12.1 Nephron Transport & Renal Physiology

12.1 Nephron Transport & Renal Physiology

Quick Answer: Map Na+ transport by segment—NHE3 (PCT), NKCC2 (TAL), NCC (DCT), ENaC (CD)—with basolateral Na/K-ATPase powering all. GFR depends on net filtration pressure and Kf; filtration fraction ≈ GFR/RPF (~20%). RAAS defends volume; ADH inserts AQP2 for free-water reabsorption; ANP opposes volume overload. The countercurrent multiplier creates medullary hypertonicity that ADH uses to concentrate urine.

Renal physiology on the CBSE is tested as integrated mechanism: which transporter, which vessel tone change, which hormone, and what happens to volume, osmolality, or acid–base. Build one nephron map, then hang clearance math and endocrine loops on it.

Nephron Architecture and Filtration Barriers

Each kidney contains roughly 1 million nephrons. Blood enters the glomerulus via the afferent arteriole, filters across the glomerular capillary, and exits via the efferent arteriole. The filtrate then traverses the proximal convoluted tubule (PCT), loop of Henle (thin descending, thin ascending, thick ascending limb [TAL]), distal convoluted tubule (DCT), and collecting duct (CD) systems (cortical then medullary).

The filtration barrier has three layers: fenestrated endothelium, glomerular basement membrane (negatively charged), and podocyte slit diaphragms. Size and charge selectivity normally keep albumin and cells out of the filtrate. Damage to charge or podocyte architecture produces proteinuria—pathology that is tested more in later renal pathology sections, but the barrier concept frames what “filtered load” means for solutes.

Filtered load of substance X = GFR × Pₓ (plasma concentration of X, corrected for protein binding when relevant).

Segment-by-Segment Transporters (High-Yield Map)

Basolateral Na+/K+-ATPase (3 Na+ out / 2 K+ in) creates the low intracellular Na+ that drives apical secondary active transport in every segment that reabsorbs Na+.

Clinical pearl for exams: Loop diuretics act on NKCC2 (same site as Bartter). Thiazides act on NCC (same site as Gitelman). Amiloride blocks ENaC (opposite of Liddle gain-of-function). Always pair transporter ↔ drug ↔ genetic “diuretic-like” syndrome.

PCT also reabsorbs the bulk of filtered bicarbonate: luminal carbonic anhydrase and intracellular CA regenerate CO2/H2O and HCO3− that exits basolaterally via NBCe1. Blocking CA with acetazolamide causes HCO3− loss and mild metabolic acidosis with diuresis of NaHCO3.

GFR Determinants

Glomerular filtration rate (GFR) is the volume of plasma filtered per unit time (normal ~90–120 mL/min/1.73 m2 in young adults; often taught as ~125 mL/min in classic physiology problems).

$\mathrm{GFR} = K_f \times \text{net filtration pressure}$

Net filtration pressure ≈ PGC − PBS − πGC, where:

• PGC = glomerular capillary hydrostatic pressure (favors filtration)
• PBS = Bowman space hydrostatic pressure (opposes filtration)
• πGC = glomerular capillary oncotic pressure (opposes filtration; rises along the capillary as protein concentrates)

Kf (ultrafiltration coefficient) reflects surface area and hydraulic conductivity of the filtration barrier. Mesangial contraction or scarring lowers Kf and can lower GFR without a change in systemic pressure.

Afferent vs Efferent Tone (Exam Pattern)

Angiotensin II preferentially constricts the efferent arteriole, raising filtration fraction and helping preserve GFR when renal perfusion is low. Prostaglandins dilate the afferent arteriole. That is why NSAIDs (↓ PGE) plus ACE inhibitors/ARBs (↓ Ang II effect) are a classic setup for acute GFR drop in low effective arterial blood volume.

Clearance Concepts and Filtration Fraction

Renal clearance of X:

$C_x = \frac{U_x \times V}{P_x}$

where Uₓ is urine concentration, V is urine flow rate, and Pₓ is plasma concentration.

Filtration fraction (FF) = GFR / RPF. Normal FF is about 0.15–0.20 (often memorized as ~20%). If GFR is 125 mL/min and RPF is 625 mL/min, FF = 0.20. Rising FF (efferent constriction, volume depletion with Ang II) increases peritubular oncotic pressure and favors proximal reabsorption—linking hemodynamics to Na+ retention.

