Renal & Urinary

The kidneys are the body’s quiet housekeepers. They filter waste, balance water and salts, and steady blood pressure. They even help make blood. This chapter starts at the nephron. From there it works outward to the ways kidneys fail fast or slowly, the stones and infections of the urinary tract, and what the labs are telling you.

Section 1The nephron: how the kidney cleans blood

Every minute, more than a liter of blood pours into your kidneys. By the time it leaves, it has been cleaned, balanced, and fine-tuned. The kidney does this with millions of tiny units called nephrons. There are roughly one million in each kidney. The nephron is the kidney's functional unit. That means it is the smallest structure that can do the whole job of making urine. So when we talk about kidney disease, we are really talking about nephrons. They get damaged, overworked, or they die.

The nephron in three steps: filter, reabsorb, secrete

Think of the nephron as a factory with three stations. First comes filtration. Blood enters a tuft of leaky capillaries called the glomerulus. The glomerulus sits inside a cup called Bowman's capsule. Blood pressure pushes water and small molecules through the filter and into the tubule. Sodium, glucose, urea, and potassium all pass through. This early fluid is called filtrate. Big things like blood cells and proteins are too large to pass, so normally they stay in the blood. That is why protein or blood in the urine is a red flag. It usually means the filter is leaking.

Next comes reabsorption along the tubule. The tubule has three parts: the proximal tubule, the loop of Henle, and the distal tubule. The body cannot afford to dump everything it just filtered. So about 99% of the water goes back into the blood. Nearly all the glucose goes back, and so does the sodium the body needs. Last comes secretion. Here the tubule actively adds unwanted substances into the filtrate to be peed out. Certain drugs, extra hydrogen ions, and extra potassium get added this way. What is left after all three steps is urine.

GFR: the number that tells you how well the kidney works

The best single measure of kidney function is the glomerular filtration rate (GFR). It is how many milliliters of filtrate all the glomeruli make each minute. A normal GFR is usually around 90-120 mL/min. When GFR falls, the kidney filters less. Wastes then start to back up in the blood. GFR is also how we stage chronic kidney disease (CKD). A GFR under 60 for three or more months defines CKD. A GFR under 15 signals kidney failure. We usually estimate it (eGFR) from a blood test instead of measuring it directly.

The kidney's other jobs (it is not just a filter)

Losing kidney function causes trouble far beyond waste buildup. That is because the kidney holds down several side jobs:

• Blood pressure, through the renin-angiotensin-aldosterone system (RAAS). When blood flow or pressure drops, the kidney releases renin. Renin sets off a chain of events that tightens vessels. It also holds on to sodium and water. Both raise blood pressure.
• Red blood cells. The kidney makes erythropoietin (EPO), the hormone that tells bone marrow to build red cells. This is why patients with failing kidneys often develop anemia.
• Activating vitamin D. The kidney turns vitamin D into its active form. Active vitamin D lets the gut absorb calcium. So kidney disease weakens bones and upsets calcium balance.
• Acid-base balance. The kidney gets rid of hydrogen ions and takes back bicarbonate. That keeps blood pH steady. Failing kidneys tend to drift toward metabolic acidosis.

Reading the labs: creatinine and BUN

Two blood tests show how the nephrons are doing. Creatinine is a waste product from normal muscle breakdown. The body makes it at a steady rate, and the kidney clears it. So when creatinine rises, it usually means filtration is falling. Creatinine is our everyday marker of GFR. BUN (blood urea nitrogen) measures urea, a waste from protein breakdown. BUN also rises when the kidney struggles. But it is thrown off more easily by dehydration, bleeding, and a high-protein diet. So it is read alongside creatinine rather than alone. Explore the nephron below to see filtration in action.

