Urinary System — revision sheet

The urinary system — two kidneys, two ureters, the urinary bladder and the urethra — filters about 180 L of plasma a day and returns all but ≈1–2 L of it, fine-tuning the volume, osmolality, electrolyte and acid–base composition of the extracellular fluid. The kidneys also excrete nitrogenous wastes and drugs, regulate blood pressure through the renin–angiotensin–aldosterone system and act as endocrine organs (erythropoietin, calcitriol, renin).

Kidney size / mass: ≈11–12 × 5–7 × 3 cm; ≈125–170 gVertebral level: T12–L3 (left slightly higher, ≈T11–L2)Nephrons per kidney: ≈1 million (wide range ≈0.2–2.5 million)Renal blood flow: ≈1.1–1.2 L/min (≈20–25% of cardiac output)Renal plasma flow: ≈600–650 mL/minGFR: ≈125 mL/min (≈180 L/day); lower in women and with ageFiltration fraction: ≈20% (GFR/RPF)Urine output: ≈1–2 L/day; oliguria <400 mL/day; anuria <100 mL/dayUrine osmolality range: ≈50–1200 mOsm/kg H₂OUreter length: ≈25–30 cm; diameter ≈3–5 mmBladder capacity: ≈400–600 mL; first sensation ≈150 mLUrethral length: Female ≈4 cm; male ≈18–20 cm

Kidney — gross anatomy

Kidney Organ: Paired, bean-shaped retroperitoneal organs on the posterior abdominal wall (≈T12–L3) that filter the blood and produce urine, regulating fluid, electrolyte and acid–base balance and secreting erythropoietin, renin and calcitriol.
Renal capsule and perirenal fascia Membrane: The layered coverings of the kidney: a tough fibrous capsule on the surface, perirenal fat, the renal (Gerota) fascia enclosing kidney and suprarenal gland, and outer pararenal fat.
Renal hilum and renal sinus Region: The vertical slit on the medial border of the kidney through which the renal vein, renal artery and renal pelvis pass (anterior → posterior), opening into the fat-filled renal sinus.
Renal cortex Organ part: The outer, granular, reddish-brown layer of the kidney parenchyma (≈1 cm thick) containing the renal corpuscles, convoluted tubules and cortical collecting ducts; it also extends between the pyramids as renal columns.
Renal medulla Organ part: The inner part of the kidney parenchyma, made of 8–18 striated, cone-shaped renal pyramids containing the loops of Henle, collecting ducts and vasa recta, which build the hypertonic gradient for concentrating urine.
Renal pyramid Organ part: A cone-shaped, striated mass of medullary tissue whose base faces the cortex and whose apex (papilla) drains urine into a minor calyx.
Renal column Organ part: An inward extension of cortical tissue between adjacent renal pyramids that carries the interlobar arteries and veins toward the cortex.
Renal papilla Organ part: The rounded apex of a renal pyramid, perforated by the openings of the papillary ducts, through which urine drips into a minor calyx.
Minor calyx Duct: A cup-shaped tube of the collecting system that embraces one (or more) renal papillae and receives urine from it; ≈7–13 per kidney.
Major calyx Duct: One of usually 2–3 (superior, middle, inferior) large channels formed by the union of minor calyces, which converge to form the renal pelvis.
Renal pelvis Organ part: The funnel-shaped, expanded upper end of the ureter, formed by the confluence of the major calyces, that collects urine and narrows at the ureteropelvic junction.

