Exam 5 Study Guide

Chapters 17 to 20 are the chapters of delivery. One system sends chemical orders through the blood. The other three build the blood, the pump and the pipes that carry those orders everywhere. Work one chapter at a time. Learn the named parts first, then the order they act in, then the numbers. Every table here is something worth copying out by hand once. Every self-test answer tells you why, not just what.

Chapter 17The Endocrine System

What to know

You have two ways to send orders. The nervous system uses electric signals along nerve cells. Each one goes to an exact spot and is over in a split second. The endocrine system uses chemical messengers called hormones. Glands drop them into the blood, and the blood carries them everywhere. You trade speed for reach. A hormone takes seconds to minutes to act, but its effect can be wide and can last a long time.

Endocrine glands are ductless. They have no tubes. They release hormones straight into the fluid around them and into the blood. Exocrine glands use a duct, a small tube, to send their product onto a surface. Sweat, spit and tear glands are exocrine. The pancreas does both jobs. Its exocrine part drips enzymes into a duct that empties into the gut. Its endocrine islands release insulin and glucagon into the blood.

A hormone reaches every cell. It only changes the cells that have the matching receptor. Those are its target cells. This is why one messenger in the blood can do a narrow job.

Chemistry decides how a hormone talks to a cell. Most hormones are amino-acid based, built from the parts that make protein. Insulin and growth hormone are peptides and proteins. The adrenal catecholamines are amines. These are water-soluble, so they mix with water but not with fat. They cannot cross the fatty cell membrane. The other family is the steroid hormones, built from cholesterol. Cortisol, aldosterone and the sex hormones are steroids. They are lipid-soluble, so they slip right through the membrane.

A lipid-soluble hormone walks in. It binds an intracellular receptor, usually in the nucleus. The pair works straight on the DNA and switches genes on or off. The cell then makes new proteins, often new enzymes. Building proteins takes time, so the start is slow, measured in hours. The new proteins stick around, so the effect is long.

A water-soluble hormone cannot get in. It binds a receptor on the cell surface and starts a relay inside. The relay runs on a second messenger, and the best known one is cyclic AMP. The hormone is the first messenger and stays outside. The second messenger switches on a chain of enzymes that were already waiting in the cell. They are ready-made, so the answer is fast, in seconds to minutes. It also fades fast once the hormone is gone.

The pituitary gland is about the size of a pea and hangs just under the brain. People call it the master gland because it directs so many others. It has a boss of its own. The hypothalamus sits just above it and turns brain signals into hormone orders. So the hypothalamus is the true master, and the pituitary is its second in command.

The posterior pituitary is the back lobe, and it makes nothing. It is nerve tissue that grew down from the hypothalamus. Two hormones are made by hypothalamic nerve cells, travel down their long arms, and are stored here until needed. They are antidiuretic hormone (ADH), which tells the kidney to save water, and oxytocin, which squeezes the uterus in childbirth and makes milk flow. The back lobe is a release station, not a factory.

The anterior pituitary is the front lobe, and it is real gland tissue. It makes six hormones, but only when the hypothalamus says so. The hypothalamus sends releasing and inhibiting hormones through a private set of vessels, the hypophyseal portal system, on the short trip down. Four of the six front-lobe hormones are tropic, which means their target is another gland. TSH drives the thyroid. ACTH drives the adrenal cortex. FSH and LH drive the gonads. Growth hormone acts on almost every tissue, and prolactin acts on the breast.

Blood sugar is one of the most closely guarded values in the body. The pancreatic islets guard it with two hormones that pull opposite ways, an antagonistic pair. Beta cells make insulin, and alpha cells make glucagon. After a meal, sugar rises, beta cells release insulin, and insulin lowers blood sugar. It tells body cells to take sugar in and tells the liver to store the extra as glycogen. Between meals, sugar drops, alpha cells release glucagon, and glucagon raises blood sugar. It tells the liver to break glycogen back down, which is glycogenolysis, and to build new sugar from other molecules, which is gluconeogenesis. Insulin is the hormone of the fed state. Glucagon is the hormone of the fasting state.

