describe the structure of the human kidney, limited to: fibrous capsule, cortex, medulla, renal pelvis, ureter, branches of the renal artery and renal vein

Homeostasis in Mammals – The Human Kidney

Learning Objective

Describe the structure of the human kidney (fibrous capsule, cortex, medulla, renal pelvis, ureter, and the main branches of the renal artery and renal vein) and explain how each part contributes to the kidney’s homeostatic functions – water‑balance, ion balance, acid‑base regulation, waste removal, hormone production and blood‑pressure control – as required by Cambridge AS & A‑Level Biology (Topic 14).

1. Overview of the Kidney

  • Bean‑shaped organ, ≈11 cm long, ≈150 g each.

  • Receives ~20 % of cardiac output (≈1.2 L min⁻¹) and produces ~1 L of urine per day.

  • Key homeostatic roles:

    • Regulation of water volume and plasma osmolality.

    • Maintenance of plasma ion concentrations (Na⁺, K⁺, Ca²⁺, Cl⁻).

    • Acid‑base balance (H⁺ secretion, HCO₃⁻ re‑absorption).

    • Removal of metabolic waste (urea, creatinine, drugs).

    • Hormone production – renin, erythropoietin (EPO), calcitriol.

    • Blood‑pressure control via the renin‑angiotensin‑aldosterone system (RAAS).

2. Structural Components (Anatomy)

StructureLocationKey Features & Homeostatic Contributions
Fibrous CapsuleOuter surfaceDense collagenous connective tissue; protects the kidney and provides a constant external pressure that assists glomerular filtration.
CortexJust beneath the capsuleContains renal corpuscles (glomeruli + Bowman’s capsule) and the proximal & distal convoluted tubules. Site of the bulk of filtration, re‑absorption and secretion.
MedullaCentral region; organised into renal pyramidsHouses the loops of Henle and collecting ducts. Counter‑current multiplication creates the medullary osmotic gradient required for water re‑absorption and urine concentration.
Renal PelvisFunnel‑shaped cavity at the hilumCollects urine from the collecting ducts and channels it into the ureter.
UreterExtends from renal pelvis to urinary bladderMuscular tube that propels urine by peristalsis; equipped with a one‑way valve at the renal pelvis to prevent reflux.

3. Vascular Supply and Drainage

3.1 Renal Artery (oxygen‑rich blood)

  1. Renal artery – enters the kidney at the hilum.

  2. Segmental branches – anterior and posterior; each supplies a distinct region of the kidney.

  3. Interlobar arteries – run between the renal pyramids.

  4. Arcuate arteries – arch over the corticomedullary junction.

  5. Cortical radiate (interlobular) arteries – branch into the cortex and give rise to afferent arterioles that feed each glomerulus.

3.2 Renal Vein (de‑oxygenated blood)

  1. Cortical radiate veins – collect blood from peritubular capillaries and vasa recta.

  2. Arcuate veins – run alongside the arcuate arteries.

  3. Interlobar veins – converge toward the hilum.

  4. Renal vein – exits the kidney and drains into the inferior vena cava.

4. Homeostatic Functions of Each Region

StructurePrimary Homeostatic RoleKey Cellular / Molecular Mechanisms (AO2)
Glomerulus (cortex)Removal of metabolic waste; first step of urine formation.Hydrostatic pressure drives ultrafiltration across a three‑layer barrier (fenestrated endothelium → basement membrane → podocyte slit diaphragms). Size‑selective (proteins retained).
Proximal convoluted tubule (PCT)Re‑absorption of ~65 % of filtered Na⁺, glucose, amino acids, HCO₃⁻ and the majority of water.Na⁺/K⁺‑ATPase (basolateral) creates Na⁺ gradient; Na⁺‑glucose cotransporter (SGLT2), Na⁺‑amino‑acid cotransporters; aquaporin‑1 water channels; Na⁺/H⁺ exchanger (NHE3) for HCO₃⁻ reclamation.
Loop of Henle (medulla)Generation of a medullary osmotic gradient → concentration of urine & water conservation.Counter‑current multiplication: active NaCl re‑absorption in thick ascending limb via NKCC2; impermeability to water; passive water re‑absorption in descending limb (AQP1).
Distal convoluted tubule (DCT) & Cortical collecting ductFine‑tuning of Na⁺, K⁺, Ca²⁺ and pH.Na⁺/Cl⁻ cotransporter (NCC); aldosterone‑stimulated ENaC (Na⁺) and ROMK (K⁺) channels; parathyroid‑hormone‑stimulated Ca²⁺‑binding protein (TRPV5) for Ca²⁺ re‑absorption.
Collecting duct (medullary)Final regulation of water balance, urine osmolality and acid‑base status.ADH‑dependent insertion of aquaporin‑2 (AQP2) water channels; H⁺ secretion by H⁺‑ATPase in α‑intercalated cells; HCO₃⁻ re‑absorption via Cl⁻/HCO₃⁻ exchanger in β‑intercalated cells.
Juxtaglomerular apparatus (JGA)Blood‑pressure control (RAAS) and Na⁺ balance.Macula densa senses NaCl delivery → signals juxtaglomerular (JG) cells; JG cells release renin → angiotensin I → angiotensin II (vasoconstriction, stimulates aldosterone release from adrenal cortex). Aldosterone ↑ Na⁺ re‑absorption (ENaC) and K⁺ secretion, increasing extracellular fluid volume and BP.
Renal artery & veinSupply oxygen‑rich blood, create filtration pressure, and return de‑oxygenated blood; part of RAAS.High perfusion pressure at afferent arterioles drives glomerular filtration; JG cells monitor this pressure and modulate renin release.

