Homeostasis in Mammals – Urine Formation in the Nephron
1. The Nephron – Functional Unit of the Kidney
2. Renal Corpuscle – Glomerular Filtration (Ultrafiltration)
2.1 Structure of the Filtration Barrier
| Component | Key Feature | Function |
|---|
| Fenestrated endothelium | Pores ≈ 70 nm | Allows plasma water & small solutes to pass; blocks cells |
| Glomerular basement membrane | Negatively charged glycoproteins | Repels most plasma proteins (size + charge selectivity) |
| Podocyte slit diaphragms | Gaps ≈ 30 nm | Final size‑selective barrier |
2.2 Starling Forces & Net Filtration Pressure (NFP)
The driving pressure for ultrafiltration is the sum of hydrostatic and oncotic forces:
NFP = PGC – PBS – πGC
- PGC = glomerular capillary hydrostatic pressure ≈ 120 mm Hg
- PBS = Bowman's space hydrostatic pressure ≈ 15 mm Hg
- πGC = oncotic pressure of glomerular plasma ≈ 30 mm Hg
- Resulting NFP ≈ 10 mm Hg → drives filtration.
2.3 Glomerular Filtration Rate (GFR)
GFR is the volume of filtrate formed per minute. In a healthy adult:
2.4 Composition of the Primary (Glomerular) Filtrate
- Water, Na⁺, K⁺, Cl⁻, HCO₃⁻, glucose, amino acids, urea, creatinine.
- Essentially plasma without proteins or cells.
- Osmolarity ≈ 300 mOsm L⁻¹ (isotonic with plasma).
2.5 Juxtaglomerular (JG) Apparatus & Renin‑Angiotensin‑II (RAII) Cascade
- Macula densa cells (distal tubule) sense NaCl concentration; low NaCl → stimulate juxtaglomerular cells.
- Juxtaglomerular cells (afferent arteriole) release renin.
- Renin converts angiotensinogen → angiotensin I; ACE (lung) → angiotensin II.
- Angiotensin II:
- Vasoconstricts afferent & efferent arterioles (maintains GFR).
- Stimulates aldosterone release from adrenal cortex.
- Increases Na⁺ reabsorption in proximal tubule (via Na⁺/H⁺ exchanger).
Figure suggestion
Cross‑section of a renal corpuscle showing the three‑layer filtration barrier, Bowman's space, and the adjacent macula densa & juxtaglomerular cells.
3. Proximal Convoluted Tubule (PCT) – Selective Reabsorption
3.1 General Features
- Reabsorbs ~65 % of filtrate volume and > 90 % of filtered solutes.
- Basolateral Na⁺/K⁺‑ATPase creates a low intracellular Na⁺ concentration – the primary driver for secondary active transport.
- Water follows solutes osmotically through abundant aquaporin‑1 (AQP1) channels.
- Microvilli (brush border) increase surface area ~30‑fold.
3.2 Key Transporters & Mechanisms
| Transporter (Location) | Substrate(s) | Transport Mode | Notes |
|---|
| Na⁺/K⁺‑ATPase (basolateral) | Na⁺, K⁺ | Active (3 Na⁺ out, 2 K⁺ in) | Maintains Na⁺ gradient for all luminal cotransporters. |
| SGLT2 (Na⁺‑glucose cotransporter, luminal) | Glucose (or galactose) + 2 Na⁺ | Secondary active (uses Na⁺ gradient) | Reabsorbs 100 % of filtered glucose; target of SGLT2 inhibitors (diabetes). |
| NHE3 (Na⁺/H⁺ exchanger, luminal) | Na⁺ in, H⁺ out | Secondary active | Facilitates HCO₃⁻ reclamation via intracellular carbonic anhydrase. |
| Amino‑acid cotransporters (Na⁺‑dependent, luminal) | Neutral & charged amino acids | Secondary active | Reabsorb ~100 % of filtered amino acids. |
| Organic anion/cation transporters (OAT/OCT, basolateral) | Phosphate, sulfate, urate, drugs | Secondary active (exchange with α‑ketoglutarate) | Important for drug clearance. |
| AQP1 (apical & basolateral) | Water | Facilitated diffusion | Allows iso‑osmotic water reabsorption. |
3.3 Quantitative Reabsorption in the PCT
| Substance | Approx. % Reabsorbed | Primary Mechanism |
|---|
| Water | ≈ 65 % | Passive osmosis via AQP1 |
| Sodium (Na⁺) | ≈ 65 % | Na⁺/K⁺‑ATPase + Na⁺‑dependent cotransporters |
| Glucose | ≈ 100 % | SGLT2 |
| Amino acids | ≈ 100 % | Na⁺‑dependent amino‑acid carriers |
| Bicarbonate (HCO₃⁻) | ≈ 100 % | NHE3 + intracellular carbonic anhydrase |
| Urea | ≈ 50 % | Passive diffusion (concentration gradient) |
| Phosphate, sulfate | ≈ 80‑90 % | Na⁺‑dependent cotransporters |
3.4 Acid‑Base Reclamation
- Lumen: NHE3 exchanges Na⁺ for H⁺ → H⁺ combines with filtered HCO₃⁻ to form H₂CO₃.