The thyroid gland is shaped like a butterfly and wraps the front of the windpipe. It makes thyroid hormone in two forms, T₃ and T₄. The numbers count iodine atoms, which is why you need iodine in your food. Thyroid hormone sets the basal metabolic rate, the speed at which almost every cell burns fuel and makes heat. It is the idle speed of the engine.

Three steps hold that speed steady. This is the hypothalamus–pituitary–thyroid axis, and it is a classic negative feedback loop. The hypothalamus releases TRH. TRH tells the front lobe to release TSH. TSH tells the thyroid to release T₃ and T₄. Then the thyroid hormones loop back and block both the hypothalamus and the pituitary. That is the brake. Plenty of hormone keeps the brake on. Falling hormone lets the brake go, TSH rises, and the thyroid works harder.

The adrenal gland caps each kidney, and it is really two glands joined. The outer adrenal cortex makes steroid hormones, and you cannot live without it. The inner adrenal medulla makes the catecholamines epinephrine and norepinephrine and is a direct part of the sympathetic nervous system. The medulla answers in seconds. The cortex takes minutes to hours.

The cortex has three zones, and you can read the job straight off the layout from the outside in. The zona glomerulosa makes mineralocorticoids, mainly aldosterone, which tells the kidney to hold sodium and water and to dump potassium. That raises blood volume and blood pressure. The wide middle zona fasciculata makes glucocorticoids, mainly cortisol, the main stress hormone. Cortisol raises blood sugar, frees up fuel and calms swelling. The inner zona reticularis makes small amounts of androgens such as DHEA.

Worth memorizing

The front lobe's six hormones fit the memory word FLAT PIG. The four tropic ones come first: FSH, LH, ACTH, TSH. Then PRL, GH and ignore the i. The two posterior hormones are the two that are left over, ADH and oxytocin.

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Chapter 18The Cardiovascular System: Blood

What to know

Blood is a connective tissue. A connective tissue has cells spread through an extracellular matrix. Blood's cells are the red cells, white cells and platelets. Its matrix is the liquid plasma they float in. What makes blood unusual is only that its matrix is a fluid, where bone has a hard one and cartilage a rubbery one.

An adult holds about 5 liters of blood, which is about 8 percent of body weight. It is about five times as thick as water and warm, at about 38 °C. It is slightly basic, with a pH held between 7.35 and 7.45.

Everything blood does fits three groups. Transport carries oxygen, nutrients and hormones out and carbon dioxide and wastes away. Regulation holds body temperature, pH and fluid balance steady. Protection fights germs with white cells and antibodies and seals leaks with clotting.

Spin a tube in a centrifuge and blood splits into three layers you can see. Plasma on top is about 55 percent. A thin buffy coat of white cells and platelets is under 1 percent. Packed red cells on the bottom are about 45 percent, and that bottom share has its own name, the hematocrit. Normal hematocrit runs about 37 to 47 percent in women and 42 to 52 percent in men.

Plasma is about 92 percent water. Most of the rest is plasma proteins. Albumin is the most common one. It holds water inside the vessels and carries fatty molecules. Globulins include antibodies and carrier proteins. Fibrinogen is the raw material for clots. One more term: let blood clot and lift the clot out, and the pale fluid left is serum, which is plasma without its clotting proteins.

The living cargo is the formed elements. Erythrocytes are red cells, leukocytes are white cells, and platelets are also called thrombocytes. Their numbers are very uneven. Red cells number about 5 million per microliter. Platelets number a few hundred thousand. White cells are rarest, at about 5,000 to 10,000 per microliter. That is why the spun tube is nearly all red on the bottom.

A red cell is a biconcave disc, a flat circle pinched in on both sides. The shape gives it plenty of surface for swapping gas and lets it fold through capillaries narrower than itself. The mature cell throws out almost everything else to make room for hemoglobin. It has no nucleus and no organelles. The cost is that it cannot repair itself, so it wears out in about 120 days, and the spleen and liver recycle it.

Every leukocyte keeps its nucleus. They split by whether you can see grains inside. Granulocytes are neutrophils, eosinophils and basophils. Agranulocytes are lymphocytes and monocytes. From most to least common, the order is neutrophil, lymphocyte, monocyte, eosinophil, basophil. The sentence Never Let Monkeys Eat Bananas holds it.