Renal vasculature

Segmental arteries of the kidney Artery: Usually five end-arteries — apical (superior), anterosuperior, anteroinferior, inferior and posterior — formed by division of the renal artery at the hilum, each supplying an independent vascular segment of the kidney.
Interlobar arteries Artery: Branches of the segmental arteries that pass through the renal columns between the pyramids toward the corticomedullary junction, where they turn to form the arcuate arteries.
Arcuate arteries Artery: Arched arteries running along the corticomedullary junction over the bases of the renal pyramids, giving rise to the cortical radiate (interlobular) arteries.
Cortical radiate arteries Artery: Arteries that radiate from the arcuate arteries toward the renal capsule through the cortex, giving off the afferent arterioles of the glomeruli.
Afferent arteriole Vessel: The small artery that carries blood from a cortical radiate artery into the glomerulus; its tone is the main regulator of glomerular blood flow and its wall contains the renin-secreting juxtaglomerular cells.
Efferent arteriole Vessel: The arteriole that drains the glomerulus and leads to a second capillary bed (peritubular capillaries or vasa recta); its resistance maintains high glomerular pressure and is preferentially raised by angiotensin II.
Peritubular capillaries Capillary bed: The low-pressure capillary network derived from efferent arterioles that surrounds the cortical tubules, taking up reabsorbed fluid and supplying solutes for secretion.
Vasa recta Capillary bed: Long, hairpin-shaped capillaries derived from the efferent arterioles of juxtamedullary glomeruli that supply the medulla and act as countercurrent exchangers, preserving the medullary osmotic gradient.

Nephron — renal corpuscle

Nephron Organ part: The structural and functional unit of the kidney (≈1 million per kidney): a renal corpuscle that filters plasma and a tubule — PCT, loop of Henle, DCT — that modifies the filtrate before it drains into a collecting duct.
Cortical nephron Organ part: The majority type of nephron (≈85%), with its glomerulus in the outer/mid cortex and a short loop of Henle that turns in the outer medulla; its efferent arteriole forms peritubular capillaries.
Juxtamedullary nephron Organ part: A nephron (≈15%) whose large glomerulus lies near the corticomedullary junction and whose long loop of Henle penetrates deep into the inner medulla; its efferent arteriole forms the vasa recta. These nephrons build the medullary osmotic gradient.
Renal corpuscle Organ part: The filtering unit at the start of each nephron (≈200 µm), formed by the glomerular capillary tuft invaginated into the double-walled glomerular (Bowman) capsule.
Glomerulus Capillary bed: A tuft of highly permeable, fenestrated capillaries interposed between the afferent and efferent arterioles, where ≈20% of the plasma entering is filtered at high pressure into the glomerular capsule.
Glomerular capsule Organ part: The double-walled epithelial cup at the start of the nephron that encloses the glomerulus and collects the filtrate in the capsular (Bowman) space before it flows into the PCT.
Glomerular filtration barrier Membrane: The three-layer sieve between glomerular blood and Bowman space — fenestrated endothelium, glomerular basement membrane and podocyte slit diaphragms — that is highly permeable to water and small solutes but restricts proteins by size and negative charge.
Podocyte Cell: Highly specialised, terminally differentiated epithelial cells of the visceral layer of Bowman capsule whose interdigitating foot processes and slit diaphragms form the outer layer of the glomerular filtration barrier.
Mesangial cells Cell: Contractile, phagocytic, smooth-muscle-like cells with surrounding matrix that support the glomerular capillary loops; extraglomerular (lacis) mesangial cells form part of the juxtaglomerular apparatus.