Section 2Acute kidney injury

Your kidneys are the body's quiet, steady filters. Every minute, about a fifth of the blood your heart pumps flows through them. Inside each kidney sit roughly a million tiny filtering units called nephrons. Each nephron starts with a knot of capillaries called the glomerulus. Blood pressure pushes fluid and waste across that filter to form urine. The tubules that follow reclaim the water, sodium, and other things your body wants to keep. Working normally, the kidneys clear metabolic garbage and balance fluid and electrolytes. They also help control blood pressure and red blood cell production. When they stumble suddenly, waste backs up fast.

Acute kidney injury (AKI) is a sudden drop in kidney function. It unfolds over hours to days. The key word is sudden. Chronic kidney disease, by contrast, creeps along for months to years. Here is the good news. Because AKI is abrupt, it is often reversible. You have to catch and fix the cause before the nephrons are damaged for good.

How we know the kidneys are failing

Two blood tests tell the story. Creatinine is a waste product from muscle that the kidneys normally clear at a steady rate. When filtering drops, creatinine rises. BUN (blood urea nitrogen) is a nitrogen waste from protein breakdown, and it climbs too. A rising creatinine means the glomerular filtration rate (GFR) is falling. GFR is the amount of blood filtered per minute. At the same time, the kidneys usually make less urine. Oliguria means putting out less than about 400 mL of urine in 24 hours. It is often described as under roughly 0.5 mL/kg/hr. Put simply: labs go up, urine goes down.

The three categories: where the problem lives

We sort AKI by where the trouble sits relative to the kidney itself. This matters because the fix is completely different for each.

Prerenal AKI is the most common. Anything that drops blood flow can cause it. Low volume from vomiting or bleeding, a weak heart, or wide-open vessels in sepsis all count. The glomerulus loses the pressure it needs to filter. The kidney tissue is still healthy. So if you restore blood flow quickly, function bounces back. Give fluids, treat the heart, stop the bleeding. If it is ignored, long-lasting low flow becomes ischemic damage. Then it tips into the intrarenal category.

Intrarenal (intrinsic) AKI means the nephron itself is injured. The classic form is acute tubular necrosis (ATN). The delicate tubule cells die. The cause is either long-lasting ischemia or toxins. Toxins include certain antibiotics (aminoglycosides), IV contrast dye, and myoglobin released in rhabdomyolysis. Dead tubule cells slough off and clog the tubules. They also cannot reabsorb properly.

Postrenal AKI is a plumbing problem downstream. Urine cannot get out. A stone, an enlarged prostate, a tumor, or even a kinked catheter blocks the flow. Pressure backs up into the kidney and filtration stops. This type is often the quickest to reverse. Relieve the blockage and urine pours out. But that only works if you catch it before back-pressure scars the tissue.

The phases of recovery

When AKI does progress, recovery classically moves through phases. The oliguric phase often lasts days to a couple of weeks. Urine output is low. Waste, potassium, and fluid pile up. This is the dangerous stretch. Watch closely for hyperkalemia, which means high potassium. It can trigger deadly heart rhythms. Watch for fluid overload and metabolic acidosis too.

As the tubules heal, the kidney enters the diuretic phase. The nephrons can filter again. But they have not regained the power to concentrate urine. So urine output surges, sometimes several liters a day. Now the risk flips. Patients can become dehydrated and lose too much sodium and potassium. Careful fluid and electrolyte replacement is essential. A slower recovery phase follows as function returns to normal. Some patients never fully get back to baseline.

Curious how the different causes stack up? Explore them in the figure below.

Section 3Chronic kidney disease

Your kidneys are quiet workhorses. Each kidney holds about a million tiny filtering units called nephrons. Every day they strain your blood. They pull out waste and extra fluid to make urine. They keep the good stuff in circulation: protein, blood cells, and most electrolytes. They also do jobs that have nothing to do with peeing. They make erythropoietin (EPO), which tells your bone marrow to build red blood cells. They activate vitamin D so your intestines can absorb calcium, which then strengthens bone. And they balance the blood's acid level. In chronic kidney disease (CKD), nephrons are slowly and permanently destroyed. So all of those jobs start to fail at once.

