatostatin, and somatostatin switches off the G cells. So acid is the brake on its own production. Anything that takes away acid takes away the brake, and gastrin climbs. That is why one lab value, a high serum gastrin, points to completely different diseases depending on the gastric pH sitting next to it.

Below the stomach, how acidic and how fatty the chyme is drives the next layer of control. S cells sense acid and release secretin. Secretin calls for bicarbonate from the pancreas and the bile ducts, which lifts the duodenal pH into the range where lipase can work. I cells sense fatty acids and amino acids and release cholecystokinin. CCK squeezes the gallbladder, pushes the acinar cells to release enzymes, and slows the stomach so the duodenum is not flooded. This is why a patient with Zollinger-Ellison syndrome has more than ulcers. They also get diarrhea and fatty stools, because the acid load overwhelms the bicarbonate and ruins pancreatic lipase right there in the gut.

Motility needs a pacemaker and a wiring diagram. The interstitial cells of Cajal make the slow waves that set the fastest possible contraction rate. The myenteric plexus runs motility. The submucosal (Meissner) plexus runs secretion and local blood flow. Between meals the migrating motor complex sweeps leftovers and bacteria downstream, about every 90 to 120 minutes. Call it the housekeeper wave. When it fails, bacteria are never swept out, and small intestinal bacterial overgrowth follows. In diabetes, autonomic neuropathy damages the vagal fibers and the ICC network. That gives gastroparesis: the patient feels full right away, vomits food recognizable from hours earlier, and has blood sugars that swing, because absorption no longer lines up with the insulin.

• Achalasia: the inhibitory NO and VIP neurons in the lower esophageal sphincter are lost. The sphincter cannot relax, and the body of the esophagus loses its squeeze. From the very start there is trouble swallowing both solids and liquids, food comes back up undigested, and barium shows a bird-beak taper.
• Hirschsprung disease: the neural crest cells never finish migrating, so the far end of the bowel has no ganglion cells and stays clamped shut. The newborn does not pass meconium within 48 hours. The bowel above is dilated and normal, and the narrow part is the abnormal one.
• Gastroparesis: the vagus and the ICC are injured, by diabetes, surgery, or medication. The stomach empties slowly, and food can clump into a bezoar.
• Ileus: the enteric nervous system is intact but switched off, by surgery, opioids, a low potassium, sepsis, or peritonitis.
• Bacterial overgrowth: the migrating motor complex or the gastric acid is lost. That brings bloating, B12 deficiency, because the bacteria eat it, and a folate level that is oddly high, because the bacteria make it.

Several parts work together to hold reflux back. There is the resting tone of the lower esophageal sphincter. There is the crural diaphragm squeezing it from outside. There is the sharp angle of His, which works like a flap valve. And there is the length of esophagus that sits inside the abdomen, so belly pressure pushes the sphincter closed instead of open. A hiatal hernia matters so much because it wrecks several of these at once. The sphincter slides up into the chest, pulls away from the crural pinch, flattens the angle of His, and leaves a small pool of acid above the diaphragm that drains back down with every swallow.

Most single reflux episodes come from one thing: a transient lower esophageal sphincter relaxation. The vagus nerve drives it. A swallow does not set it off. A stretched stomach does. That one fact explains most of the advice patients get. Big meals, late meals, extra weight, and a stomach that empties slowly all stretch the fundus more, so there are more reflux episodes. It also explains why raising the head of the bed, staying upright after eating, and losing weight work. They work mechanically, not through any drug.

• More pressure inside the abdomen: extra weight, pregnancy, fluid in the belly, tight clothing, and straining.
• Weaker sphincter tone: nicotine, alcohol, chocolate, peppermint, high-fat meals, caffeine, calcium channel blockers, nitrates, anticholinergics, and progesterone in pregnancy.
• Slower clearing: less saliva with its bicarbonate, as in sleep, dry mouth, and scleroderma, plus a weak squeeze in the esophagus.
• Slower stomach emptying: gastroparesis, a narrowed stomach outlet, and large fatty meals.
• Anatomy: a hiatal hernia, or earlier surgery at the gastroesophageal junction.

Under the microscope, the first change is that the refluxed material widens the spaces between the lining cells. That is why a patient can have real symptoms with a normal-looking endoscopy, which is called non-erosive reflux disease. Keep the injury going and you see thickening of the basal layer, long stretched vascular papillae, and eosinophils and neutrophils inside the lining. As the injury goes deeper, the complications climb a ladder: erosive esophagitis, then ulcers and bleeding, then a scarred peptic stricture, which gives worsening trouble swallowing solids in a patient with a long heartburn history, and then metaplasia.

Barrett esophagus is metaplasia. The layered squamous lining of the lower esophagus is swapped for a gut-type column lining that contains goblet cells. On endoscopy it looks like salmon-pink tongues reaching up above the gastroesophageal junction. Metaplasia is an adaptive change, because the column lining handles acid far better than squamous. But there is a price. The new lining divides fast and its genes are unstable, and that is the soil where dysplasia and then adenocarcinoma grow. The risk of moving forward is small each year, around half a percent, but it adds up over decades. That is why surveillance endoscopy with biopsy, not symptom relief, decides how the patient does.

Reflux also causes trouble outside the esophagus, because what comes up can reach the throat and the airway. A lasting cough, a hoarse voice with an inflamed back of the larynx, asthma that starts or worsens in adulthood, a lump-in-the-throat feeling, and worn tooth enamel on the tongue side of the teeth are all known effects. In a frail patient, or one with nerve damage, reflux at night is a real risk for aspiration. It can show up as repeated pneumonia rather than heartburn at all.

The lining's defense comes in layers. A mucus and bicarbonate gel holds a pH gradient, so the surface facing the stomach can sit at pH 2 while the cell surface underneath stays near neutral. Phospholipids on the surface make that layer water-repellent. Damaged cells are replaced within minutes by restitution, where neighboring cells flatten out and slide across the gap. Under all of that, blood flow in the lining carries away hydrogen ions that leak back in, and delivers the bicarbonate and oxygen the repair needs. Prostaglandin E2 keeps nearly all of these layers running at once, meaning the mucus, the bicarbonate, the blood flow, and the restitution. That is why one drug that blocks prostaglandin production can take the whole system apart.

Helicobacter pylori is a spiral organism that makes urease. It survives in the mucus layer by making ammonia to buffer the acid right around it. Where it settles decides the disease, and this is the highest-value idea in the topic. Infection mostly in the antrum inflames the D cells. Somatostatin falls, so gastrin rises, the parietal cell mass grows, and acid output climbs. That extra acid pours into the duodenum, which responds by growing stomach-type lining in the bulb, and the organism then settles there too. That is the duodenal ulcer road. Infection mostly in the body, or throughout the stomach, destroys the acid-making glands instead. That gives atrophy, too little acid, and intestinal metaplasia. That is the road to gastric ulcer, gastric adenocarcinoma, and MALT lymphoma. One organism, opposite effects on acid, and location decides which.

NSAIDs injure by blocking COX-1. That cuts prostaglandin E2 and strips away the mucus, the bicarbonate, and the blood flow in the lining. The effect works through the whole body, not just where the pill touches. That is the point people miss most often. Enteric-coated, rectal, and IV NSAIDs still cause ulcers, because the prostaglandin shortage arrives through the bloodstream. The risk climbs with age over 65, a high dose or two NSAIDs at once, steroids, blood thinners or SSRIs taken with them, a past ulcer, and H. pylori on top. NSAIDs also dull the pain through that same prostaglandin pathway. So a fair number of NSAID users show up for the first time with bleeding or a perforation instead of pain.

The complications follow the anatomy exactly. Bleeding is the most common. A back-wall ulcer in the duodenal bulb sits right against the gastroduodenal artery, so it bleeds fast and hard from an artery. A gastric ulcer on the lesser curve eats into branches of the left gastric artery. Perforation belongs to the front wall of the duodenum, which has nothing behind it but peritoneum. You get sudden severe pain, an abdomen as hard as a board, no bowel sounds, and free air under the diaphragm. Penetration is a perforation that has walled itself off into a nearby organ, classically the pancreas. The pain then turns into a steady, boring ache that goes through to the back. Gastric outlet obstruction comes from swelling or scarring near the pylorus. The patient vomits undigested food, and you can hear a splash when you shake the belly.

Blood that enters the stomach is broken down by acid, then by colon bacteria. Hemoglobin turns into hematin and related pigments, and the stool comes out black, sticky, tarry, and famously foul-smelling. That takes two things: enough blood, usually 50 to 100 mL, and enough time, roughly 14 hours of transit. Blood that reaches the rectum without that exposure stays red or maroon. So black tarry stool tells you the blood sat in acid and moved slowly. Red blood tells you it did not, either because the source is further down, or because the bleed was so fast that it swept through before digestion could happen.

The urea-to-creatinine ratio is a neat piece of physiology. Blood in the small intestine is a big protein meal. It gets digested and absorbed, and its nitrogen is turned into urea, which raises the BUN. At the same time, low blood volume cuts blood flow to the kidney and pushes the proximal tubule to reabsorb urea. Both effects push the same direction. So a BUN to creatinine ratio above about 20 or 30 to 1, with a normal creatinine, points strongly to an upper source. A lower GI bleed does not raise the urea this way, because the blood never passes through the absorbing small bowel.