Platelets are not whole cells. They are cell fragments pinched off giant marrow cells called megakaryocytes. They have no nucleus and live about 10 days. They rush to a wound, pile up and start a clot.

Your body makes hundreds of billions of blood cells a day. The factory is red bone marrow, and the process is hematopoiesis. All of them come from one kind of parent, the hematopoietic stem cell, also called a hemocytoblast. It is multipotent, which means it can become any blood cell but only a blood cell. Save pluripotent for embryonic and induced stem cells, which can build tissues from all three germ layers.

Early on, the offspring pick one of two branches. The myeloid line is the broad one. It makes red cells, platelets by way of megakaryocytes, and most white cells: neutrophils, eosinophils, basophils and monocytes. The lymphoid line makes only the lymphocytes, which are the T, B and NK cells. Leukemias are named myeloid or lymphoid after which branch went wrong.

Hormones set the pace. Erythropoietin (EPO) comes from the kidney when it senses low oxygen, and it drives red cell making. Thrombopoietin comes from the liver and drives platelet making. Colony-stimulating factors and interleukins boost white cells during an infection. Raw materials matter too: iron for hemoglobin, and vitamin B₁₂ and folate to build DNA. Unfinished new red cells are reticulocytes, and counting them shows how hard the marrow is working.

Hemoglobin is why red cells matter. Each one is built from four protein chains called globins, two alpha and two beta. Each chain holds a heme group with an iron atom at its centre, and oxygen binds that iron. Four hemes means four O₂ per hemoglobin. One red cell holds about 270 million hemoglobins, so it can carry over a billion oxygen molecules.

Hemoglobin grabs oxygen where there is plenty and lets go where there is little. In the lungs the oxygen pressure is high, so it binds O₂ and becomes oxyhemoglobin, a bright scarlet red. In the tissues the oxygen pressure is low, so it releases its load and becomes deoxyhemoglobin, a darker brick red. Binding is also cooperative: once the first O₂ is on, the next ones bind more easily. Warmth, extra carbon dioxide and more acid all push hemoglobin to let go more oxygen, and those are exactly the conditions in a hard-working muscle.

Carbon dioxide rides home three ways, and the shares are worth knowing. About 70 percent travels as bicarbonate in the plasma. About 23 percent rides on hemoglobin as carbaminohemoglobin. About 7 percent is simply dissolved. Oxygen is lopsided the other way: about 98.5 percent on hemoglobin and about 1.5 percent dissolved. Carbon monoxide beats oxygen to hemoglobin and blocks delivery.

Hemostasis means blood standing still. Three overlapping steps seal a leak, and each buys time for the next. First, vascular spasm: smooth muscle in the vessel wall tightens and the vessel narrows. Second, the platelet plug: platelets stick to bare collagen, switch on and pile up into a soft plug. Third, coagulation: a chain of clotting factors builds a fibrin mesh that locks the plug in place.

The plug snowballs. A torn vessel exposes the tough collagen under its smooth lining. Platelets grab the collagen, helped by a linking protein called von Willebrand factor. Contact turns them from smooth discs into spiky, sticky, active platelets, and they release ADP, thromboxane A₂ and serotonin. Those chemicals call in and switch on more platelets, which is positive feedback. Aspirin blocks thromboxane, which is why low-dose aspirin makes platelets less sticky.

Coagulation turns a weak plug into a strong clot. Plasma proteins called clotting factors switch each other on in a cascade, like falling dominoes. Two starting paths, the intrinsic and the extrinsic, join into one common pathway. The last two moves are the ones to remember: the enzyme thrombin is switched on, and thrombin turns fibrinogen, which dissolves, into threads of fibrin, which do not. That net traps red cells and platelets, and the net is the clot. Calcium, which is clotting factor IV, is needed at several steps, and the liver needs vitamin K to build several factors in the first place. Later an enzyme called plasmin breaks the clot up, which is fibrinolysis.

Your blood type depends on flags on your red cells called antigens, or agglutinogens. In the ABO system the flags are A and B. Here is the rule that drives everything: your plasma holds antibodies against any antigen you lack. Type A has the A antigen and anti-B. Type B has B and anti-A. Type AB has both antigens and no ABO antibodies. Type O has neither antigen and both antibodies.