What CKD is and how we stage it

CKD is a slow, permanent loss of nephrons over months to years. Kidneys have huge reserve. So a person can lose a lot of function before feeling sick. We measure the damage with the glomerular filtration rate (GFR). Roughly, that is how many milliliters of blood the kidneys clean per minute. A normal GFR is usually around 90-120 mL/min. CKD is defined as kidney damage or a GFR below 60 for at least 3 months. It is staged from 1 (mild) to 5 (kidney failure).

Why it happens: diabetes and hypertension

The two leading causes are the same two conditions seen everywhere. In diabetes, long-term high blood glucose damages the delicate glomerular capillaries. That damage is called diabetic nephropathy. One of the earliest warning signs is small amounts of protein leaking into the urine (albuminuria). In hypertension, the constant high pressure batters those same tiny vessels. They thicken and scar. Cruelly, failing kidneys also raise blood pressure. They hold on to salt and water and fire the renin-angiotensin system. So hypertension both causes CKD and gets worse because of it. It is a vicious cycle.

Uremia: when wastes build up

As filtering fails, nitrogen wastes climb in the blood. Urea and creatinine are the main ones. This toxic buildup is called uremia. Blood reaches every organ, so the symptoms are body-wide. Patients often feel exhausted, sick to the stomach, and itchy. The itch is pruritus, from waste left in the skin. They may lose their appetite, taste metal, and think slowly. In severe uremia, wastes irritate the heart lining (pericarditis) and the brain. Brain involvement is uremic encephalopathy. It causes confusion and even seizures. Uremia is why the kidneys become a whole-body problem.

The complications and why each one happens

To understand CKD, trace each problem back to a kidney job that was lost:

• Anemia. Fewer working cells make less EPO, so the marrow builds fewer red cells. Patients are pale, tired, and short of breath.
• Bone disease (renal osteodystrophy). Kidneys can no longer activate vitamin D or get rid of phosphate. Low calcium and high phosphate drive the parathyroids to pump out PTH. PTH pulls calcium out of bones and leaves them weak.
• Hyperkalemia. The kidneys normally dump extra potassium. When they cannot, potassium rises. That threatens dangerous cardiac arrhythmias. This is often the most urgent lab.
• Fluid overload. Salt and water that are not excreted back up. The result is edema, hypertension, and fluid in the lungs.
• Metabolic acidosis. Kidneys normally excrete acid and make new bicarbonate. As that fails, acid builds up. It adds to fatigue, bone loss, and rapid breathing.

When the kidneys fail

Stage 5 is often called end-stage renal disease (ESRD). The kidneys can no longer keep the body's chemistry safe. There are two options. Dialysis filters the blood artificially, either with a machine (hemodialysis) or through the lining of the belly (peritoneal dialysis). The other option is a kidney transplant. Dialysis manages the disease but does not cure it. It replaces only a fraction of true kidney function. That is why prevention and slowing the disease matter so much. Explore the stages in the interactive figure below.

Section 4Glomerular disease: nephrotic vs nephritic

Every minute, your kidneys pull a huge amount of blood through roughly a million tiny filters called glomeruli. Picture each glomerulus as a wadded-up tuft of capillaries wrapped in a cup. Its filtering wall has three layers. First is the capillary lining (endothelium). Next is a middle mesh called the glomerular basement membrane (GBM). Last are special cells with interlocking foot processes, called podocytes. This wall is a smart sieve. It lets water, salts, and small wastes pass into the urine. But it holds back big things the body wants to keep, above all albumin (the main blood protein) and red blood cells. Two features make that possible. The pores sort by size. And the wall carries a negative charge that pushes away albumin, which is negative too. When this filter breaks, it tends to fail in one of two patterns. Telling them apart is one of the most important ideas in kidney disease.