Each upper cause has its own mechanical signature. A peptic ulcer is the most common overall, and it bleeds when the base of the ulcer digs into a vessel. Esophageal varices bleed with no pain, in huge amounts, in someone with long-term liver disease. That is the bleed that kills fastest. A Mallory-Weiss tear is a lengthwise split in the lining at the gastroesophageal junction, caused by forceful retching. So the classic story is several rounds of vomiting with no blood, and then blood. It usually stops on its own. A Dieulafoy lesion is an unusually large artery just under the lining that wears through otherwise normal tissue, which gives dramatic bleeding with no ulcer to find. Think of an aortoenteric fistula in anyone with an old aortic graft who has a small warning bleed before a catastrophic one.

The lower causes sort out by pattern too. Diverticulosis is the most common cause of major lower bleeding. An artery, the vasa recta, is draped over the dome of the diverticulum and bursts. That gives painless, fast, often large bleeding that usually stops by itself. Angiodysplasia is a worn-out, widened cluster of small vessels in the lining. It favors the right colon and older adults, and it typically causes slow, repeated bleeding that may be invisible. It has a known tie to aortic stenosis. Ischemic colitis strikes the areas with the poorest blood supply, such as the splenic flexure, and it gives pain first, blood second, in modest amounts. Hemorrhoids and fissures give small amounts of bright red blood on the paper or coating the stool, and a fissure also hurts. Slow, hidden bleeding that shows up only as iron deficiency anemia in an adult over 50 is colorectal cancer until colonoscopy proves otherwise. In a child, painless rectal bleeding should bring up Meckel diverticulum with stomach-type lining in it.

• Upper endoscopy: it both finds the bleed and treats it. It is the first test in any suspected upper bleed, once the patient is stable.
• Colonoscopy: the test for a lower bleed, but it needs a cleaned-out colon and a reasonably stable patient.
• CT angiography: it finds bleeding at about 0.3 to 0.5 mL per minute, and it is fast, so it fits the unstable patient who is bleeding right now.
• Tagged red cell scan: it picks up slower bleeding, around 0.1 mL per minute, but it only shows roughly where.
• Capsule endoscopy: for a hidden small bowel bleed, in the stretch neither scope can reach.

The genes that raise the risk differ between the two. NOD2/CARD15 variants belong to Crohn disease, and they predict ileal disease with strictures. Ulcerative colitis goes with a different set of genes, including HLA class II and barrier genes. NOD2/CARD15 weakens how the cell senses and handles bacterial products, so the lining and the first-line immune response cannot keep normal gut bacteria at a polite distance. The adaptive immune system then mounts a long, self-feeding attack on things in the gut lumen. Both diseases peak twice, once from late adolescence into the thirties and again later in life, and both come and go in flares. Smoking does something strange here. It makes Crohn disease worse and raises the chance it comes back after surgery. But it is tied to a lower risk of ulcerative colitis, and UC sometimes shows up soon after a patient quits.

Crohn's effects on nutrition follow from its favorite spot. The terminal ileum is the only place that takes bile salts back, and the only place with receptors for intrinsic factor and B12. So disease or surgery there causes B12 deficiency and a shrunken bile salt pool. Without enough bile salts, fat cannot be packaged into micelles, so fat is poorly absorbed. That gives fatty stools and shortages of vitamins A, D, E and K. Bile salts that are not reabsorbed reach the colon, irritate the lining, and drive a watery diarrhea. A shrunken bile salt pool also leaves the bile prone to forming stones, so gallstones are more common. And unabsorbed fat ties up the calcium in the gut, calcium that would normally have grabbed oxalate. Free oxalate is then absorbed by the colon, and calcium oxalate kidney stones follow.

Ulcerative colitis acts like a disease of the rectum that spreads. The rectum is inflamed and will not stretch, so patients feel urgency, go often, and have tenesmus, which is the feeling of needing to pass stool when the rectum is already empty. Bleeding stands out, because the inflamed lining is fragile and ulcerated on the surface. How far it reaches predicts how sick the patient is. Disease limited to the rectum may just be a nuisance, while disease through the whole colon is a body-wide illness, with fever, weight loss, and anemia. Long-standing widespread colitis carries a real colorectal cancer risk, starting about 8 to 10 years after diagnosis. That is why surveillance colonoscopy with biopsies looking for dysplasia, not symptom relief, decides the long-term outcome. Crohn colitis carries a similar risk when a similar amount of colon is involved.

• Skin: erythema nodosum, which is tender red lumps on the shins and usually tracks how active the disease is, and pyoderma gangrenosum, a fast-growing ulcer with a purple, undercut edge that often ignores disease activity.
• Eye: uveitis, episcleritis, and iritis. Eye pain, redness, or a change in vision in a patient with IBD is urgent.
• Joints: arthritis in the arms and legs, which follows the bowel activity, and spine disease such as ankylosing spondylitis and sacroiliitis, which does not.
• Liver and bile ducts: primary sclerosing cholangitis, strongly tied to ulcerative colitis, with a rising alkaline phosphatase and a beaded look to the bile ducts. It also pushes the risk of bile duct cancer and colorectal cancer higher still.
• Blood: the anemia of long-term illness, plus iron deficiency, and a real tendency to clot during flares. So think about a clot in the legs or lungs in any sick IBD inpatient.

Appendicitis

The appendix has a narrow tube and one blind end, so anything that blocks it traps a lining that keeps secreting. In adults the block is usually a hard pellet of stool, called a fecolith. In children and adolescents it is usually swollen lymph tissue after a viral illness. In older adults it is occasionally a tumor. The lining keeps making mucus into a closed space, so the pressure inside climbs above the pressure in the veins. Drainage fails, the wall swells and then runs short of blood, the bacteria already living there invade the dying wall, and gangrene turns into a perforation, usually within 48 to 72 hours of the first symptom.

• Rovsing sign: pressing on the left lower belly causes pain in the right lower belly. The pain follows the peritoneum, not the hand.
• Psoas sign: pain when you straighten the hip, which suggests an appendix lying behind the cecum against the psoas muscle.
• Obturator sign: pain when you rotate the bent hip inward, which suggests an appendix down in the pelvis.
• Rebound and guarding: both mean the parietal peritoneum is irritated. Guarding the patient cannot control is the more reliable of the two.
• Labs: a mildly high white count with a shift to the left. A normal white count does not rule out early appendicitis, and a very high one suggests it has perforated.

Diverticular disease

Colonic diverticula are pseudodiverticula pushed out by pressure. The mucosa and submucosa squeeze through the muscle wall exactly where the vasa recta vessels pass through it, because those are the weak spots. They are called "pseudo" because they contain no muscle layer. High pressure inside the bowel drives them. That is why the classic setting is a low-fiber diet, with small hard stool and strong squeezing contractions in the narrow, high-pressure sigmoid colon. Diverticulosis just means the pouches are there. Diverticulitis is what happens when one gets plugged with dried stool, the trapped lining becomes inflamed and short of blood, and it springs a tiny leak.

Bowel obstruction: mechanical versus functional

A mechanical obstruction means something is physically in the way. In the small bowel the order of frequency is scar bands from earlier surgery, then a hernia with bowel trapped in it, then cancer. In the large bowel it is colorectal cancer, then a twisted loop, then a diverticular stricture. A functional obstruction, called ileus, means the tube is open but the gut has stopped squeezing. It follows abdominal surgery, opioids and anticholinergic drugs, a low potassium or other electrolyte problem, sepsis and peritonitis, a spinal injury, or bleeding behind the abdominal lining. Acute colonic pseudo-obstruction, or Ogilvie syndrome, is the colon's version in a frail inpatient, where the colon balloons with nothing blocking it at all.

The physiology of a mechanical obstruction is a fluid problem long before it is a surgical one. Above the block, the bowel fills with swallowed air and with its own secretions. The blocked gut keeps secreting while losing its ability to reabsorb, and several liters a day can pile up. The rising tension in the wall then pushes plasma into the wall itself and out onto the surface of the peritoneum. All of that fluid is inside the body but outside the circulation. That is the classic third space. The result is a patient whose blood volume is low. The heart races, the urine output falls, and the labs look prerenal. Yet the total body water may be almost normal, and there is no blood loss and no diarrhea to explain it. Vomiting stomach contents again and again also loses hydrogen ions, chloride, and potassium. That gives a metabolic alkalosis with a low chloride and a low potassium, and the volume loss itself keeps it going. That is the direct link back to Module 1.

Strangulation is where a plumbing problem turns into dying bowel. As the stretching raises the pressure inside the wall, the thin-walled veins collapse first. Blood cannot get out while blood keeps coming in. The wall gorges and swells, which raises the pressure further, and in the end the arteries fail too. The lining's barrier breaks down, bacteria and endotoxin cross into the portal blood and the peritoneum, and the patient turns septic. A closed loop obstruction carries the highest risk. That means a twisted loop, or a hernia pinched at both ends of a loop. There is no way for the pressure to escape upstream, and the mesentery is often twisted at the same time, which cuts the blood supply directly.

