The Urinary System: How Your Body Filters Blood

Kidney Anatomy

The kidneys are paired, bean-shaped organs roughly the size of a fist, each weighing about 120 to 170 grams. They sit in the retroperitoneal space against the posterior abdominal wall, flanking the vertebral column at approximately the T12 to L3 level. The right kidney sits slightly lower than the left because of the liver above it. Each kidney is enclosed in a fibrous renal capsule and surrounded by a cushion of perirenal fat that protects it from impact.

A cross-section of the kidney reveals three distinct zones. The outer renal cortex, about 1 centimeter thick, contains the glomeruli and convoluted tubules of the nephrons, where filtration and most reabsorption occur. The renal medulla, beneath the cortex, consists of 8 to 18 cone-shaped renal pyramids, whose striped appearance results from parallel loops of Henle and collecting ducts. The tips of the pyramids (renal papillae) project into cup-shaped chambers called minor calyces, which merge into major calyces, which drain into the renal pelvis, a funnel-shaped structure continuous with the ureter.

The kidneys receive an extraordinary blood supply. The renal arteries, branching directly from the abdominal aorta, deliver approximately 1,200 milliliters of blood per minute, roughly 20% to 25% of cardiac output. This massive blood flow is not for the kidneys' own metabolic needs but to enable efficient filtration. Blood flows through a unique sequence of two capillary beds in series: the glomerular capillaries (where filtration occurs) and the peritubular capillaries (where reabsorbed substances re-enter the blood). This arrangement, found nowhere else in the body, allows fine-tuned regulation of both filtration pressure and reabsorption.

The Nephron: Functional Unit of the Kidney

Each kidney contains approximately 1 million nephrons, the microscopic tubular structures that perform all the kidney's filtering and processing work. A nephron consists of two main components: a renal corpuscle (where blood is filtered) and a renal tubule (where the filtrate is processed into urine).

The renal corpuscle contains the glomerulus, a dense ball of fenestrated (porous) capillaries, enclosed within Bowman's capsule, a double-walled cup. Blood pressure in the glomerular capillaries (about 55 mmHg, much higher than in typical capillaries) forces water and small dissolved molecules through the filtration membrane into Bowman's capsule. This membrane consists of three layers: the fenestrated capillary endothelium, the basement membrane, and the podocytes of Bowman's capsule, whose interdigitating foot processes create filtration slits. The membrane freely passes water, glucose, amino acids, urea, creatinine, ions, and small proteins, but blocks blood cells and most plasma proteins.

The renal tubule extends from Bowman's capsule through four distinct segments, each with specialized transport functions. The proximal convoluted tubule (PCT) reabsorbs about 65% of the filtrate, including virtually all filtered glucose and amino acids, most sodium, bicarbonate, and water, and many other substances. The loop of Henle, a U-shaped segment that dips into the medulla, establishes the osmotic gradient in the medulla that enables the kidney to concentrate urine. The distal convoluted tubule (DCT) fine-tunes sodium, potassium, and calcium balance under hormonal control. The collecting duct, shared by multiple nephrons, makes the final determination of urine concentration under the influence of antidiuretic hormone (ADH).

Urine Formation: Three Processes

Urine formation involves three processes occurring simultaneously across different nephron segments. Glomerular filtration produces about 180 liters of protein-free plasma filtrate per day (the glomerular filtration rate, or GFR, averages about 125 mL/min). GFR is determined by the balance of hydrostatic and osmotic pressures across the filtration membrane and is tightly regulated by autoregulatory mechanisms that keep GFR stable despite fluctuations in blood pressure between approximately 80 and 180 mmHg.

Tubular reabsorption recovers useful substances from the filtrate back into the blood. The PCT is the workhorse, using active transport (sodium-potassium ATPase pumps on the basolateral membrane), co-transporters, and channels to reabsorb sodium, glucose, amino acids, bicarbonate, phosphate, and water. In a healthy person, 100% of filtered glucose is reabsorbed in the PCT. However, if blood glucose exceeds approximately 180 mg/dL (the renal threshold), the glucose transporters become saturated and glucose spills into the urine, producing glucosuria, a hallmark of uncontrolled diabetes.

Tubular secretion moves substances from the peritubular capillaries into the tubular fluid, effectively adding them to the urine. This process eliminates substances like hydrogen ions (for pH regulation), potassium ions (regulated by aldosterone), creatinine, certain drugs (penicillin, aspirin metabolites), and toxins. Secretion is particularly important for substances that are protein-bound in the blood and therefore not filtered at the glomerulus, because secretion gives the kidney a second mechanism for clearing them.

Concentrating and Diluting Urine

The kidney's ability to produce concentrated urine (up to about 1,200 mOsm/L, four times the concentration of blood plasma) depends on the countercurrent multiplier system in the loop of Henle and the countercurrent exchanger in the vasa recta (specialized capillaries that supply the medulla). The descending limb of the loop of Henle is permeable to water but not to sodium, so water leaves by osmosis as the tubule descends into the increasingly salty medulla. The ascending limb is impermeable to water but actively pumps sodium and chloride out into the medullary interstitium, building the osmotic gradient.