Nephron — tubule

Proximal convoluted tubule Organ part: The first, longest (≈14 mm) segment of the renal tubule, lined by brush-bordered cuboidal cells rich in mitochondria, which isosmotically reabsorbs ≈65–67% of filtered Na⁺ and water and nearly all filtered glucose, amino acids and most bicarbonate.
Nephron loop (loop of Henle) Organ part: The U-shaped part of the nephron between the PCT and DCT that dips into the medulla — descending limb, (thin ascending limb) and thick ascending limb — reabsorbing ≈25% of filtered NaCl and ≈15–20% of water and generating the medullary osmotic gradient.
Thin descending limb of the nephron loop Organ part: A thin, water-permeable segment of the loop of Henle lined by squamous epithelium that concentrates the tubular fluid by losing water to the hypertonic medullary interstitium.
Thin ascending limb of the nephron loop Organ part: A thin, water-impermeable segment present only in the long loops of juxtamedullary nephrons, in which NaCl leaves the concentrated tubular fluid passively into the inner medullary interstitium.
Thick ascending limb of the nephron loop Organ part: The water-impermeable "diluting segment" of the loop of Henle that actively reabsorbs ≈25% of filtered Na⁺ via NKCC2, drives the countercurrent multiplier and paracellular Ca²⁺/Mg²⁺ reabsorption, and ends at the macula densa.
Distal convoluted tubule Organ part: A short (≈5 mm) cortical segment after the macula densa that reabsorbs ≈5–7% of filtered NaCl via the thiazide-sensitive NCC, is impermeable to water and is the main site of PTH-regulated Ca²⁺ reabsorption.
Collecting duct Duct: The final, hormonally regulated segment of the urine-forming system, receiving fluid from several nephrons and running from cortex to papilla; principal cells adjust Na⁺, K⁺ and water (aldosterone, ADH) and intercalated cells adjust acid–base.
Principal cell (collecting duct) Cell: The majority cell type of the connecting tubule and collecting duct, which reabsorbs Na⁺ through ENaC and secretes K⁺ under aldosterone control and reabsorbs water through ADH-regulated aquaporin-2.
Intercalated cell (collecting duct) Cell: Mitochondria-rich cells scattered among principal cells in the collecting duct that fine-tune acid–base balance: α (type A) cells secrete H⁺ and reabsorb K⁺; β (type B) cells secrete HCO₃⁻.

Juxtaglomerular apparatus

Juxtaglomerular apparatus Organ part: A sensory–secretory complex at the vascular pole of each glomerulus, formed by the macula densa of the distal tubule, renin-secreting juxtaglomerular cells of the afferent arteriole and extraglomerular mesangial cells; it regulates GFR (tubuloglomerular feedback) and renin release.
Macula densa Cell: A plaque of tall, densely packed epithelial cells in the wall of the terminal thick ascending limb, facing the vascular pole, that senses tubular NaCl and signals to the afferent arteriole and JG cells.
Juxtaglomerular cells Cell: Modified, renin-granule-containing smooth muscle cells in the wall of the afferent arteriole at the vascular pole — the only significant source of circulating renin.

Lower urinary tract

Ureter Organ: A muscular, retroperitoneal tube ≈25–30 cm long that carries urine from the renal pelvis to the urinary bladder by peristalsis, with three physiological narrowings where stones lodge.
Urinary bladder Organ: A hollow, distensible muscular reservoir in the anterior pelvis that stores urine at low pressure (capacity ≈400–600 mL) and expels it by detrusor contraction during micturition.
Detrusor muscle Muscle: The smooth muscle of the bladder wall, arranged in interlacing bundles, that relaxes to accommodate filling and contracts as a single unit, under parasympathetic (M3) control, to empty the bladder.N Parasympathetic pelvic splanchnic nerves S2–S4 (contraction, M3); sympathetic hypogastric nerve T11–L2 (relaxation, β3).
Trigone of the bladder Region: The smooth, triangular area of the internal surface of the bladder base bounded by the two ureteric orifices and the internal urethral orifice.
Internal urethral sphincter Muscle: An involuntary smooth muscle sphincter at the bladder neck, contracted by sympathetic α1 stimulation, which maintains continence at rest and (in males) prevents retrograde ejaculation.N Sympathetic T11–L2 via the hypogastric nerve and inferior hypogastric plexus (α1-adrenoceptors).
External urethral sphincter Muscle: A voluntary, skeletal muscle sphincter surrounding the membranous urethra (males) or middle third of the urethra (females), innervated by the pudendal nerve (S2–S4), which provides conscious control of continence.N Pudendal nerve (perineal branch), S2–S4 — somatic, nicotinic cholinergic.
Urethra Organ: The fibromuscular tube that conveys urine from the bladder to the exterior: short (≈4 cm) and purely urinary in females; long (≈18–20 cm), with prostatic, membranous and spongy parts, and shared with the reproductive tract in males.