Give someone red cells carrying an antigen their plasma has antibodies against, and the antibodies clump the donor cells together. That is agglutination. Then the cells burst, which is hemolysis. The pieces clog small vessels and overload the kidneys, and a serious reaction can kill. So blood is always typed and cross-matched first.

On top of ABO sits the Rh system, named for the rhesus monkey used in the tests that found it. It has several antigens, C, c, D, E and e, but D triggers the immune system by far the most, so D is the only one tested as a routine. Have D and you are Rh-positive. Lack it and you are Rh-negative. ABO antibodies are present from early in life, but an Rh-negative person only makes anti-D after meeting Rh-positive blood. That matters in pregnancy. An Rh-negative mother carrying an Rh-positive baby can become sensitized to D. In a later Rh-positive pregnancy her antibodies may cross the placenta and attack the baby's red cells, which is hemolytic disease of the newborn. A shot of Rh immune globulin clears the baby's cells from her blood before her immune system reacts.

Worth memorizing

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The traps people fall into

Chapter 19The Cardiovascular System: The Heart

What to know

The heart sits in the mediastinum, the space between the two lungs, resting on the diaphragm. It is tilted, so about two-thirds of it lies left of the midline. The broad top where the big vessels attach is the base. The blunt tip pointing down and to the left is the apex, and that is where you feel the beat against the chest.

A tough two-walled sac wraps it, the pericardium. The outer fibrous pericardium holds the heart in place and keeps it from overfilling. Inside that the serous pericardium has two thin layers and makes a slippery fluid in the pericardial cavity, so the beating heart glides without rubbing. Fill that space with fluid or blood and the heart is squeezed, which is cardiac tamponade, an emergency.

Here is the most useful idea in the chapter. The heart is two pumps bolted together, side by side, working in step. The right heart takes blood low in oxygen back from the body and pushes it a short way to the lungs. The left heart takes fresh blood back from the lungs and sends it to the whole rest of the body. A wall of muscle, the septum, keeps the sides from mixing. Each pump has a thin-walled atrium on top that receives and a thick-walled ventricle below that pumps. Blood always moves atrium, then ventricle, then out an artery.

The wall has three layers. The endocardium is the smooth inner lining, and it joins the lining of the vessels so blood never meets a rough surface. The myocardium is the thick middle layer of heart muscle that does the pumping, and it is much thicker on the left. The epicardium is the outer layer, which is the serous pericardium layer stuck to the heart.

The left ventricle wall is much thicker than the right one because it must make about five times as much pressure. It pushes blood all the way around the body, while the right side only has to reach the nearby lungs. All that muscle needs its own supply, which the coronary arteries bring from the aorta. Block one and the muscle past the block starves and dies, which is a myocardial infarction.

Now follow one drop. It comes back from the body low in oxygen through the superior and inferior venae cavae into the right atrium. It drops through the tricuspid valve into the right ventricle, which squeezes it through the pulmonary valve into the pulmonary trunk and on to the lungs. There it gives off carbon dioxide and picks up oxygen. It returns through the pulmonary veins into the left atrium, passes the mitral valve into the left ventricle, and is pushed through the aortic valve into the aorta.

That one loop is really two loops end to end. The pulmonary circuit runs from the right heart to the lungs and back, and its only job is to swap gases. The systemic circuit runs from the left heart to every other tissue and back. They are linked in series, so every drop must pass through both, and the two ventricles must move the same amount of blood over time. If the right side pumped more for long, blood would back up in the lungs. If the left side pumped more, blood would back up in the body. That is why failure on one side slowly drags down the other.

Four one-way valves keep blood moving forward. The two atrioventricular valves sit between each atrium and the ventricle below. The tricuspid is on the right with three cusps. The mitral, also called bicuspid, is on the left with two. The two semilunar valves guard the artery exits: the pulmonary valve at the pulmonary trunk and the aortic valve at the aorta.

The two kinds stop backflow differently. A hard ventricular squeeze could push the AV flaps up into the atria, so each flap is tied down by tough strings called the chordae tendineae. They attach to bumps of muscle on the ventricle floor, the papillary muscles, which tighten just before the squeeze. The flaps close flat but cannot flip inside out, like the ropes on a parachute. The semilunar valves need no strings. Each has three cup-shaped pockets that catch the blood falling back, and its weight snaps them shut.