ADH, released from the posterior pituitary in response to increased blood osmolarity or decreased blood volume, controls the final concentration of urine by inserting aquaporin-2 water channels into the collecting duct walls. When ADH is present, water flows from the collecting duct into the hyperosmotic medullary interstitium, producing concentrated urine. When ADH is absent (for example, after drinking a large volume of water), the collecting duct remains impermeable to water, and dilute urine is produced. This mechanism allows urine osmolarity to range from about 50 mOsm/L (very dilute) to 1,200 mOsm/L (maximally concentrated).

Kidneys and Blood Pressure

The kidneys regulate blood pressure through both fluid volume control and the renin-angiotensin-aldosterone system (RAAS). When blood pressure or sodium delivery to the kidney decreases, specialized cells in the juxtaglomerular apparatus release renin, an enzyme that catalyzes the conversion of angiotensinogen (produced by the liver) to angiotensin I. Angiotensin-converting enzyme (ACE), primarily in the pulmonary capillaries, converts angiotensin I to angiotensin II, a potent vasoconstrictor that also stimulates aldosterone secretion from the adrenal cortex and ADH release from the posterior pituitary.

Aldosterone promotes sodium reabsorption in the DCT and collecting duct, and water follows sodium osmotically, increasing blood volume. The combined effects of vasoconstriction, sodium retention, and water retention raise blood pressure. When pressure returns to normal, renin secretion decreases and the system downregulates. This is why ACE inhibitors, angiotensin receptor blockers (ARBs), and diuretics are first-line treatments for hypertension: they target different components of the same regulatory pathway.

Other Kidney Functions

The kidneys produce erythropoietin (EPO), a hormone that stimulates red blood cell production in the bone marrow. When the kidneys detect reduced oxygen delivery (from anemia, altitude, or lung disease), they increase EPO secretion, prompting the bone marrow to produce more red blood cells and improve oxygen-carrying capacity. Chronic kidney disease (CKD) impairs EPO production, causing the anemia that affects most CKD patients and is treated with synthetic EPO (epoetin alfa).

The kidneys activate vitamin D by converting its inactive circulating form (25-hydroxyvitamin D) to the active form (1,25-dihydroxyvitamin D, or calcitriol), which promotes intestinal calcium absorption. This function links the urinary, skeletal, and endocrine systems: parathyroid hormone stimulates the kidney to produce calcitriol, which increases calcium absorption, which raises blood calcium, which suppresses further PTH release. CKD disrupts this pathway, contributing to the bone disease (renal osteodystrophy) common in kidney failure patients.

The Lower Urinary Tract

Urine formed in the kidneys drains through the ureters, two muscular tubes about 25 to 30 centimeters long, to the urinary bladder. Peristaltic waves in the ureter walls propel urine downward, and the oblique angle at which the ureters enter the bladder wall creates a valve-like mechanism that prevents urine from refluxing back toward the kidneys during bladder contraction.

The urinary bladder, a hollow muscular organ, stores urine until voluntary voiding (micturition). The detrusor muscle in the bladder wall can stretch to accommodate 400 to 600 milliliters of urine, though the urge to void typically begins at about 200 to 300 milliliters. Voiding involves coordinated contraction of the detrusor muscle (parasympathetic control) and relaxation of the external urethral sphincter (voluntary, somatic control). In infants, voiding is purely reflexive. Voluntary control of the external sphincter develops between ages 2 and 4 as cortical inhibition of the voiding reflex matures.

Common Kidney Conditions

Chronic kidney disease affects approximately 850 million people worldwide and is the 12th leading cause of death globally. It is defined as reduced kidney function (GFR below 60 mL/min) or kidney damage persisting for three or more months. Diabetes and hypertension account for roughly two-thirds of CKD cases. The disease progresses through five stages based on GFR, from stage 1 (kidney damage with normal GFR above 90) to stage 5 (end-stage kidney disease, GFR below 15), which requires dialysis or kidney transplantation for survival.

Kidney stones (nephrolithiasis) affect approximately 1 in 11 people during their lifetime. Most stones are composed of calcium oxalate (about 80%), though uric acid, struvite, and cystine stones also occur. Stones form when dissolved minerals in the urine crystallize, often when urine is too concentrated from inadequate fluid intake. Small stones may pass through the urinary tract with intense pain (renal colic) but no lasting damage. Larger stones may require intervention: shock wave lithotripsy (external sound waves to fragment the stone), ureteroscopy (endoscopic retrieval), or percutaneous nephrolithotomy (surgical removal through a small incision). Drinking 2.5 to 3 liters of water daily is the single most effective prevention strategy.