• High small bowel obstruction: early, heavy, bile-colored vomiting, little swelling, and dehydration and alkalosis that come on fast.
• Low small bowel obstruction: a lot of swelling, vomiting that starts later and turns smelly from bacterial overgrowth, and more cramping.
• Large bowel obstruction: a hugely swollen belly and nothing passing. Vomiting comes late or not at all.
• A closed loop with a tight ileocecal valve: the colon cannot vent backward, so the thin-walled cecum stretches the most, and that is where it usually tears.

The normal bilirubin pipeline

Red cells live about 120 days. When they are cleared, macrophages in the spleen and liver strip the heme ring and turn it into biliverdin and then into unconjugated, also called indirect, bilirubin. That molecule dissolves in fat and travels through plasma stuck tightly to albumin. Two things follow at once. It can cross fatty membranes, including the blood-brain barrier. And the glomerulus cannot filter it, so it never shows up in urine. The liver cell takes it in, and the enzyme UDP-glucuronosyltransferase attaches glucuronic acid. That makes conjugated, also called direct, bilirubin, which now dissolves in water. The conjugated pigment is pumped into the bile canaliculi, runs down the bile ducts, is stored and concentrated in the gallbladder, and is released into the duodenum after a meal. Colon bacteria then turn it into urobilinogen. Most becomes stercobilin, which is what makes stool brown. A small part of the urobilinogen is reabsorbed, cycles back through the liver, and a trace leaves in the urine as urobilin, which is what makes urine yellow.

Prehepatic jaundice: too much pigment arriving

Here the liver is healthy, but it is being flooded. A sickle cell crisis, G6PD deficiency, hereditary spherocytosis, an autoimmune attack on red cells, a hemolytic transfusion reaction, or the breakdown of a large bruise can all deliver heme faster than the conjugating enzyme can handle. Unconjugated bilirubin rises. Because it is stuck to albumin, the urine bilirubin is negative. More pigment does eventually reach the gut, so more urobilinogen is made and reabsorbed. That means urine urobilinogen goes up, and the stool is normal or even darker than usual. The supporting labs are the hemolysis panel: a high LDH, a low haptoglobin, a high reticulocyte count, and anemia. In a newborn, the UGT enzyme is still immature and red cells turn over faster, which makes this the common kind of jaundice. The danger is that fat-soluble unconjugated bilirubin crosses the newborn's immature blood-brain barrier and settles in the basal ganglia, causing kernicterus.

Intrahepatic jaundice: the liver cell itself is sick

Viral hepatitis, alcoholic hepatitis, acetaminophen poisoning, drug reactions, and cirrhosis all damage the very cell that both takes the pigment in and ships it out. So you get a mixed picture. Uptake and conjugation are impaired, so unconjugated bilirubin rises. But the injured cell also leaks pigment it had already conjugated back into the blood, so conjugated bilirubin usually rises more. The giveaway is the enzymes. ALT and AST climb into the hundreds or thousands, because those enzymes sit in the liver cell's cytoplasm and spill out when cells die, while the ALP stays fairly modest. Urine bilirubin is positive and the urine darkens. The stool runs from normal to pale, depending on how much bile still gets through.

Posthepatic jaundice: the pipe is blocked

A stone in the common bile duct, a cancer in the head of the pancreas, a scarred bile duct, or a bile duct cancer stops conjugated bile from reaching the gut, so it backs up into the blood. Conjugated bilirubin dissolves in water, so the kidney filters it freely and the urine turns dark. Nothing reaches the colon, so no stercobilin is made. The stool turns pale, putty-colored, and greasy, which is called acholic steatorrhea, and urine urobilinogen disappears. Without bile salts, fat and the fat-soluble vitamins A, D, E and K are poorly absorbed. So the prothrombin time lengthens from vitamin K deficiency, and unlike liver cell failure, it will correct once you give vitamin K. Bile salts settling in the skin cause intense itching. The enzyme pattern is cholestatic: ALP and GGT rise a lot, far out of proportion to ALT and AST. The GGT is what proves the ALP is coming from bile duct lining and not from bone.

How the architecture fails

Long-term injury from alcohol, hepatitis B or C, or fatty liver disease wakes up the hepatic stellate cells that sit in the space of Disse. Normally they sit quietly and store vitamin A. Now they turn into squeezing myofibroblasts and lay down thick collagen. Two things happen at the same time. First, the sinusoids stiffen, and the liver cells that regrow organize themselves into regenerative nodules that press on the vessels. So the resistance to portal blood flow rises. Second, working liver cells are steadily replaced by scar, so the liver's ability to build things and to clear things falls. On top of the first problem, the arterioles in the gut widen under the influence of nitric oxide, which pushes even more blood into a portal system that is already blocked. So cirrhosis is a permanent change in the liver's architecture, not just dead cells.

Failure mode 1: blood cannot get through, which is portal hypertension

The portal vein normally runs at a low pressure. Portal hypertension exists once the hepatic venous pressure gradient passes about 5 mmHg. Above roughly 10 mmHg it starts to matter clinically, and above about 12 mmHg the varices are at real risk of bleeding. Blood that cannot get through the liver looks for any connection that already exists between the portal and the systemic circulation. Those portosystemic collaterals are thin-walled veins with no support, and they were never built for that pressure. Each one gives a sign you can recognize. Meanwhile the spleen, which drains into the portal system, gets congested and swells, and a big spleen traps and destroys blood cells.

Failure mode 2: the liver cannot do its jobs, which is liver cell failure

The liver builds albumin and nearly all the clotting factors. It conjugates bilirubin and ships it out. It breaks down hormones and drugs. It turns ammonia into urea. It stores glycogen and makes new glucose. And it holds the Kupffer cells that filter portal blood. Lose liver cell mass and you lose each of those jobs in turn. Because factor VII has the shortest half-life of the clotting factors, the prothrombin time and the INR are the earliest and most sensitive signs of failing production. Albumin has a half-life of about 20 days, so it moves slower and tells you how long this has been going on. Estrogen that the liver normally clears builds up, which gives the classic feminizing findings in men.

Colon bacteria strip the nitrogen off protein in food and in secretions, which makes ammonia, and that ammonia is absorbed into the portal vein. In a healthy person nearly all of it is pulled out on the first pass through the liver and turned into urea for the kidney to pass. In cirrhosis, blood detours through collaterals, and the liver cells that are left cannot keep up. So ammonia and other gut-made nerve toxins, such as mercaptans, short-chain fatty acids, and false neurotransmitters, reach the general circulation. Ammonia is small and dissolves in fat, so it crosses the blood-brain barrier easily.

Inside the brain, astrocytes are the only cells that can clear ammonia. Glutamine synthetase joins the ammonia to glutamate and makes glutamine. Glutamine pulls water in, so the astrocyte swells. Under the microscope these are called Alzheimer type II astrocytes. The results are mild brain swelling, astrocytes that can no longer support neurons, less glutamate available as the main excitatory signal, and stronger GABA inhibition. In the patient that looks like a level of consciousness and movement control that rises and falls and can be reversed, rather than a fixed one-sided deficit. That is exactly how you tell it apart from a stroke.

The grades, and what the exam findings mean

Asterixis is worth understanding rather than memorizing. Ask the patient to hold the arms out and bend the wrists back. The metabolic encephalopathy keeps cutting out the steady signal that holds the posture, so the hands drop and are jerked back. It is a brief loss of muscle tone, called negative myoclonus, not a true tremor. That is why it vanishes at grade IV: a patient in a coma cannot hold the posture that shows it off. Fetor hepaticus, a sweet musty smell on the breath, comes from sulfur compounds that skip past the liver and come out in the air.

Triggers: something almost always tipped the patient over

• Bleeding in the GI tract, which is the single most important trigger. A liter of blood in the gut is a huge protein meal, and colon bacteria turn it into ammonia
• An infection of any kind, especially spontaneous bacterial peritonitis, which speeds up breakdown and raises the nitrogen load
• Constipation, which gives bacteria more time with the protein in the gut
• Dehydration and too much diuretic, which concentrate the toxins and cut how much ammonia the kidney can clear
• A low potassium, which makes the kidney produce more ammonia, and a metabolic alkalosis, which shifts the balance from the charged NH4+ form toward the NH3 form that can cross membranes and the blood-brain barrier
• Sedatives, opioids, and benzodiazepines. A failing liver cannot clear them, and they add to the already strong GABA inhibition
• A high urea and kidney failure, which send more urea to gut bacteria, and they break it right back down into ammonia
• A TIPS procedure, or shunts that formed on their own and keep getting bigger

Why lactulose works

Lactulose is a two-sugar molecule the gut cannot absorb, so it arrives in the colon intact, and bacteria ferment it into lactic and acetic acid. Making the colon acidic shifts the ammonia balance toward NH4+, the charged ammonium ion, which cannot cross the gut wall. So the nitrogen is trapped and leaves in the stool. The osmotic pull at the same time speeds things along, which gives bacteria less time to make ammonia and physically carries nitrogen out of the gut. The goal is two to three soft stools a day. Fewer means the dose is too low. More risks dehydration and a low potassium, and both of those make encephalopathy worse. Rifaximin works a different way. It is an antibiotic the gut does not absorb, and it cuts down the colon bacteria that make urease. It is usually added to lactulose rather than used instead of it.