Valves fail two ways. A stenotic valve is stiff and narrow and will not open all the way, so the heart strains to push through a smaller hole. A regurgitant or insufficient valve is leaky and will not close all the way, so blood sloshes backward each beat. Both make blood swirl roughly, and that swirl is a murmur.

Your arm muscles wait for the brain. The heart does not. Special cells fire on their own, over and over, which is autorhythmicity. The fastest of them form the sinoatrial node in the wall of the right atrium. It fires about 60 to 100 times a minute and sets the pace for everything else, which is why it is the pacemaker. Cut every nerve to the heart and it keeps beating. The nerves only speed it up or slow it down.

The spark follows a set order so the chambers squeeze in sequence. It starts at the SA node and sweeps across both atria, which squeeze and top off the ventricles. It reaches the atrioventricular node, the only electrical doorway between atria and ventricles, where it pauses on purpose for about a tenth of a second. From there it races down the AV bundle, also called the bundle of His, splits into the right and left bundle branches along the septum, and fans out through the Purkinje fibers into the ventricle walls. The ventricles squeeze from the apex up, which pushes blood toward the exits like squeezing a tube from the bottom.

The AV pause is not a flaw. It gives the atria just enough time to finish emptying into the ventricles. Without it all four chambers would squeeze at once and the heart would move far less blood. The system also has backups. If the SA node quits, the AV node takes over at 40 to 60 a minute. The Purkinje fibers can take over more slowly still, at 20 to 40. The autonomic nervous system is the speed dial. Sympathetic nerves speed the SA node and make each squeeze stronger. The parasympathetic vagus nerve slows the heart at rest.

One beat switches between two states. Systole is the squeeze, when a chamber tightens to push blood out. Diastole is the rest, when it loosens and fills. At about 75 beats a minute, one full cycle takes about 0.8 seconds, and most of that time is diastole. The rest is not laziness. It is when the ventricles refill, and it is the only time the coronary arteries can push blood into the thick heart muscle. Beat too fast and you cut that short.

The phases run in order. Atrial systole is a gentle squeeze that tops off ventricles already mostly full, adding the last 20 to 30 percent. Ventricular systole has two parts: isovolumetric contraction, where all four valves are shut and pressure builds but no blood moves, then ventricular ejection, where the semilunar valves pop open and blood rushes out. Ventricular diastole also has two parts: isovolumetric relaxation, with all valves shut again, then ventricular filling, where the AV valves open and blood pours in mostly on its own.

The two heart sounds come from valves closing, not opening. S1 is the low lub, the AV valves snapping shut as the ventricles start to squeeze. S2 is the sharper dub, the semilunar valves snapping shut as the ventricles start to relax. Between the two sounds the heart is pumping out. After S2 it is filling.

Four numbers tie it together. End-diastolic volume is what is in a ventricle when filling is done. End-systolic volume is what is left after it pumps. Stroke volume is what actually leaves in one beat, so stroke volume equals EDV minus ESV. Cardiac output is heart rate times stroke volume, and at rest it is about 5 liters a minute.

Each firing spreads a wave of electrical change called depolarization, strong enough to reach the skin. Sensors pick it up and the plot over time is an electrocardiogram. It does not record the beat. It records the electrical events that set off the beat, so each bump lines up with a step in the wiring. The small P wave is atrial depolarization. The tall, spiky QRS complex is ventricular depolarization, big because the ventricles hold so much muscle. The atria reset at the same time, but that signal hides inside the QRS. The rounded T wave is ventricular repolarization, the ventricles resetting so they can relax and fill.

The flat parts matter too. The PR interval runs from the start of P to the start of QRS and holds the atrial firing plus the AV pause. Too long and the person has a heart block. The QRS duration shows how fast the ventricles are being fired. The QT interval covers the whole span of ventricular electrical activity. Because the paper moves at a fixed speed, the space between two QRS peaks gives the rate. Too fast is tachycardia and too slow is bradycardia. A messy shaking line with no real QRS is ventricular fibrillation, where the ventricles only twitch and no blood moves, and a defibrillator is built to shock that back into order. ST elevation is a warning sign of a heart attack.