5. Acid‑Base Regulation (Explicit AO2 Requirement)

  • H⁺ secretion – α‑intercalated cells of the collecting duct use an apical H⁺‑ATPase (and H⁺/K⁺‑ATPase) to excrete H⁺ into the tubular lumen.

  • HCO₃⁻ re‑absorption – intracellular carbonic anhydrase converts filtered HCO₃⁻ + H⁺ to CO₂ + H₂O; CO₂ diffuses into the cell, is reconverted to HCO₃⁻, which is transported across the basolateral membrane by a Na⁺‑HCO₃⁻ cotransporter.

  • Buffer generation – NH₄⁺ excretion and titratable acid (phosphate) excretion provide additional means of acid removal.

  • Renal compensation – In chronic respiratory acidosis (elevated CO₂), the kidney increases H⁺ secretion and HCO₃⁻ re‑absorption; in chronic respiratory alkalosis, the opposite adjustments occur. This demonstrates the kidney’s role in long‑term pH homeostasis.

6. Renin‑Angiotensin‑Aldosterone System (RAAS) – Blood‑Pressure Control

  1. ↓ renal perfusion pressure or ↓ NaCl delivery to macula densa → juxtaglomerular cells release renin.

  2. Renin cleaves angiotensinogen (liver) → angiotensin I.

  3. Angiotensin‑converting enzyme (ACE, lung endothelium) converts angiotensin I → angiotensin II.

  4. Angiotensin II actions:

    • Vasoconstriction of arterioles → ↑ systemic blood pressure.

    • Stimulates aldosterone release from adrenal cortex.

    • Promotes ADH release (enhances water re‑absorption).

  5. Aldosterone binds to receptors in principal cells of the distal nephron → ↑ ENaC activity → ↑ Na⁺ (and water) re‑absorption, ↑ K⁺ secretion → ↑ extracellular fluid volume and blood pressure.

7. Link to Topic 8 – Transport in Mammals

  • Filtration – Bulk flow driven by hydrostatic pressure across the glomerular barrier (passive transport).

  • Re‑absorption – Combination of primary active transport (Na⁺/K⁺‑ATPase), secondary active transport (SGLT, NKCC2), facilitated diffusion (GLUT), and passive diffusion (urea, water through aquaporins).

  • Secretion – Primary active transport of H⁺, K⁺, organic anions/cations from peritubular capillaries into the tubular lumen.

  • Osmotic gradients – Counter‑current multiplication in the loop of Henle creates a high‑osmolarity medulla, a classic example of passive water movement driven by an osmotic potential.

  • Hormonal control of transport – ADH, aldosterone and atrial natriuretic peptide modify the number or activity of specific transport proteins, illustrating endocrine regulation of membrane transport.

8. Summary Table (Structure → Location → Main Homeostatic Role)

StructureLocationPrimary Homeostatic Role
Fibrous CapsuleOuter surfaceProtection; maintains intrarenal pressure for efficient filtration.
Cortex (glomeruli, PCT, DCT)Beneath capsuleFiltration of plasma; bulk re‑absorption of Na⁺, glucose, amino acids, HCO₃⁻; initial secretion of waste.
Medulla (loops of Henle, medullary collecting ducts)Central regionCreates and uses the osmotic gradient for water re‑absorption and urine concentration.
Renal PelvisHilum, funnel‑shapedCollects urine from collecting ducts and channels it into the ureter.
UreterFrom pelvis to bladderPropels urine to the bladder by peristalsis; prevents reflux.
Renal artery (segmental → interlobar → arcuate → cortical radiate → afferent)Hilum → cortex & medullaDelivers oxygen‑rich blood; generates hydrostatic pressure for filtration; initiates RAAS for blood‑pressure regulation.
Renal vein (cortical radiate → arcuate → interlobar → renal)HilumDrains de‑oxygenated blood; returns filtered plasma constituents to systemic circulation.

Suggested diagram: transverse section of a human kidney showing the fibrous capsule, cortex, medulla (renal pyramids), renal pelvis, ureter, and the branching pattern of the renal artery and renal vein.