Hepatitis A

This is an RNA picornavirus with no envelope, spread by the fecal-oral route: contaminated water, undercooked shellfish, food handled by an infected person, daycare centers, and travel to places where it is common. Incubation runs about two to six weeks. The virus is shed in stool for the two weeks before symptoms start, so most of the spreading is already done by the time the patient turns yellow. Hepatitis A causes a short illness that resolves on its own. It never becomes chronic and never causes cirrhosis, though rarely it can be severe in older adults or in people whose livers are already damaged. IgM anti-HAV confirms a current infection. IgG anti-HAV means a past infection or a vaccine, and it gives lifelong protection. There is a killed-virus vaccine. Hepatitis E behaves much the same. It is fecal-oral and clears on its own, but it kills a strikingly high share of pregnant women in the third trimester.

Hepatitis B

This is the only DNA virus among the hepatitis viruses. It spreads through blood, sex, and from mother to baby around birth. Think needle sticks, shared injection gear, unscreened transfusions, and passage at delivery. Incubation is long, roughly one to six months, so the exposure is often long forgotten. About 5 to 10 percent of infected adults fail to clear it and become chronic, but that figure rises to about 90 percent for babies infected at birth. Chronic infection brings a risk of cirrhosis and liver cancer, and hepatitis B stands out because it can cause liver cancer without causing cirrhosis first. The viral DNA inserts itself into the host's own genes and acts as a cancer-driving insertion. Immune-complex disease outside the liver can come with it, such as polyarteritis nodosa and membranous nephropathy. The vaccine contains surface antigen only, and that is the key to reading the serology.

Hepatitis C

This is a blood-borne RNA flavivirus. Injection drug use is now the main route. Transfusion before 1992 and needle-stick injuries are the historical and on-the-job sources. It can pass through sex and from mother to baby, but far less easily than hepatitis B does. The first infection is usually silent, and that is exactly the problem. Roughly 75 to 85 percent of those infected become chronic, the highest rate in the group. The reason sits in the virus. Its RNA polymerase does not proofread, so the virus mutates fast and circulates as a swarm of quasispecies that keeps outrunning the antibody response. That same shiftiness is why there is no hepatitis C vaccine. Over decades, chronic infection brings cirrhosis and liver cancer. Outside the liver it can cause mixed cryoglobulinemia, membranoproliferative glomerulonephritis, and porphyria cutanea tarda. Direct-acting antiviral pills now cure well over 95 percent of cases in eight to twelve weeks, so the focus has shifted firmly to screening and finding cases.

How an acute attack runs

Acute viral hepatitis runs in three phases. The early phase brings feeling unwell, a low fever, aching muscles, no appetite, nausea, and classically a sudden dislike of cigarettes in smokers. Discomfort under the right ribs comes from the liver capsule being stretched. The yellow phase follows as the bilirubin climbs: jaundice, dark urine, pale stool, itching, and often a strange improvement in how the patient feels overall. The recovery phase clears the jaundice, though the tiredness hangs on for weeks. In the labs, the ALT and AST run in the hundreds to thousands, with the ALT usually higher than the AST. That pattern flips in alcoholic hepatitis, where the AST runs more than twice the ALT and the enzymes rarely pass 400.

Why stones form

About 80 percent of stones are cholesterol stones. They form when bile holds more cholesterol than the bile salts and phospholipids can keep dissolved, and when a sluggish gallbladder gives the crystals time to seed and grow. Estrogen makes the liver push more cholesterol into bile, and progesterone slows the gallbladder down. That is why being female, pregnancy, and birth control pills all raise the risk. Extra weight raises the cholesterol in bile. Fast weight loss, long fasting, and IV feeding cause stones through stasis, because a gallbladder that is never asked to squeeze never empties. Pigment stones come in two kinds. Black stones form with long-term hemolysis, sickle cell disease, and cirrhosis, where extra unconjugated bilirubin settles out as calcium bilirubinate. Brown stones form in ducts that are infected or carry parasites, where bacterial enzymes unhook the bilirubin right there. Crohn disease and ileal surgery cause stones by cutting off the reabsorption of bile salts in the terminal ileum, which drains the pool that keeps cholesterol dissolved.

The range of bile duct blockage

Biliary colic is the mildest end, and the name is wrong. The pain is steady, not crampy, because it comes from a stretching organ and not from waves of squeezing. A fatty meal releases cholecystokinin, the gallbladder squeezes against a stone briefly wedged in the cystic duct, the pressure climbs, and the pain builds over an hour and then fades once the stone drops back. Nothing is inflamed, so there is no fever and the labs are normal. If the stone stays put, the trapped bile turns into an irritant, the wall becomes inflamed and swollen, bacteria move in, and the picture becomes acute cholecystitis. Ultrasound is the first imaging test all along. It finds stones, a thickened wall, and fluid around the gallbladder, and it is fast, cheap, and free of radiation.

Acute pancreatitis: the gland digests itself

The pancreas makes enzymes strong enough to dissolve tissue, so it stores them switched off as zymogens and does not turn them on until they reach the duodenum. Acute pancreatitis begins when trypsinogen is turned into trypsin too early, inside the acinar cell. Trypsin is the master switch. Once it is active, it switches on chymotrypsinogen, proelastase, prophospholipase A2, and the kallikrein-kinin cascade. Elastase eats vessel walls and causes bleeding. Lipase and phospholipase digest fat. And the inflammatory messengers that are released set off a body-wide inflammatory response. The two dominant causes, which together account for about 80 percent, are gallstones blocking the ampulla, and alcohol, which thickens the protein in pancreatic juice and poisons the acinar cells directly.

• Hypovolemic shock. The inflamed area behind the abdomen and the leaky capillaries everywhere pull liters of fluid out of the vessels. That is why giving IV fluid hard and early is the single most important treatment
• A low calcium. Lipase frees fatty acids from the fat around the pancreas, and those fatty acids grab calcium to form insoluble soaps. That is called saponification, and it gives Chvostek and Trousseau signs, and tetany when severe
• Acute respiratory distress syndrome. Phospholipase A2 circulating in the blood breaks down the surfactant in the lungs, so the air sacs collapse and the oxygen falls, often on day two or three
• Grey Turner sign, which is bruising in the flank, and Cullen sign, which is bruising around the navel. Blood from dying, bleeding tissue tracks through the planes behind the abdomen. Both mean severe disease
• A pancreatic pseudocyst. This is a pocket of enzyme-rich fluid walled off by healing tissue rather than a true lining. It usually shows up weeks later, as a lasting mass, pain, or feeling full early
• Infected dead tissue. Gut organisms settle in the dead pancreas, and this is the leading cause of late death
• A high blood sugar, from islet cells being destroyed plus a stress surge of glucagon

Chronic pancreatitis

Repeated injury, most often from years of alcohol use, replaces working acinar tissue with scar, calcium, and narrowed ducts. The clinical story follows the anatomy. The exocrine pancreas, which is 80 to 90 percent of the gland, fails first. So patients get fatty stools that are bulky, greasy, foul-smelling, and float, along with weight loss despite eating enough, and shortages of the fat-soluble vitamins A, D, E and K. The endocrine islets are a small part of the gland and fail later, which gives a diabetes that is hard to control, because the glucagon cells are lost alongside the beta cells. The pain is chronic, below the breastbone and into the back, and it may oddly ease as the gland burns out. The lab point that feels backward is this: the amylase and lipase are often normal in chronic pancreatitis, because so few acinar cells are left to release them. The diagnosis rests on imaging showing calcium deposits and duct changes, and on a stool elastase test.

The menstrual cycle in two halves

The follicular phase, also called proliferative, runs from day 1 to ovulation. FSH from the front of the pituitary calls up a group of follicles, and their granulosa cells make estradiol. The rising estradiol does two things. It drives the endometrium to thicken, building up the functional layer and regrowing the spiral arteries. And it feeds back to turn FSH down, which starves the smaller follicles, so only the dominant follicle, the one with the most FSH receptors, survives. Then the feedback flips. Once the estradiol has stayed high for about 48 hours, it switches from turning the pituitary down to turning it up, and that gives the LH surge. Ovulation follows the surge by about 36 hours, which is day 14 in a textbook 28-day cycle.

The luteal phase, also called secretory, starts when the burst follicle turns into the corpus luteum, which makes progesterone. Progesterone turns the thickened endometrium into a secretory bed rich in glycogen, ready to support an implanting embryo. It also thickens the cervical mucus into a barrier and raises the body's resting temperature by about half a degree. The corpus luteum has a fixed lifespan of about 14 days, unless hCG from an implanting embryo rescues it. With no hCG it shrinks away, the progesterone and estrogen fall, the spiral arteries clamp down, the functional endometrium runs out of blood and sloughs, and the period begins.

Polycystic ovary syndrome

PCOS is diagnosed with the Rotterdam criteria, which need two out of three: rare or absent ovulation, too much androgen, either on exam or in the labs, and ovaries that look polycystic on ultrasound. The mechanism is a loop that feeds itself. The GnRH pulses come faster, which favors LH over FSH, so the LH to FSH ratio rises. LH pushes the ovary's theca cells to overproduce androgens, while the relatively low FSH means the granulosa cells never finish maturing a dominant follicle. The follicles stall at the small antral stage. Those are the "cysts," and they are really immature follicles lined up around the edge of the ovary, not true cysts. With no dominant follicle, nothing triggers the LH surge, so there is no ovulation and no corpus luteum. Layered on top is insulin resistance, present in most patients and not just a matter of weight. High insulin pushes the theca cells to make androgens and at the same time tells the liver to make less sex hormone-binding globulin. So more of the testosterone in the blood runs free and active. Fat tissue converts androgens into estrone, which keeps the endometrium bathed in estrogen.