Worth memorizing

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The traps people fall into

Chapter 20Blood Vessels & Circulation

What to know

Every artery and vein is built the same way, with layers called tunics wrapped around a hollow channel called the lumen. From the inside out they are the tunica intima, the tunica media and the tunica externa. Every vessel has all three. What changes from an artery to a vein to a capillary is how thick each layer is and how much muscle or stretchy tissue it holds.

The tunica intima is mostly one sheet of flat cells, the endothelium. That sheet lines the heart and every vessel in one unbroken surface. It is so smooth that blood cells glide past without clotting. It also sends out signals, such as nitric oxide, that tell the muscle below to relax or tighten. Under it is a thin base layer, and in arteries a springy internal elastic lamina as well.

The tunica media is usually thickest, and it is the vessel's muscle: rings of smooth muscle mixed with stretchy fibers. Tighten the muscle and the lumen narrows, which is vasoconstriction. Relax it and the lumen widens, which is vasodilation. Nerves and hormones work this layer, so the media is the dial the body turns to send blood where it is needed.

The tunica externa is a tough sleeve of collagen. It anchors the vessel and keeps it from stretching too far. Big vessels have walls too thick to feed from the inside, so their externa carries its own small vessels, the vasa vasorum, meaning vessels of the vessels.

An artery carries blood away from the heart, which is a rule about direction, not oxygen. Arteries take a hard push with every beat, so they have a thick, muscular media and strong stretchy walls. The biggest ones near the heart, like the aorta, are elastic arteries. Their walls stretch as the ventricle pumps and spring back between beats, turning a pulsing flow into a steadier stream. Farther out, muscular arteries bring blood to each organ. The smallest branches are the arterioles, which act like nozzles you can adjust.

A vein carries blood back toward the heart. By then nearly all the pressure is spent in the capillaries, so veins need no thick muscular wall. They have a thin middle layer, a wide floppy lumen and a large outer layer. Their key feature is one-way valves, flaps of the inner lining that snap shut to stop backsliding. Veins stretch easily, so they are the body's blood reservoir. About two-thirds of all your blood sits in your veins at rest, ready to be squeezed back into use.

Between the smallest arteriole and the smallest venule are the capillaries, the reason the whole system exists. A capillary wall is just one layer of endothelium on a thin base layer, which is the tunica intima with everything else stripped away. That paper-thin wall is the point, because gases, sugar and wastes slip right across it. There are so many that no cell is more than about a tenth of a millimeter from one. They come in three levels of leakiness. Continuous capillaries are tightest, in muscle and the brain. Fenestrated capillaries have small windows, in the kidney and intestine. Sinusoidal capillaries have the widest gaps, in the liver and bone marrow.

Blood pressure is the force of moving blood pushing on the vessel wall, measured in millimeters of mercury. In the big arteries it rises and falls with each beat. The peak during the ventricular squeeze is the systolic pressure. The low point while the ventricles relax and refill is the diastolic pressure. A typical healthy reading is about 120/80 mm Hg, systolic over diastolic.

The gap between the two numbers is the pulse pressure, which is systolic minus diastolic. For 120/80 that is 40 mm Hg, and it is the surge you feel at the wrist. Because the heart rests longer than it squeezes, the true average pushing pressure is not the middle of the two numbers. The mean arterial pressure is estimated as the diastolic pressure plus one-third of the pulse pressure. For 120/80 that is about 80 plus 13, so about 93 mm Hg. MAP matters because it is the pressure that actually pushes blood through the organs, and below about 60 mm Hg the brain and kidneys go short.

Follow blood outward and its pressure drops the whole way. That drop is the pressure gradient, and it is exactly what keeps blood moving, because fluid always runs from high pressure to low. Pressure is high and pulsing in the aorta and large arteries. It falls off a cliff across the arterioles. By the venae cavae it is near zero.

Flow follows one rule: flow equals the pressure difference divided by resistance. Resistance is how hard a vessel pushes back, and it depends most on the vessel's radius. Resistance rises as radius falls, and very steeply, with the fourth power of the radius. Halve the radius and resistance goes up 16 times. Double it and resistance drops to one sixteenth. Because arterioles have muscular walls that tighten or widen on command, they are the body's main resistance vessels. They set both how much blood each organ gets and your overall blood pressure.