Renal physiology

Urine Fluid: The aqueous solution of wastes, excess water and electrolytes produced by the kidneys (≈1–2 L/day), whose volume, osmolality and pH vary widely to keep plasma composition constant.
Glomerular filtration Physiological process: The bulk flow of protein-free plasma from the glomerular capillaries into Bowman space, driven by a net filtration pressure of ≈10 mmHg across a very permeable barrier; normal GFR ≈125 mL/min (≈180 L/day).
Tubular reabsorption Physiological process: The selective return of filtered water and solutes from the tubular lumen to the peritubular blood — >99% of filtered water and Na⁺, essentially all glucose and amino acids — largely powered by the basolateral Na⁺/K⁺-ATPase.
Tubular secretion Physiological process: The transfer of substances from the peritubular capillary blood into the tubular lumen — mainly H⁺, K⁺, NH₄⁺, organic anions and cations including many drugs — allowing excretion beyond what filtration alone achieves.
Countercurrent mechanism Physiological process: The system by which loops of Henle (multiplier), urea recycling and vasa recta (exchanger) build and preserve a corticomedullary osmotic gradient of ≈300 → 1200 mOsm/kg, allowing ADH-regulated production of concentrated urine.
Renal autoregulation Physiological process: Intrinsic mechanisms — the myogenic response and tubuloglomerular feedback — that keep renal blood flow and GFR nearly constant over a mean arterial pressure of ≈80–180 mmHg.
Renal clearance Physiological process: The volume of plasma from which a substance is completely removed by the kidneys per unit time, C = (U × V) / P; used to measure GFR (inulin, creatinine) and renal plasma flow (PAH).
Renal regulation of acid–base balance Physiological process: The kidney maintains plasma HCO₃⁻ (≈24 mmol/L) by reclaiming virtually all filtered bicarbonate and generating new bicarbonate through excretion of ≈50–100 mmol/day of acid as titratable acid and ammonium.
Micturition reflex Physiological process: The spinobulbospinal reflex, coordinated by the pontine micturition centre and permitted by the cortex, that switches the lower urinary tract from storage (detrusor relaxed, sphincters closed) to voiding (detrusor contracts, sphincters relax).

Mnemonics

Hilum of the kidney, anterior → posterior: VAP — Vein, Artery, Pelvis (renal vein most anterior, renal pelvis/ureter most posterior).
Ureter and its crossings: Water under the bridge — the ureter (water) passes under the uterine artery in females and under the ductus (vas) deferens in males. Risk to the ureter when ligating the uterine artery at hysterectomy.
Functions of the kidney: A WET BED — Acid–base balance, Water balance, Erythropoietin, Toxin and waste removal, Blood pressure (renin), Electrolytes, vitamin D activation.
Indications for urgent dialysis: AEIOU — Acidosis (severe metabolic), Electrolytes (refractory hyperkalaemia), Intoxication with dialysable drugs (SLIME: Salicylates, Lithium, Isopropanol/methanol, Metformin, Ethylene glycol), Overload of fluid (refractory pulmonary oedema), Uraemia (pericarditis, encephalopathy, bleeding).
Parts of the male urethra, proximal → distal: PMS — Prostatic (widest, most dilatable), Membranous (shortest; passes through the external sphincter; injured in pelvic fractures), Spongy/penile (longest; injured in straddle injuries).
Causes of renal papillary necrosis: POSTCARDS — Pyelonephritis, Obstruction, Sickle cell disease, Tuberculosis, Cirrhosis, Analgesics (NSAIDs), Renal vein thrombosis, Diabetes mellitus, Systemic vasculitis.
Micturition: autonomic roles: Parasympathetic = Pee (S2–S4 pelvic splanchnics, M3 → detrusor contracts). Sympathetic = Storage (T11–L2 hypogastric, β3 relaxes detrusor, α1 closes the bladder neck). "S2, 3, 4 keeps the pee off the floor" — the pudendal nerve to the external sphincter.

From the Corpus anatomy atlas — for education only.