- Inside the PCT cell: carbonic anhydrase catalyses H₂CO₃ → CO₂ + H₂O.
- CO₂ diffuses into blood; in plasma, carbonic anhydrase reforms HCO₃⁻, which is added back to circulation.
Figure suggestion
Schematic of a PCT cell showing apical SGLT2, NHE3, amino‑acid cotransporters, basolateral Na⁺/K⁺‑ATPase, AQP1, and carbonic anhydrase.
4. Loop of Henle – Counter‑Current Multiplication & Urea Recycling
4.1 Segments & Permeability
| Segment | Key Transport | Water Permeability |
|---|
| Descending limb (thin) | Passive water loss (osmotic) | High |
| Ascending limb (thin) | Passive NaCl loss | Low |
| Ascending limb (thick) | Active Na⁺/K⁺/2Cl⁻ cotransporter (NKCC2) | Impermeable to water |
4.2 Counter‑Current Multiplication
- Active NaCl removal from the thick ascending limb creates a hyperosmotic interstitium.
- Descending limb loses water to this gradient, concentrating tubular fluid.
- The process repeats along the length of the loop, establishing a corticomedullary osmotic gradient up to ~1200 mOsm L⁻¹.
4.3 Urea Recycling
- Urea is passively reabsorbed in the inner medullary collecting duct (under ADH influence) and secreted back into the thin descending limb.
- This recycling adds to the medullary osmolarity, enhancing water reabsorption in the presence of ADH.
Figure suggestion
Diagram of the Loop of Henle showing the direction of water and solute movement, NKCC2 location, and the urea recycling loop.
5. Distal Convoluted Tubule (DCT) & Collecting Duct – Fine‑Tuning & Hormonal Control
5.1 Distal Convoluted Tubule
- Na⁺/Cl⁻ cotransporter (NCC) reabsorbs ~5 % of filtered NaCl.
- Secretion of K⁺ (ROMK channels) and H⁺ (H⁺‑ATPase) – essential for K⁺ balance and acid‑base regulation.
- Regulated by aldosterone (↑ NCC activity, ↑ ENaC, ↑ K⁺/H⁺ secretion).
- Clinical link: Thiazide diuretics block NCC → natriuresis, ↓ blood pressure.
5.2 Collecting Duct – Final Water & Electrolyte Adjustments
- ADH (vasopressin) → V2 receptors → cAMP → insertion of aquaporin‑2 (AQP2) into the apical membrane → water reabsorption ↑ (concentrated urine).
- Aldosterone → ↑ ENaC (apical) & Na⁺/K⁺‑ATPase (basolateral) → Na⁺ reabsorption ↑, K⁺ secretion ↑.
- ANP → ↓ Na⁺ reabsorption (inhibits ENaC & NCC) → natriuresis & diuresis.
- Collecting duct permeability to water is the major determinant of final urine volume.
5.3 Prostaglandins & Autoregulation (Brief)
- Renal vasodilatory prostaglandins (e.g., PGE₂) counteract excessive vasoconstriction, preserving GFR.
- Non‑steroidal anti‑inflammatory drugs (NSAIDs) inhibit prostaglandin synthesis → may reduce renal blood flow and GFR.
Figure suggestion
Combined diagram of DCT and collecting duct showing NCC, ENaC, ROMK, AQP2 insertion, and sites of hormonal action.
6. Vasa Recta – Counter‑Current Exchange
- Hairpin‑loop capillaries that run parallel to the Loop of Henle.
- Blood flows in opposite directions in the descending and ascending limbs, allowing solutes to diffuse out while water diffuses in, preserving the medullary osmotic gradient.
- Essential for maintaining the concentration gradient needed for water reabsorption under ADH.