Capillary exchange is a tug-of-war between two pressures. Hydrostatic pressure is the blood pressure inside the capillary, and it pushes fluid out through the wall into the tissue spaces. Colloid osmotic pressure, also called oncotic pressure, comes mostly from albumin, a protein too big to leave the blood. Because those proteins stay behind, they pull water back in by osmosis. Fluid moves whichever way the stronger pressure wins.

The balance flips along the capillary. At the arteriolar end, hydrostatic pressure is about 35 mm Hg against an osmotic pull of about 25 mm Hg, so fluid is pushed out. That is filtration, and it brings water, oxygen and nutrients to the cells. As blood moves along, hydrostatic pressure fades while the osmotic pull holds steady. By the venule end hydrostatic pressure has dropped below the osmotic pull, so fluid is drawn back in. That is reabsorption, and it picks up carbon dioxide and wastes. The difference between push and pull at any point is the net filtration pressure.

The two sides do not quite balance. Capillaries push out a little more than they take back. About 85 percent returns to the blood and about 15 percent stays in the tissues, and the lymphatic vessels mop that up and return it later. If any part of this fails, fluid builds up and the tissue swells, which is edema. Three things tip the balance: too much hydrostatic pressure, as in heart failure or a leg you stand on too long; too little albumin to pull fluid back, as in liver disease, kidney disease or too little protein in food; or blocked lymph drainage.

Blood runs in set loops called circuits. The pulmonary circuit is the short loop to the lungs: the right heart pumps blood low in oxygen out the pulmonary arteries, the lung capillaries load oxygen and dump carbon dioxide, and the pulmonary veins bring it back to the left heart. The systemic circuit is the long loop to everything else: the left heart pumps blood rich in oxygen out the aorta, and the veins bring it back low in oxygen to the right heart. The two run in series, so the same amount of blood must pass through both.

Most blood passes one capillary bed between leaving the heart and coming back. A portal system is different: blood passes two capillary beds in a row, joined by a vein. The big example is the hepatic portal system. Blood leaving the stomach and intestines is full of nutrients just absorbed, and it may carry drugs or poisons too. Instead of going straight back to the heart, the hepatic portal vein sends it into a second capillary bed inside the liver. There nutrients are stored, harmful chemicals cleaned out and blood sugar adjusted. Only then does the blood move on through the hepatic veins.

A few organs have special setups. The coronary circulation is the heart feeding itself, with arteries branching off the aorta the moment blood leaves. The cerebral circulation feeds the brain and includes a protective ring of linked arteries, the circle of Willis. If one artery into the ring is blocked, others send blood around the gap. Links between vessels like that are anastomoses, natural detours that give a backup supply. Where there are few of them, a blockage is far more dangerous, which is one reason a clot in a coronary artery so often causes a heart attack.

By the time blood reaches the veins it has almost no pressure left, often around 15 mm Hg, falling toward zero near the heart. Yet the blood in your feet must climb more than a meter against gravity. The amount coming back each minute is venous return, and over time it must exactly match cardiac output. With so little pressure, veins cannot do it alone.

The main helper is the skeletal muscle pump. Deep veins run between your muscles, and every tightening squeezes them and forces blood along. The one-way valves make the squeeze work: the valve above opens to let blood rush up, and the valve below slams shut so it cannot fall back. Tighten, and blood moves toward the heart. Relax, and the valves hold the ground you gained. The respiratory pump helps too. Breathing in makes the chest bigger and drops the pressure inside it, while pressure in the belly rises and squeezes the belly veins, drawing blood up into the low-pressure chest veins. A third helper is venoconstriction, where nerves tighten the vein walls and push stored blood back into use.

When the helpers fail, the results are familiar. Stand still a long time and the muscle pump stops, blood pools in the legs, and you may faint. Weak or leaky valves let blood collect and stretch the veins into bulging varicose veins. Blood that sits still, as on a long flight or after surgery, can clot in a deep vein, which is a deep vein thrombosis.

Worth memorizing

How to reason through the question types

The traps people fall into

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