Nephrotic syndrome: a very leaky filter

In nephrotic syndrome, the filter becomes too leaky without much inflammation. The podocytes and the charge barrier are damaged, so albumin pours out. The hallmark is massive proteinuria. That usually means more than 3.5 grams of protein in the urine per day. Losing that much protein drops the blood albumin. Low blood albumin is hypoalbuminemia. Albumin normally holds fluid inside vessels by oncotic (pulling) pressure. Lose it and fluid leaks into the tissues. That causes the heavy, all-over edema classic for this syndrome. Think puffy eyes in the morning and swollen legs and belly by evening. The liver senses the low protein and makes more of many proteins, including lipoproteins. So cholesterol and triglycerides climb (hyperlipidemia), and fatty casts can show up in urine. Patients also lose proteins that keep clotting in check, plus immunoglobulins. That raises the risk of thrombosis and infection. A classic cause in children is minimal change disease. In adults, membranous nephropathy and diabetic or lupus-related disease are common.

Nephritic syndrome: an inflamed filter

In nephritic syndrome, the problem is inflammation. Usually immune complexes lodge in the glomerulus and set off an immune attack. That inflammation physically breaks the capillary wall. Red blood cells then escape into the urine. The hallmark is hematuria, often tea- or cola-colored. Sometimes you also see red blood cell casts, which are tiny cell-molds shaped in the tubules. Those casts all but prove the bleeding comes from the glomerulus. Protein leaks too, but usually in smaller amounts, typically less than 3.5 g/day. Inflammation also swells and clogs the filter, so filtration drops. The kidney holds on to salt and water. That drives hypertension and a milder edema, often in the face. Creatinine also rises (azotemia) and urine output drops. The teaching classic is post-streptococcal glomerulonephritis (PSGN). A child develops cola-colored urine, puffy eyes, and high blood pressure roughly 1 to 3 weeks after a strep throat or skin infection. Immune complexes deposit in the glomeruli.

How to keep the two straight

Here is a simple anchor. NephrOtic = prOtein, which means a huge protein loss. NephrItic = Inflammation and hematurIa, which means blood. Nephrotic is a quiet, leaky sieve. Nephritic is an angry, bleeding, high-pressure one. Real patients can overlap. But the dominant pattern usually declares itself.

Explore the interactive figure below. It puts the leaky nephrotic filter next to the inflamed nephritic one.

Section 5Kidney stones

Your kidneys filter your blood all day long. The leftover minerals and salts dissolve into your urine. Think of urine as a river carrying dissolved cargo. As long as enough water is flowing, the minerals stay dissolved and wash out harmlessly. Kidney stones (nephrolithiasis) form when that balance tips. The urine becomes supersaturated. That means it holds more mineral than it can keep dissolved. The extra starts to crystallize, and those tiny crystals clump together into a hard stone. It is the same physics as making rock candy. The difference is that it happens inside the collecting system of the kidney, and nobody wants it.

Why the urine becomes supersaturated

Two things push urine past its saturation point. First, too little water (dehydration) concentrates the minerals. The same amount of solute is packed into less fluid. Second, too much mineral overloads the urine. That extra load comes from diet, genetics, or metabolic conditions. When calcium, oxalate, uric acid, or other salts climb high enough, crystals drop out of solution. Sluggish, concentrated, standing urine is the perfect place for a stone to grow. That is why low fluid intake over time is the single biggest risk factor you can change.

The pain: renal colic

A stone sitting quietly in the kidney may cause nothing at all. The trouble starts when it drops into the ureter, the narrow tube that drains urine to the bladder. The stone lodges and blocks the flow. Urine backs up. The ureter stretches and spasms as it tries to squeeze the stone along. That produces renal colic. It is sudden, awful, wave-like flank pain that classically radiates to the groin as the stone travels downward. Patients are famously restless and cannot find any position that helps. Compare that with peritonitis, where the patient lies still and guards the belly. The stone also scrapes the lining of the ureter. So you often see hematuria, which is blood in the urine. Sometimes you can see it, and sometimes only the dipstick finds it. Nausea, vomiting, and urinary urgency are common companions, because the kidney and gut share nerve pathways.