7. Hormonal Regulation & Integrated Feedback Loops
| Hormone | Source | Primary Renal Effect | Homeostatic Role |
|---|
| ADH (vasopressin) | Posterior pituitary | Inserts AQP2 → ↑ water reabsorption in collecting duct | Raises blood volume & plasma osmolarity when osmolarity ↑ |
| Aldosterone | Adrenal cortex (zona glomerulosa) | ↑ ENaC & Na⁺/K⁺‑ATPase → ↑ Na⁺ reabsorption, K⁺ secretion | Maintains Na⁺ balance & blood pressure; stimulated by low BP or high K⁺ |
| ANP | Cardiac atria (stretch receptors) | ↓ Na⁺ reabsorption in DCT & collecting duct; ↑ GFR | Reduces blood volume & pressure when atrial pressure is high |
| Renin‑Angiotensin‑II | Juxtaglomerular cells (kidney) | Vasoconstriction of afferent/efferent arterioles; ↑ Na⁺ reabsorption (proximal & distal); stimulates aldosterone | Restores GFR & blood pressure during hypovolemia |
| Prostaglandins (e.g., PGE₂) | Renal interstitial cells | Vasodilation → ↑ renal blood flow & GFR | Protects kidney during states of reduced perfusion |
Feedback Example – Low Blood Volume
- ↓ renal perfusion pressure → ↑ renin release.
- Renin → Ang‑II → vasoconstriction + aldosterone release.
- Aldosterone → ↑ Na⁺ & water reabsorption (PCT, DCT, collecting duct).
- Blood volume & pressure rise → renin secretion falls – negative feedback.
8. Practical Skills – Experiments & Data Analysis (AO3)
8.1 Designing a Filtration Experiment
- Use isolated mammalian kidneys (e.g., rabbit) perfused with a known concentration of inulin.
- Collect urine at timed intervals; measure inulin concentration in plasma and urine.
- Calculate GFR using the clearance formula.
- Vary perfusion pressure (e.g., by changing arterial resistance) to observe changes in GFR – demonstrates Starling forces.
8.2 Investigating PCT Reabsorption
- Incubate isolated proximal tubule segments with radiolabelled glucose or amino acids.
- Add specific inhibitors (e.g., phlorizin for SGLT2) and compare uptake.
- Plot uptake vs. inhibitor concentration to determine kinetic parameters (Vmax, Km).
8.3 Calculating Osmolarity Changes
Given: Initial filtrate volume 120 mL, Na⁺ concentration 140 mmol L⁻¹. After the PCT, volume = 42 mL and Na⁺ reabsorbed = 65 %.
- Remaining Na⁺ = 0.35 × 140 mmol L⁻¹ × 0.120 L = 5.88 mmol.
- New Na⁺ concentration = 5.88 mmol / 0.042 L ≈ 140 mmol L⁻¹ (iso‑osmotic), illustrating iso‑osmotic water reabsorption.
8.4 Data Interpretation – Hormonal Effects
Provide students with a table of urine volume and osmolality under four conditions: (a) baseline, (b) ADH infusion, (c) aldosterone infusion, (d) ANP infusion. Ask them to explain the observed changes in terms of transporter activity and water permeability.
9. Summary of Urine Formation (Key Points for the Syllabus)
- Glomerular filtration – Size‑ and charge‑selective ultrafiltration driven by NFP produces an isotonic primary filtrate free of proteins.
- Proximal convoluted tubule – Reclaims ~65 % of water and > 90 % of solutes via Na⁺/K⁺‑ATPase‑driven secondary active transport (SGLT2, NHE3, amino‑acid carriers) and passive water movement (AQP1).
- Loop of Henle – Counter‑current multiplication creates a corticomedullary osmotic gradient; urea recycling augments this gradient.
- Distal convoluted tubule – Fine‑tunes NaCl reabsorption (NCC) and K⁺/H⁺ secretion; aldosterone enhances Na⁺ uptake and K⁺ loss.
- Collecting duct – Final water reabsorption controlled by ADH‑dependent AQP2 insertion; aldosterone and ANP modify Na⁺/K⁺ handling.
- Vasa recta – Counter‑current exchange preserves the medullary gradient.
- Hormonal integration – ADH, aldosterone, ANP, renin‑angiotensin‑II, and prostaglandins act in coordinated feedback loops to maintain fluid, electrolyte, and acid‑base homeostasis.
These notes cover all the content required by Cambridge International AS & A Level Biology (9700) Topic 14 – Control & Coordination, and provide the experimental and quantitative skills expected for the AO3 component.