• Periods: few periods, none at all, or sudden heavy bleeding when a thick, unstable lining finally breaks down
• Androgen effects: hair growth in a male pattern, acne, thinning scalp hair, and rarely an enlarged clitoris. Full virilization should send you looking for an androgen-making tumor instead
• Metabolic: weight around the middle, acanthosis nigricans as a visible sign of high insulin, abnormal lipids, trouble handling glucose, type 2 diabetes, and sleep apnea
• Reproductive: infertility from not ovulating, plus higher rates of miscarriage, gestational diabetes, and preeclampsia
• Long-term: overgrowth of the endometrium, and endometrial cancer, from years of estrogen with nothing to balance it
• The treatment follows the mechanism. Weight loss and metformin go after the insulin resistance. A combined birth control pill brings back a regular withdrawal bleed, protects the endometrium, and raises SHBG so less testosterone runs free. Letrozole brings on ovulation for women who want to get pregnant

BPH versus prostate cancer: it all comes down to zones

The prostate has separate zones, and the two major diseases start in different ones. That is why they look so different. The transition zone wraps right around the prostatic urethra, and that is where benign overgrowth happens. So BPH squeezes the urethra early and announces itself with urinary symptoms. The peripheral zone sits at the back, against the rectum, and gives rise to roughly 70 to 80 percent of cancers. So cancer grows away from the urethra and stays quiet early, but you can feel it on a rectal exam. BPH is driven by dihydrotestosterone, which 5-alpha-reductase makes from testosterone inside the prostate's stromal cells, and it is an overgrowth of both gland and stromal tissue. It is not pre-cancerous. Having BPH neither causes cancer nor protects against it. The two simply show up together often in aging men.

Testicular torsion

Most patients are born with a bell-clapper deformity. The tunica vaginalis attaches too high on the spermatic cord, so the testis hangs free inside it like the clapper in a bell, instead of being anchored at the back. The testis can then spin on the cord. The order matters. The thin-walled veins collapse first, so blood cannot get out while blood keeps coming in. That brings fast congestion, heavy swelling, and pain far worse than the exam looks. As the pressure builds, the arteries are finally choked off too, and the tissue starts to die. That is why the clock is so unforgiving. More than 90 percent of testicles are saved within 6 hours, roughly 50 percent by 12 hours, and under 10 percent after 24 hours. It peaks at two ages: in newborns, and in boys 12 to 18, when a fast-growing testis and more activity raise the risk.

The three compartments and the 60-40-20 rule

About 60% of an adult's body weight is water. Two-thirds of it sits inside cells (ICF) and one-third sits outside (ECF). The ECF then splits roughly three to one into interstitial fluid and plasma. That last slice matters far more than its size suggests, because plasma is the only compartment that feeds organs. Water crosses between compartments on its own, by osmosis, chasing solute. It does not move because we want it to. So the compartments stay in osmotic balance all the time, and the only way to shift water is to change the osmolality on one side.

Tonicity: what a fluid does to a cell

Tonicity is not a property of the bag. It is a prediction about the cell. Compare the fluid you are giving to the ECF it is entering, then ask which way water has to move to even the two out.

The four causes of edema

Edema is extra fluid sitting in the interstitium, the space around cells, and there are only four ways to put it there. Three are problems with the Starling forces at the capillary: the push-out force is too high, the pull-in force is too low, or the wall has turned leaky. The fourth is a drainage problem, where the lymph vessels that normally mop up the small daily surplus are blocked.

Electrolyte problems and their signature findings

Sodium problems are almost always water problems in disguise, and they show up in the brain, because neurons swell or shrink with the osmolality. Potassium and calcium problems show up on the ECG, because both ions set the resting membrane voltage and how fast the cell resets.

Four moves, every single time

• Move 1, the pH (7.35 to 7.45). Under 7.35 is acidosis. Over 7.45 is alkalosis. If it is inside the range but not exactly 7.40, note which side of 7.40 it sits on. That side names the primary problem, even when compensation has hidden it.
• Move 2, the pCO2 (35 to 45 mmHg). A high CO2 means acid held in, from breathing too little. A low CO2 means acid blown off, from breathing too much.
• Move 3, the HCO3 (22 to 26 mEq/L). A low bicarbonate means acid was gained or base was lost. A high bicarbonate means acid was lost or base was gained.
• Move 4, match it up and look for compensation. Whichever value explains the pH is the primary problem. If the other value has moved the opposite way, compensation has started. If the pH is still abnormal, it is partly compensated. If the pH is back in range, it is fully compensated. If the other value has not moved at all, it is uncompensated.

The anion gap sorts out the acidoses

Once you know there is a metabolic acidosis, one calculation tells you the mechanism: anion gap = Na minus (Cl + HCO3), normally about 8 to 12. A wide gap means an acid you did not measure was added to the blood, such as ketones, lactate, uremic acids, or toxic alcohols, and it used up bicarbonate on the way in. A normal gap means bicarbonate was simply lost, most often in the stool with diarrhea or through the kidney tubules in renal tubular acidosis. Chloride is then reabsorbed to fill the empty slot, which is why this is also called a hyperchloremic acidosis.

Inside the adaptive arm, split humoral from cell-mediated. Humoral immunity is antibody made by plasma cells, and it goes after things floating in the fluid around cells: bacteria, toxins, and free virus. Cell-mediated immunity is CD8 killer T cells destroying the body's own cells once they have been taken over, and it handles viruses inside cells, fungi, tuberculosis, tumor cells, and transplanted tissue. That split explains the infections you see when each arm fails. Losing antibody brings repeated infections with encapsulated bacteria. Losing T cells brings opportunistic and fungal disease.

The four hypersensitivity reactions

Hypersensitivity is not broken immunity. It is normal immune machinery aimed at the wrong target, or turned up too far. Sort the four types by asking two questions. Which thing is doing the damage: IgE, antibody against a cell, immune complexes that settled out, or T cells? And how fast does it show up?

Autoimmunity versus immunodeficiency

Adaptation is the cell buying time

A cell under stress has three choices: adapt, die, or sit injured somewhere in between. Adaptation is a reversible, controlled change in size, number, or type that lets the cell keep working under a new load. The key point is that adaptation is not disease. It is compensation that worked. It turns dangerous only when the stress lasts so long that the adapted cell is itself abnormal, as when metaplasia drifts into dysplasia. Or when the compensating tissue creates a new problem, as when a thickened ventricle outgrows its own blood supply.

Injury that can be undone versus injury that cannot: where the line sits

Almost every cell injury on an exam starts with too little oxygen, because oxygen is what the cell needs to make ATP. When the ATP falls, the Na+/K+ pump stops, sodium and water pour into the cell, and the cell swells. That is the earliest change, and it can still be undone. The cell also switches to burning fuel without oxygen, lactate builds up, the inside of the cell turns acidic, and ribosomes fall off the rough ER, so protein building stalls. All of that reverses if blood flow comes back. The point of no return is the membranes. Once the lysosome and mitochondrial membranes tear and calcium floods the inside of the cell, the proteases, phospholipases, and endonucleases that calcium switches on digest the cell from within, and nothing brings it back.

Necrosis versus apoptosis

• Coagulative. The proteins set like a cooked egg, so the tissue outline stays. This is the pattern in an infarct of the heart, kidney, or spleen
• Liquefactive. Enzymes dissolve the tissue into fluid. This is brain infarcts and any bacterial abscess
• Caseous. A cheesy, crumbly center inside a granuloma. This is tuberculosis and some fungi
• Fat. Lipase frees fatty acids that bind calcium, which is saponification. This is acute pancreatitis and breast trauma
• Gangrenous. This is a clinical word. Dry gangrene is coagulative, in a limb starved of blood. Wet gangrene adds bacteria, so it is liquefactive. Gas gangrene comes from Clostridium perfringens

What actually makes a tumor malignant

People lose points here by grading tumors on size or on how fast they grow. A uterine fibroid can be the size of a grapefruit, and a basal cell carcinoma can be 4 mm across. Neither fact settles the question. What defines malignancy is invasion: the ability to break through the basement membrane, chew through the tissue around it, and seed distant sites. Everything else on the comparison table, such as poor differentiation, a big nucleus for the size of the cell, no capsule, and odd mitoses, is a clue under the microscope that predicts invasive behavior. It is not the definition of it.

Oncogenes versus tumor suppressor genes

Staging versus grading, the pair people flip most often

How spread happens, and where it goes

Every cardinal sign traces back to two changes in the vessels

Inflammation is how tissue with a blood supply answers injury, and it is a vessel event before it is a cell event. Injured cells and the mast cells living there release histamine, bradykinin, prostaglandins, leukotrienes, nitric oxide, and pieces of complement. Two things follow. The arterioles widen, and the capillaries and small veins turn leaky. Hold those two changes in mind and you can work out all five cardinal signs instead of memorizing them.