Types of stones

Calcium stones are by far the most common. They are usually calcium oxalate, and they make up roughly 75 to 80 percent of cases. Knowing the type matters, because prevention differs.

Here is a common point of confusion. Cutting calcium in the diet usually backfires. Normal calcium intake binds oxalate in the gut, so the oxalate leaves in the stool instead of the urine. That actually lowers the risk of calcium-oxalate stones. The real fix is fluid.

Care priorities

Management rests on three pillars while the stone passes or is treated:

• Pain control. Renal colic is genuinely one of the worst pains people report. NSAIDs like ketorolac are often first-line, because they reduce ureteral spasm and inflammation. Opioids may be added. Treat pain hard and early.
• Hydration. Push generous oral or IV fluids. Dilute, flowing urine helps flush the stone along and stops new crystals from forming. This is prevention and treatment in one.
• Strain all urine. Have the patient void through a fine mesh strainer to catch the stone. This confirms the stone passed. It also lets the lab analyze what the stone is made of, so prevention can be targeted.

Watch for warning signs that need escalation. Fever is one, because a blocked, infected kidney is an emergency. So are signs of complete obstruction, and pain or vomiting you cannot control. Larger stones that will not pass may need lithotripsy, which uses shock waves to break the stone apart. Some need surgical removal. The interactive figure below shows how a stone lodges in the ureter and produces colic. Explore it once you have the mechanism in mind.

Section 6UTIs & incontinence

Your urinary tract is basically a plumbing system with one-way flow and a built-in rinse. The kidneys make urine. The ureters carry it down. The bladder stores it. The urethra lets it out. From the bladder up, that flow is normally sterile. Every time you empty your bladder, you flush out stray bacteria trying to swim upstream. The lining fights back too, with a slightly acidic pH and mucus that makes it hard for microbes to stick. A urinary tract infection (UTI) happens when bacteria settle in the urethra and climb upward faster than the body can rinse them away. The usual culprit is Escherichia coli from the bowel.

Lower UTI (cystitis) vs upper UTI (pyelonephritis)

Where the bacteria land decides how sick the patient looks. In a lower UTI (cystitis), the infection stays in the bladder and urethra. The bladder wall becomes inflamed and touchy. So patients feel dysuria (burning when they pee), frequency, and urgency. Urine is often cloudy or foul-smelling, and there may be low suprapubic discomfort just above the pubic bone. The infection is local, so patients usually stay afebrile, meaning they run no fever. They do not feel sick all over.

An upper UTI (pyelonephritis) means bacteria have climbed the ureters into one or both kidneys. Kidney tissue is rich with blood, so the infection now reaches the bloodstream. The body mounts a full systemic response: fever, chills, nausea, and the classic flank pain. You will find costovertebral angle (CVA) tenderness when you tap over the kidney. Pyelonephritis is the more dangerous picture. It can progress to urosepsis, and over time it can scar the kidney.

Why women get more UTIs, and the catheter problem

Anatomy is the main reason women get far more UTIs than men. The female urethra is much shorter, roughly 4 cm versus about 20 cm in men. So bacteria have a shorter climb to the bladder. The opening also sits close to the vagina and anus, which gives perineal E. coli an easy route. Sexual activity pushes bacteria toward the urethra. And after menopause, lower estrogen thins the tissue and shifts the protective vaginal flora. That raises the risk again.

An indwelling (Foley) catheter is a major risk, because it bypasses every one of those natural defenses at once. The tube is a direct highway into the bladder. Bacteria form a biofilm along its surface, where antibiotics and immune cells cannot reach. Nonstop drainage also removes the flushing effect of a normal void. This is why catheter-associated UTIs (CAUTIs) are among the most common hospital-acquired infections. It is also why the single best prevention is simple: remove the catheter as early as possible.