Renal blood flow (RBF) ≈ RPF / (1 − Hct). Autoregulation keeps RBF and GFR relatively stable across mean arterial pressures roughly 80–180 mmHg via:

1. Myogenic response — afferent smooth muscle contracts when stretched by higher pressure.
2. Tubuloglomerular feedback (TGF) — macula densa senses increased NaCl delivery (via NKCC2), signals afferent constriction (adenosine/ATP pathways), lowering GFR back toward set point.

Outside the autoregulatory range, GFR falls with hypotension and can rise with severe hypertension until secondary injury intervenes.

RAAS Cascade

Juxtaglomerular cells release renin when they sense low perfusion pressure, increased sympathetic (β1) tone, or decreased NaCl delivery to the macula densa.

ACE inhibitors block Ang II formation and raise bradykinin (cough/angioedema risk). ARBs block AT1 receptors. Both reduce aldosterone drive and can raise serum K+.

ADH, Aquaporins, and ANP

Antidiuretic hormone (ADH, vasopressin) is released from the posterior pituitary when plasma osmolality rises (dominant day-to-day stimulus via osmoreceptors) or when effective arterial volume falls markedly (baroreceptor override).

In principal cells of the collecting duct, V2 receptor → Gs–cAMP → trafficking of aquaporin-2 (AQP2) to the apical membrane. Water is reabsorbed down the medullary osmotic gradient. Basolateral AQP3/AQP4 provide exit pathways. Without ADH (central DI) or without response (nephrogenic DI), large volumes of dilute urine are excreted.

Atrial (and B-type) natriuretic peptide rises with atrial/ventricular stretch (volume overload). ANP/BNP increase GFR slightly (afferent dilation/efferent constriction pattern), inhibit renin and aldosterone, and promote natriuresis—opposing RAAS. Exam items often contrast RAAS (volume down) vs ANP (volume up).

Countercurrent Multiplier and Exchanger

Multiplier (loop of Henle): Active salt reabsorption by NKCC2 in the water-impermeable TAL adds solute to the medullary interstitium. The thin descending limb loses water to the hypertonic interstitium, so fluid at the tip of the long loop becomes highly concentrated. Fluid ascending the TAL becomes progressively dilute (“diluting segment”), enabling excretion of free water when ADH is low.

Exchanger (vasa recta): Medullary blood flow is arranged so solute is not washed out: descending vasa recta pick up solute and lose water; ascending vasa recta reverse the process. Slow medullary blood flow preserves the gradient. High medullary flow (e.g., osmotic diuresis states) can dissipate the gradient and impair concentrating ability.

Urea recycling under ADH (UT-A1 in inner medullary CD) contributes substantially to deep medullary osmolality—another reason low-protein states can mildly impair maximal concentration.

Typical corticomedullary gradient teaching numbers: cortex ~300 mOsm/kg, inner medulla up to ~1200–1400 mOsm/kg in humans under maximal ADH.

Free-Water Clearance Concepts

Osmolar clearance Cosm = (Uosm × V) / Posm.

Free-water clearance:

$C_{H_2O} = V - C_{osm}$

Exam logic: inability to generate positive CH2O means inability to dilute (e.g., loop diuretic blocking the diluting segment, or SIADH inappropriately concentrating). Inability to generate negative CH2O means inability to concentrate (DI, washout of gradient).

Putting It Together for CBSE Items

When a vignette gives volume depletion, predict ↑ renin → ↑ Ang II → ↑ aldosterone → Na+ retention, K+/H+ wasting risk, ↑ FF, and ADH release if osmolality or baroreceptors demand it. When a vignette gives a diuretic, name the transporter and the expected urinary electrolytes and acid–base pattern. When asked about concentrating ability, invoke NKCC2-dependent gradient + AQP2. When asked about “which clearance,” match inulin/creatinine to GFR and PAH to RPF, then compute FF as their ratio.

Master this map before electrolyte disorders and acid–base: those chapters assume you already know where Na+, K+, water, and H+ are handled along the nephron.