Acute versus chronic inflammation

Wound healing, and what gets in its way

Healing by primary intention is a clean surgical cut held together: little tissue lost, little granulation, and a fine scar. Secondary intention is an open wound with tissue missing, such as a pressure injury or an abdomen that opened back up. It has to fill in from the bottom with granulation tissue, it pulls itself smaller as myofibroblasts contract, and it leaves a big scar after a longer course with more risk of infection. Tertiary intention, also called delayed primary, is a dirty wound left open on purpose and closed later, once the infection risk has dropped.

• Poor blood flow or low oxygen. Think PAD, bad veins, pressure, and smoking, since nicotine narrows vessels and carbon monoxide pushes oxygen off the hemoglobin. Making collagen needs oxygen
• Diabetes and high blood sugar. Neutrophils cannot find or eat invaders as well, the small vessels are sugar-coated, and nerve damage hides the injury
• Infection or a foreign body. It keeps the wound stuck in the inflammatory phase and eats the oxygen and nutrients repair needs
• Poor nutrition. Protein is the raw material for collagen. Vitamin C is needed to cross-link it. Zinc is needed for skin and fibroblast growth. And vitamin A works against the effects of steroids
• Steroids and drugs that hold the immune system down. They dull the inflammatory phase, hold back fibroblasts and collagen, and hide the fever and redness that would announce an infection
• Chemotherapy and radiation. They hit the fast-dividing fibroblasts and surface cells that repair depends on
• Older age and extra weight. The surface closes more slowly, and fat has a poor blood supply and pulls on the wound edges
• Mechanical strain. Coughing, straining, extra weight, or lifting too soon, on a wound that is still at a fraction of its final strength

The Monro-Kellie doctrine: why compensation runs out all at once

The skull holds roughly 1,400 to 1,700 mL of three things: brain tissue, about 80%, blood, about 10%, and cerebrospinal fluid, about 10%. The Monro-Kellie doctrine says that if any one of the three grows, another has to shrink, or the pressure climbs. The brain buys time by pushing CSF down into the spinal space, absorbing CSF faster, and squeezing blood out of the brain's veins. Those tricks are limited, and they do not fail gradually. While they hold, a growing blood clot barely moves the ICP at all. Once they are used up, a few more milliliters send the pressure off a cliff. That is exactly why a head-injured patient can look deceptively stable and then crash within minutes.

A normal ICP is 5 to 15 mmHg. But what actually pushes blood through the brain is the cerebral perfusion pressure: CPP = MAP minus ICP, and it has to stay above roughly 60 mmHg. Below 50 the brain can no longer regulate its own flow and the tissue starts to starve. Below 30, death of tissue is nearly certain. Every ICP treatment is really a CPP treatment. Raise the head of the bed to 30 degrees with the head straight so the neck veins can drain. Avoid bending the neck or the hips. Keep suctioning short and grouped together. Treat fever, pain, and seizures, because each one raises the brain's demand. Keep the CO2 normal, because carbon dioxide is the most powerful thing there is for widening brain vessels. And pull water out of the brain with mannitol or hypertonic saline. Never hang a hypotonic fluid. Free water follows the osmotic pull straight into injured tissue.

Stroke: a blocked pipe versus a burst pipe

Around a blocked vessel, blood flow does not switch off. It falls off in a gradient. In the middle, called the infarct core, the flow is too low to keep the membranes running. The ATP fails, the sodium-potassium pump stops, sodium and water rush in and swell the cell, the cell loses its charge and dumps glutamate, the glutamate opens calcium channels on its neighbors, and the calcium switches on the proteases, lipases, and free radicals that finish the cell off. Around that core sits the penumbra. Collateral vessels give it just enough flow to stay alive, but not enough to fire. It is electrically silent, which is why the deficit you see is bigger than the dead tissue. And it is the whole target of opening the vessel back up.

Traumatic brain injury: the first hit, and the second one you can prevent

TBI splits in two. The primary injury is the tearing, bruising, and bleeding that happened at impact, and nothing undoes it. The secondary injury is the chain of low oxygen, low blood pressure, swelling, seizures, fever, and rising ICP that unfolds over the next hours to days. Secondary injury is the entire reason for repeated neuro checks. It can be prevented, and it is what actually decides how the patient ends up.

Seizures: excitement outrunning the brakes

A seizure is a sudden, synchronized firing of cortical neurons. Glutamate-driven excitement overwhelms the GABA brakes in a group of cells that normally fire on their own schedule. Focal seizures start in one half of the brain, and awareness may or may not be kept. Generalized seizures involve both halves from the start, and awareness is always lost. The tonic-clonic pattern is stiffening first, often with a cry as air is forced through the voice box, then rhythmic jerking. After it comes a postictal period of confusion, headache, and heavy sleepiness, while the worn-out cortex recovers. That postictal state is itself a clue, because events that are not true seizures usually lack it. Absence seizures in children are the opposite picture: a blank stare for three to ten seconds, with no warning and no postictal phase, and they are routinely mistaken for not paying attention.

• A new seizure is metabolic until proven otherwise. Look for a low sodium, a low glucose, a low calcium, low oxygen, a fever in a small child, WITHDRAWAL from alcohol or benzodiazepines, too much stimulant or local anesthetic, and eclampsia. Fix the cause and the seizure stops. Anticonvulsants alone will not hold it.
• Epilepsy means two or more seizures that nothing provoked. It is a lasting tendency, not a single event.
• During the event: nothing in the mouth, no holding the patient down, roll them on their side to protect the airway, pad the area and lower them to the floor, and note the time plus how it started. Where it starts tells you where the focus is.
• Check the glucose and the sodium at the bedside first. An EEG finds the abnormal firing, and imaging looks for a structural cause afterward.

The remodeling balance, and why estrogen matters

Bone is never still. Osteoclasts dig it out and osteoblasts fill it back in, and the peak bone mass is banked by about age 30. The osteoblasts actually control their own demolition crew with two signals. RANKL switches osteoclasts on. Osteoprotegerin is a decoy receptor that soaks the RANKL up. Estrogen keeps the osteoprotegerin high and keeps osteoclasts from living long. After menopause that brake comes off. Breakdown outruns building, and the thin struts of spongy bone go first: the vertebrae, the femoral neck, and the far end of the radius. Glucocorticoids reach the same place by a different road. They hold osteoblasts back, cut calcium absorption in the gut, and send more calcium out in the urine. That is why steroid treatment expected to run past three months calls for bone protection from day one.

After the fracture: three emergencies, three clocks

A fracture heals in overlapping stages. First a blood clot forms, then inflammation, then a soft cartilage-like callus, then a hard woven-bone callus, and finally remodeling into layered bone. That takes weeks. The complications that kill or cripple do not wait for it. Each of the three owns its own time window, and knowing the clock is usually enough to name the diagnosis before any test comes back.

Osteoarthritis versus rheumatoid arthritis versus gout

Pressure injuries: staged by what you can see

The pressure that closes a capillary in the skin is roughly 32 mmHg, and lying or sitting on a bony point passes that easily. The pressure flattens the capillaries, the tissue starves, and waste piles up. When the pressure comes off, blood rushes back in and the area reddens. Redness that BLANCHES when you press a finger on it and fades within minutes is healthy compensation, not an injury. Pressure held past that point kills cells. Three accomplices speed it up. Shear, where the skeleton slides down the bed while the skin stays put, kinks and tears the deep vessels. Friction strips the top layer of skin. And moisture from incontinence or sweat softens the barrier until it breaks down. Muscle handles starvation far worse than skin does, so serious injuries often start at the bone-muscle line and only reach the surface later.

Burns: depth, and the two phases of danger

A burn is dangerous twice, in opposite directions, and the electrolytes flip between the two. The early, resuscitation phase runs from the injury through roughly the first 24 to 48 hours. Histamine, bradykinin, prostaglandins, and oxygen radicals make the capillaries leak, plasma pours into the tissue, and the patient goes into burn shock that is both low-volume and poorly distributed at once. The blood concentrates, so the hematocrit runs HIGH even as volume is lost. The blood pressure falls, the urine drops off, the potassium runs HIGH from burst cells, the sodium runs low as it follows water into the tissue, and the blood turns acidic. The second, diuretic phase starts around 48 to 72 hours, once the capillaries seal again. Fluid in the tissue moves back into the vessels, and the danger flips to overload and fluid in the lungs, with a LOW potassium as it moves back into cells and is lost in the urine. Behind both sits the hypermetabolic phase, where the body's resting energy use can double, protein is broken down relentlessly, immunity is held down, and infection becomes the leading cause of death after the resuscitation is over.

Reading a family tree in four moves

Work the chart in a fixed order rather than hunting for the pattern you already have in mind. The order matters, because each move knocks out whole categories, and the fourth move is the single most powerful one on the exam.

• Does it show up in every generation? An unbroken line of affected people, generation after generation, means one bad copy is enough. That is dominant. If the trait disappears for a generation and then shows up among the children of two unaffected parents, those parents were quiet carriers. That is recessive.
• What is the sex ratio? Roughly equal numbers of men and women point to an autosome. A family tree that is almost all male, where the disease travels through unaffected women, is X-linked recessive. A male has only one X, so a single bad copy has nothing to balance it.
• Did a father pass it to a son? A father gives his son a Y, never his X. Even one case of father to son rules out both X-linked patterns for good.
• Look at an affected father's daughters. In X-linked dominant disease, an affected man passes it to all of his daughters and none of his sons, because every daughter gets his one X. That fan-shaped pattern is unmistakable once you look for it.