The older adult who just gets confused

Older adults often skip the textbook burning-and-fever story entirely. A UTI may show up instead as new-onset confusion, delirium, or a sudden functional decline. That decline can look like new incontinence, falls, or refusing to eat. Part of the reason is a blunted immune response, which mutes fever. Part is that an aging brain is very sensitive to the systemic inflammation and dehydration that infection brings. The key point: an acute change in mental status in an older adult always deserves a workup that includes a urinalysis.

Urinary incontinence at a glance

Incontinence is leaking urine you did not mean to pass. It is a symptom, not a normal part of aging. The main types:

• Stress incontinence. Leaking with a cough, a laugh, or lifting. Weak pelvic floor support lets pressure overcome the sphincter.
• Urge incontinence. A sudden need to void that you cannot control. The detrusor muscle contracts on its own.
• Overflow incontinence. Constant dribbling from a bladder that never fully empties. The cause is often an obstruction, such as an enlarged prostate, or a weak, underactive detrusor.
• Functional incontinence. The urinary tract is normal. But a physical or thinking barrier, like poor mobility or dementia, keeps the person from reaching the toilet in time.

You can explore the layered urinary tract in the interactive figure below. It shows exactly where cystitis and pyelonephritis take hold.

ReferenceWords worth knowing

A quick reference for the terms in this chapter. Skim it now, or circle back whenever a word trips you up.

Acute kidney injury (AKI)

A sudden drop in kidney function over hours to days. It is often reversible. Waste products like creatinine and BUN build up in the blood, and urine output usually falls.

Acute tubular necrosis (ATN)

The most common intrarenal cause of AKI. The tubule cells are injured and die. The usual causes are long-lasting ischemia (low blood flow) or nephrotoxins such as certain antibiotics or contrast dye.

BUN (blood urea nitrogen)

A blood test that measures urea, a nitrogen waste from protein breakdown that the kidneys normally clear. A rising BUN often signals reduced kidney function or dehydration.

Chronic kidney disease (CKD)

A slow, permanent loss of kidney function lasting three months or longer. It is staged by the GFR and is usually caused over time by diabetes or hypertension.

Creatinine

A waste product from normal muscle metabolism that the kidneys filter and excrete steadily. So a rising serum creatinine is one of the most reliable signs that kidney function is declining.

Dialysis

A treatment that artificially filters wastes, extra fluid, and electrolytes from the blood, by hemodialysis or peritoneal dialysis, when the kidneys can no longer do the job well enough.

Erythropoietin (EPO)

A hormone made mostly by the kidneys that tells the bone marrow to produce red blood cells. That is why failing kidneys often cause anemia.

GFR (glomerular filtration rate)

The volume of blood the glomeruli filter each minute, normally about 90–120 mL/min. It is the best overall measure of how well the kidneys are working.

Glomerulus

The tiny tuft of capillaries at the start of each nephron, where blood is first filtered. Water and small wastes pass through, while blood cells and large proteins normally stay behind.

Nephron

The working filter unit of the kidney, with about one million per kidney. It filters blood and fine-tunes the urine through its glomerulus and connected tubules.

Nephritic syndrome

A pattern of glomerular inflammation. It is marked by blood in the urine (hematuria), mild to moderate protein loss, hypertension, and often a reduced GFR.

Nephrotic syndrome

A pattern of glomerular damage marked by heavy protein loss in the urine, usually more than 3.5 g/day. It also brings low blood albumin, widespread edema, and high blood lipids.

Oliguria

Urine output that is too low, usually defined as less than about 400 mL in 24 hours. It is a key warning sign of acute kidney injury.

Pyelonephritis

A bacterial infection of the kidney and its collecting system, often from a bladder infection that ascended upward. It typically causes fever, chills, and flank pain.

Renal colic

Severe, cramping, wave-like flank pain that radiates toward the groin. It is classically caused by a kidney stone that blocks and stretches the ureter.

Uremia

The toxic illness of advanced kidney failure, in which retained nitrogen wastes cause symptoms such as nausea, fatigue, itching, and confusion.

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