The Urinary System
Kidney anatomy, the nephron, filtration, reabsorption, secretion and micturition
By the end of this unit you can…
- ✓List the functions of the kidneys and describe their gross anatomy and blood supply
- ✓Describe the nephron, its two capillary beds and the juxtaglomerular complex
- ✓Explain glomerular filtration, net filtration pressure and the control of GFR
- ✓Describe tubular reabsorption and secretion along the nephron
- ✓Explain how the countercurrent mechanism and ADH produce dilute or concentrated urine
- ✓Describe renal clearance, urine composition, the ureters, bladder, urethra and micturition
Key terms
Practice →1Kidney functions & anatomy
The kidneys filter ~180 L of blood plasma per day, returning almost all of it — producing only ~1.5 L of urine. Functions: regulate water volume and solute concentration, ion concentrations (Na⁺, K⁺, Ca²⁺), acid–base balance, excrete metabolic wastes (urea, uric acid, creatinine) and foreign substances, and make renin (BP), erythropoietin (RBCs) and calcitriol (active vitamin D); also gluconeogenesis during fasting.
The bean-shaped kidneys are retroperitoneal, from about T12 to L3; the right kidney is slightly lower (displaced by the liver). Each is surrounded by a fibrous capsule, a perirenal fat capsule and renal fascia. Vessels, nerves and the ureter enter/leave at the hilum.
- Renal cortex
- Outer granular region (contains most nephron parts — glomeruli and convoluted tubules).
- Renal medulla
- Cone-shaped renal pyramids (striped by collecting ducts and loops) separated by renal columns; the pyramid tips are papillae.
- Minor & major calyces → renal pelvis
- Collect urine draining from the papillae and funnel it into the ureter.
- 1Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate (interlobular) arteries.
- 2Afferent arteriole → glomerulus → efferent arteriole.
- 3Peritubular capillaries (or vasa recta in the medulla) around the tubules.
- 4Cortical radiate veins → arcuate veins → interlobar veins → renal vein → IVC.
The kidneys receive ~25% of cardiac output (~1200 mL/min) even though they're < 1% of body mass.
2The nephron
Each kidney has over a million nephrons — the structural and functional units that form urine. Each nephron has a renal corpuscle and a renal tubule:
| Part | Features | Main job |
|---|---|---|
| Glomerulus | Tuft of fenestrated capillaries | Filtration |
| Glomerular (Bowman's) capsule | Parietal layer + visceral layer of podocytes whose filtration slits between foot processes form part of the filtration membrane | Collects filtrate |
| Proximal convoluted tubule (PCT) | Cuboidal cells with dense microvilli and many mitochondria | Most reabsorption (and secretion) |
| Nephron loop (loop of Henle) | Descending limb (permeable to water) and ascending limb (permeable to solutes, impermeable to water) | Creates the medullary osmotic gradient |
| Distal convoluted tubule (DCT) | Cuboidal cells, few microvilli | Hormonally regulated reabsorption (aldosterone, PTH); secretion |
| Collecting duct | Receives filtrate from many nephrons; principal cells (Na⁺, water — ADH, aldosterone) and intercalated cells (acid–base) | Final adjustment of urine |
~85% of nephrons are cortical nephrons; ~15% are juxtamedullary nephrons with long loops plunging deep into the medulla — key for making concentrated urine.
Two capillary beds: the glomerulus (high pressure — fed and drained by arterioles; the efferent arteriole is narrower, keeping pressure high for filtration) and peritubular capillaries (low pressure, porous — readily absorb reabsorbed solutes and water). The juxtaglomerular complex (JGC) where the DCT touches the afferent arteriole contains macula densa cells (sense NaCl in the filtrate) and granular (juxtaglomerular) cells (secrete renin).
3Glomerular filtration
Urine formation has three steps: glomerular filtration, tubular reabsorption and tubular secretion.
Filtration is a passive process driven by hydrostatic pressure. The filtration membrane (fenestrated endothelium, basement membrane, podocyte filtration slits) lets water and small solutes through but holds back blood cells and most proteins. Protein or blood in the urine signals damage to this membrane.
| Pressure | mmHg | Direction |
|---|---|---|
| Glomerular hydrostatic pressure (HP_gc) | 55 | Out of the glomerulus (promotes filtration) |
| Capsular hydrostatic pressure (HP_cs) | 15 | Into the glomerulus (opposes) |
| Colloid osmotic pressure of glomerular blood (OP_gc) | 30 | Into the glomerulus (opposes) |
| Net filtration pressure (NFP) | 55 − (15 + 30) = 10 | Drives filtration |
Glomerular filtration rate (GFR) — volume of filtrate formed per minute by both kidneys — is about 120–125 mL/min. GFR is held fairly constant:
- Intrinsic (renal autoregulation)
- Myogenic mechanism (stretch of the afferent arteriole → it constricts) and tubuloglomerular feedback (macula densa senses high NaCl/flow → afferent arteriole constricts). Keeps GFR stable over MAP 80–180 mmHg.
- Extrinsic — sympathetic
- In emergencies (hemorrhage), strong sympathetic stimulation constricts afferent arterioles → GFR falls, conserving blood volume for vital organs.