When whole chromosomes go wrong: nondisjunction

Nondisjunction is a mechanical failure to separate, not an inherited allele. That is why these disorders usually show up in families with no history at all. The mother's age matters because her eggs stop partway through meiosis during her own time in the womb and then sit there for decades. The protein clamps holding the chromosome pairs together wear out over time, so separation fails more often in an older egg. And because an extra chromosome misdoses hundreds of genes at once, the result is always multi-system: the face, the heart, the brain, and growth all together.

The risk next time: what carries over to the next pregnancy

The recurrence risk is figured per pregnancy, and it has no memory. Three affected children do not use up the 25% for a carrier couple. Every conception is an independent roll of the dice. For an extra chromosome caused by a one-off nondisjunction, the recurrence risk is low, roughly 1% or the mother's age-related risk, whichever is higher. That is precisely because the error was a one-time mechanical event and not an allele being passed down. The one exception that changes the counseling completely is a parent who carries a balanced Robertsonian translocation. That parent looks perfectly normal but makes unbalanced eggs or sperm over and over, so the recurrence risk is substantial, and the parents' own karyotypes should be checked.

• Penetrance is the share of people with the genotype who show any sign of it at all. Incomplete penetrance makes a dominant family tree look like it skipped a generation, which is a classic trap.
• Expressivity is how severe the disease is in the people who do show it. The same neurofibromatosis mutation may give one relative a few coffee-colored spots and another relative disfiguring tumors.
• Anticipation is when an unstable three-letter repeat grows longer as it passes to the next generation, so each generation is affected earlier and more severely. Huntington disease and fragile X work this way.
• Mosaicism is a mutation that appeared after fertilization, so only some cell lines carry it. The findings are patchy and usually milder.
• Heterozygote advantage is why a lethal recessive allele can stay common. Sickle trait resists falciparum malaria, so in regions where malaria is common the allele keeps its high frequency.

Work through it once and it becomes automatic. A high free T4 with a flattened TSH means the thyroid is making hormone on its own while a healthy pituitary tries to shut it off. That is primary hyperthyroidism. A high free T4 with a normal or high TSH makes no sense for a working loop, so the pituitary must be the problem, as with a TSH-making adenoma. A low cortisol with a high ACTH means the adrenal cortex has failed while the pituitary keeps shouting at it. That is primary adrenal insufficiency. A low cortisol with a low ACTH means the shouting has stopped, so the problem is at the top. The same arithmetic works for every axis you will be asked about.

ADH and the two water disorders

Antidiuretic hormone works on V2 receptors in the collecting duct and slots aquaporin-2 channels into the membrane. It moves water, not sodium, and that is the whole key to both diseases. Too much ADH means free water is reabsorbed and kept, which waters the plasma down. The sodium falls while the urine comes out oddly concentrated. Too little ADH, or a duct that cannot answer it, means free water pours out. You get liters of watery urine, a rising blood sodium and osmolality, and a thirst that never quits, which is the only thing keeping the patient afloat. An awake patient with a working thirst and a glass of water can hide diabetes insipidus for a long time. Sedate that patient, or put a tube in, and the sodium climbs fast.

Graves versus Hashimoto: same organ, opposite antibodies

Both are autoimmune, both can enlarge the gland, and both can start with an overactive phase. So what tells them apart is what the antibody does. In Graves disease, a thyroid-stimulating antibody binds the TSH receptor and switches it on. The gland acts as if the TSH were permanently high. Hormone output soars, the pituitary's TSH is shut off, and the whole gland grows. That same TSH receptor also sits on the fibroblasts behind the eye and in the shin. Stimulated, those cells lay down thick sugar-protein molecules, which gives bulging eyes and thickened skin over the shins. No other thyroid disease does that. In Hashimoto thyroiditis, antibodies against thyroid peroxidase and thyroglobulin come along with lymphocytes destroying the follicles. Stored hormone leaking from dying cells can cause a brief overactive spell, called hashitoxicosis. After that the gland burns out into permanent hypothyroidism.

That shared POMC parent molecule is worth holding on to, because it explains a physical sign. Whenever the ACTH runs high for a long time, either from primary adrenal failure or from a tumor making ACTH where it should not, the extra parent molecule also yields MSH. The patient darkens in the creases of the palms, inside the cheeks, in old scars, and over pressure points. So the darkening is not a random finding. It is a direct readout that the pituitary is fine and shouting at a cortex that cannot answer.

Diabetes: why type 1 makes ketones and type 2 usually does not

In type 1 diabetes the immune system destroys the beta cells, with anti-GAD and islet cell antibodies, so there is no insulin at all. With no insulin to hold back the enzyme that breaks down fat, fat breakdown runs free, fatty acids flood the liver, and ketoacids pile up. In type 2 diabetes there is insulin resistance with a partial shortage. There is not enough insulin to move glucose into cells, but there is still enough to hold fat breakdown back. That one remaining job is the whole reason type 2 patients usually do not make ketoacids. Instead they drift over days into a hyperosmolar state, as the glucose climbs and the osmotic diuresis dries them out.

Sorting anemia by cell size (MCV)

Cell size is not cosmetic. It is a fingerprint of the step that failed. A developing red cell keeps dividing until its cytoplasm is packed with hemoglobin. If the hemoglobin cannot be built, because there is no iron, no heme, or too few globin chains, the cell keeps dividing while it waits, and the finished product is small. If instead the DNA cannot be built, from a shortage of B12 or folate, the nucleus cannot keep up with a cytoplasm that keeps maturing, so the cell grows large before it manages to divide. If the cells that come out are a normal size, then the factory is fine, and the problem is loss, destruction, or a marrow that is never told to get to work.

Reading the iron studies as a set

The four numbers only make sense together. Serum iron is what is circulating right now. Ferritin is the stored form, and it mirrors the body's total iron. TIBC, meaning transferrin, is the delivery fleet. When the liver senses the stores are empty it builds more transferrin, so the TIBC moves the opposite way from the ferritin. Saturation is just how much of that fleet is loaded. The anemia of long-term illness is the great mimic. Inflammation drives hepcidin, which locks iron inside macrophages. So the serum iron falls even though the stores are full. The ferritin is normal or high, since it also rises with inflammation, and the TIBC is low, because the body is not short of iron. It is blocked from using it.

The hemolysis signature

When red cells die early, two things happen everywhere. The marrow ramps up, so the reticulocytes rise. And the contents of the broken cells spill out. The LDH rises, because it lives inside the cell. The heme is broken down into unconjugated, or indirect, bilirubin. And free hemoglobin in the plasma is mopped up by haptoglobin, which gets used up and falls. That is the whole signature: three up, one down. Where the cells die then decides the rest of the picture.

Sickle cell disease

A single point mutation swaps valine for glutamic acid at spot 6 of the beta-globin chain. The HbS that results behaves normally while it is carrying oxygen. But once it lets that oxygen go, the changed surface lets HbS molecules stack into stiff rods that bend the cell into a crescent. Those cells are rigid, sticky to the vessel lining, and short-lived, 10 to 20 days instead of 120. So the patient has two diseases at once. One is blocked vessels, which brings pain crises from starved tissue, plus stroke, priapism, and dying kidney tissue. The other is long-term hemolytic anemia. Anything that raises the share of hemoglobin without oxygen, or concentrates the hemoglobin inside the cell, sets off a crisis: low oxygen, dehydration, acidosis, fever and infection, cold that narrows vessels, and physical stress.

Repeated infarcts in the spleen leave the patient with a spleen that no longer works by early childhood. That strips away the body's main defense against encapsulated bacteria. It is why vaccines and quick antibiotics matter so much, and why a fever is treated as an emergency rather than a symptom. Hydroxyurea works by raising fetal hemoglobin, which physically dilutes the HbS and breaks up the stacking.

What each number actually measures

The PT and INR test the tissue-factor pathway, also called extrinsic, plus the shared final pathway, and factor VII dominates it. The aPTT tests the contact pathway, also called intrinsic, meaning factors XII, XI, IX, and VIII, plus the shared final pathway. So a long aPTT by itself points at factor VIII or IX, meaning hemophilia, or at heparin. A long PT by itself points at factor VII, so warfarin, vitamin K deficiency, or early liver disease. And both long means either the shared final pathway, meaning X, V, prothrombin, or fibrinogen, or a disaster affecting everything, such as DIC, liver failure, or massive transfusion. Fibrinogen is the raw material itself, and it stays normal in almost everything except DIC and end-stage liver failure. That is exactly what makes it so useful. D-dimer is not a clotting test at all. It is a piece of broken-down fibrin, so it tells you a clot was formed and then dissolved.

Disseminated intravascular coagulation

DIC is not a disease. It is a complication of something else: sepsis, which is the most common, a disaster in childbirth such as the placenta tearing loose, amniotic fluid entering the blood, or a retained fetus, plus major trauma or a crush injury, burns, cancer, and severe transfusion reactions. Each one floods the blood with tissue factor, which makes thrombin everywhere at once. Tiny clots lodge in the small vessels of the kidneys, lungs, brain, and fingers, starving those organs, and they shred passing red cells into fragments called schistocytes. Meanwhile the platelets and clotting factors used to build all that fibrin are used up faster than the liver can replace them, and plasmin switches on to dissolve the pile of clot. The result is the paradox at the center of DIC: the patient is clotting and bleeding at the same time.

Virchow's triad: why clots form where they do

An abnormal clot needs at least one of three conditions. Stasis: not moving, long flights, atrial fibrillation, varicose veins, or a stretched, failing ventricle. Slow blood lets clotting factors pile up in one spot instead of being washed away. Injury to the vessel lining: surgery, a catheter left in, trauma, smoking, high blood pressure, high blood sugar, or a torn atherosclerotic plaque. A healthy lining actively prevents clotting, so exposing the collagen and tissue factor underneath takes that protection away. A blood that clots too easily: pregnancy and the weeks after delivery, birth control with estrogen, cancer, factor V Leiden, antiphospholipid syndrome, nephrotic syndrome, where antithrombin III is lost in the urine, and the rise in fibrinogen after surgery.

The determinants and their levers

From atherosclerosis to a heart attack

It starts with injury to the vessel lining, from high blood pressure, smoking, high blood sugar, or a high LDL. The damaged lining turns leaky and sticky. LDL slips into the wall and is oxidized, monocytes move in and become macrophages that gorge on that oxidized LDL to become foam cells, and the fatty streak that results pulls in smooth muscle, which lays a fibrous cap over a soft fatty core. As long as that cap holds, the plaque only narrows the opening. The patient has symptoms only when demand outruns a fixed supply, which is stable angina, and that usually takes about 70 percent narrowing. The catastrophe is the rupture. A thin cap tears, exposing the clot-triggering fatty core and tissue factor to flowing blood. Platelets stick and clump within seconds, and a clot of fibrin and platelets can shut the vessel completely.

Angina versus a heart attack, and what the troponin actually tells you

Cardiac troponin I and T are structural parts of the heart muscle's contracting machinery. They can only reach the blood if the cell membrane has failed, which means the cell has died. So the troponin draws the line between unstable angina, where the tissue is starved but can still recover, and NSTEMI, where muscle has actually died. The two look identical at the bedside. It rises within about 3 to 4 hours, peaks around 24 hours, and stays up for 7 to 14 days. That is great for catching a late presentation but poor for spotting a second heart attack. And here is the key point: a high troponin means heart muscle injury, not a blocked coronary artery. It also rises in sepsis, a lung clot, myocarditis, worsening heart failure, and kidney failure.

Left versus right heart failure

Heart failure is easiest to understand as a plumbing problem. Whichever ventricle fails, blood dams up behind it while the output in front of it falls. The left ventricle takes blood from the lungs, so left failure floods the lungs. The right ventricle takes blood from the body, so right failure floods the body's veins.

Systolic versus diastolic failure

When the compensation becomes the disease

A falling cardiac output triggers three ancient reflexes that were built for bleeding, not for a weak pump. The sympathetic system switches on, which raises the heart rate and tightens the vessels. That raises the afterload and the heart's oxygen demand while shortening the very part of the beat when the coronary arteries actually get blood. RAAS switches on, which holds on to salt and water. That adds preload to a ventricle already stretched past anything useful, and angiotensin II and aldosterone drive scarring in the muscle. Then remodeling follows. Thickening makes a wall that outgrows its own blood supply and stiffens, while stretching pulls the sarcomeres past their best overlap, so each fiber generates less force. Every compensation raises the tension in the wall, and that wall tension is the thing killing the ventricle.

The kinds of shock

All shock is one thing: oxygen delivery falling below what cells need. That forces cells to burn fuel without oxygen, and they make lactate. What separates the categories is which part of the delivery system failed, and the pattern of pressures tells you which.

Where the GFR actually comes from

Each kidney holds about a million nephrons, and the glomerulus is the one capillary bed in the body sandwiched between two arterioles instead of between an arteriole and a vein. That setup is the whole trick, because the kidney can adjust the flow in and the flow out separately. The net filtering pressure is the pressure inside the glomerulus, about 55 mmHg, minus the pressure in the capsule around it, about 15, minus the pulling pressure of the plasma proteins, about 30. That leaves roughly 10 mmHg of usable push, and a normal GFR near 125 mL/min. To protect that number when the blood supply drops, the kidney widens the incoming arteriole using prostaglandins it makes on the spot, and tightens the outgoing arteriole using angiotensin II. In effect it dams the blood up inside the tuft.

This is why two very common drugs turn dangerous in a dehydrated patient. NSAIDs block prostaglandin production, so the incoming arteriole cannot widen. ACE inhibitors and ARBs block angiotensin II, so the outgoing arteriole cannot tighten. Either one alone is usually fine. Put them together in someone who is already low on volume and both protective arms are cut off, and the pressure in the glomerulus collapses. Downstream, the proximal tubule takes back about 65 percent of the filtrate plus all of the glucose. The loop of Henle builds the salt gradient in the medulla, and that gradient is what the strong loop diuretics attack. The distal tubule fine-tunes sodium and calcium. And the collecting duct has the final say, under hormone control, where ADH slots in water channels and aldosterone drives the sodium-for-potassium trade.

The three acute kidney injuries

AKI means a creatinine rise of at least 0.3 mg/dL within 48 hours, or 1.5 times the baseline within 7 days, or a urine output under 0.5 mL/kg/hr for 6 hours. Sorting it out is just asking where the problem sits relative to the nephron. Prerenal is a blood supply problem. The tubules are intact but starving, so they hoard sodium and water desperately. Intrarenal, most often acute tubular necrosis, means the tubule cells are dead or dying, so they can neither reclaim sodium nor build a salt gradient. Postrenal is a blockage that passes back-pressure up to the glomerulus and cancels the pressure that drives filtering.

Acute tubular necrosis runs a predictable three-act course, and each act has its own danger. The oliguric phase lasts one to three weeks, and it is where patients die. Potassium, acid, and fluid pile up with nowhere to go. The diuretic phase is deceptive good news, because filtering recovers before reabsorbing does. The patient pours out liters of watery urine and can crash into low volume and a low potassium while everyone is celebrating. The recovery phase takes months, and the ability to concentrate urine is the last thing to come back. The heavy urine output after a blocked bladder is relieved behaves exactly the same way.

Nephritic versus nephrotic

The filter has three layers. There is a vessel lining full of windows, a basement membrane studded with negatively charged heparan sulfate, and the foot processes of podocytes, joined by slit bridges made of nephrin. In nephritic disease, immune complexes lodge in the capillary wall, complement is switched on, and inflammatory cells multiply into the tuft. The wall turns leaky enough for red cells to squeeze through, but the channel also narrows, so filtering falls, sodium and water are held back, and the blood pressure rises. In nephrotic disease there is no inflammatory invasion at all. The podocytes simply flatten out and the negative charge barrier is lost, so albumin, which is small and negatively charged and would normally be pushed away, streams into the urine at more than 3.5 grams a day.

Chronic kidney disease: what dies when nephrons die

CKD means a GFR under 60 mL/min for three months or more. It also counts if damage markers such as albumin in the urine persist. It is staged G1 through G5, and dialysis is usually considered near a GFR of 15. Diabetes and high blood pressure together cause about two-thirds of cases. The engine that drives it forward is hyperfiltration. As nephrons drop out, the ones left grow bigger and filter harder to hold total GFR steady. That is exactly why creatinine looks normal for so long. It is also why the disease keeps feeding itself. The higher pressure inside those surviving filters scars them too. This is the whole reason we use ACE inhibitors, ARBs, and SGLT2 inhibitors in CKD. They lower the pressure inside the glomerulus. You accept a small dip in GFR now to save nephrons over years.

Obstructive versus restrictive

Spirometry answers the first question mechanically. In obstruction the airways narrow or collapse as you breathe out. So forced expiratory volume in one second falls much more than forced vital capacity, and the ratio drops below 0.70. Air that cannot get out stays behind, so residual volume and total lung capacity rise and the chest is overinflated. In restriction the lung or chest wall simply will not expand. Every volume shrinks together, total lung capacity falls, and the ratio stays normal or even rises above 0.80, because a stiff lung empties fast. Diffusing capacity then sorts out the cause. It is low in emphysema, where alveolar surface is destroyed, and in fibrosis, where the membrane is thickened. It is normal or high in asthma and in restriction from outside the lung, such as obesity or kyphoscoliosis.

How low blood oxygen happens

Asthma versus COPD

ARDS versus cardiogenic pulmonary edema

Tension pneumothorax

A tension pneumothorax is a one-way flap valve. Air enters the pleural space when you breathe in and cannot get out when you breathe out. So the pressure in that space climbs above atmospheric, and it rises with every single breath. The lung on that side collapses first. But the deadly step comes next. The mediastinum is pushed toward the other side of the chest, and the shift kinks the superior and inferior vena cava. Venous return falls, the right ventricle has nothing to pump, and cardiac output collapses. Be precise about this, because it changes what you do first. The patient is dying of obstructive shock, from missing preload, not mainly from failed oxygenation.