- Extrinsic — renin–angiotensin–aldosterone
- Low BP → granular cells release renin → angiotensin II → systemic vasoconstriction, aldosterone, ADH and thirst → BP and GFR restored.
4Reabsorption & secretion
Tubular reabsorption returns ~99% of the filtrate to the blood. The PCT reabsorbs the most: all glucose and amino acids, ~65% of Na⁺ and water, most bicarbonate, K⁺, Cl⁻ and other ions, plus small proteins. The key driver is active transport of Na⁺ by the Na⁺–K⁺ pump on the basolateral membrane; glucose, amino acids and others follow by secondary active transport (cotransport with Na⁺); water follows by osmosis (obligatory water reabsorption, through aquaporins).
Carriers have a transport maximum (Tm): when a substance exceeds it, the excess appears in urine. When blood glucose exceeds ~180 mg/dL, glucose spills into urine (glycosuria) — a sign of diabetes mellitus.
| Segment | Reabsorbs | Regulated by |
|---|---|---|
| PCT | Glucose, amino acids, ~65% Na⁺ and water, HCO₃⁻, K⁺ | Not hormonally regulated (obligatory) |
| Descending limb | Water only | — |
| Ascending limb | Na⁺, K⁺, Cl⁻ (Na⁺–K⁺–2Cl⁻ symporter) — no water | — |
| DCT & collecting duct | Na⁺ (aldosterone), Ca²⁺ (PTH), water (ADH) | Hormones — facultative reabsorption; ANP inhibits Na⁺ reabsorption |
Tubular secretion moves substances from peritubular capillaries into the filtrate: H⁺ (pH control), K⁺ (mostly in the collecting duct, driven by aldosterone), creatinine, ammonium, and many drugs (penicillin, phenobarbital). It disposes of substances not filtered and controls blood pH.
5Diluting & concentrating urine
The kidneys keep plasma osmolality near 300 mOsm by producing urine from ~50 (dilute) to ~1200 mOsm (concentrated). This depends on the medullary osmotic gradient — interstitial fluid gets saltier deeper into the medulla, from 300 to 1200 mOsm.
- Countercurrent multiplier (loops of juxtamedullary nephrons)
- Fluid flows in opposite directions in the two limbs. The descending limb loses water to the salty interstitium; the ascending limb pumps out NaCl but is impermeable to water. Each limb's activity reinforces the other, multiplying the gradient.
- Countercurrent exchanger (vasa recta)
- Blood vessels follow the loop, so they remove reabsorbed water and solutes without washing away the gradient.
- Urea recycling
- Urea leaving the collecting duct in the deep medulla contributes to the high osmolality.
Without ADH, the collecting ducts stay impermeable to water → large volumes of dilute urine. With ADH, aquaporins are inserted, water leaves the ducts into the salty medulla → small volumes of concentrated urine.
6Renal clearance & urine
Renal clearance is the volume of plasma the kidneys clear of a substance per minute: C = UV / P (U = urine concentration, V = urine flow rate, P = plasma concentration). Inulin is freely filtered and neither reabsorbed nor secreted, so its clearance equals GFR (~125 mL/min). Clinically, creatinine clearance estimates GFR.
| Normal urine | Abnormal findings |
|---|---|
| Clear, pale to deep yellow (urochrome from bilirubin) | Glucose (glycosuria — diabetes mellitus) |
| Slightly aromatic; pH ~6 (range 4.5–8) | Proteins (proteinuria/albuminuria — glomerular damage, hypertension) |
| Specific gravity 1.001–1.035 | Ketone bodies (ketonuria — starvation, diabetes) |
| 95% water; urea (largest solute), Na⁺, K⁺, PO₄³⁻, SO₄²⁻, creatinine, uric acid | RBCs (hematuria — kidney stones, infection, trauma); WBCs/pus (pyuria — UTI); bilirubin |
7Ureters, bladder, urethra & micturition
Ureters carry urine from the renal pelvis to the bladder by peristalsis, entering the bladder at an oblique angle (pressure in the filling bladder closes their openings, preventing backflow). Wall: transitional epithelium, smooth muscle, adventitia.
The urinary bladder is a collapsible muscular sac in the pelvis. Its smooth muscle is the detrusor; it's lined by transitional epithelium that stretches. The trigone — triangle between the two ureteral openings and the urethral opening — is a common site of infection. A full bladder holds ~500 mL (capacity up to ~1000 mL).
| Urethra | Female | Male |
|---|---|---|
| Length | 3–4 cm — short | ~20 cm — prostatic, intermediate (membranous) and spongy parts |
| Carries | Urine only | Urine and semen |
Sphincters: the internal urethral sphincter (involuntary smooth muscle at the bladder–urethra junction) and the external urethral sphincter (voluntary skeletal muscle of the pelvic floor).
- 1Bladder fills to ~200 mL → stretch receptors send signals to the spinal cord and pons.
- 2Pontine micturition center (when appropriate) triggers parasympathetic signals → the detrusor contracts and the internal sphincter relaxes.
- 3The person voluntarily relaxes the external urethral sphincter → urine flows out.
- 4If urination is postponed, reflex contractions subside and return as filling continues.