14.1 Renal Anatomy, Glomerular Ultrafiltration & Tubular Transport

14.1 Renal Anatomy, Glomerular Ultrafiltration & Tubular Transport

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Renal Macro- and Micro-Anatomy

The kidneys are paired retroperitoneal organs receiving approximately 20% to 25% of resting cardiac output (~1.0–1.2 L/min) via the renal arteries. Structurally, the kidney is partitioned into an outer, isosmotic renal cortex (~300 mOsm/L) and an inner renal medulla, organized into renal pyramids whose apices (renal papillae) project into minor calyces, emptying into major calyces, the renal pelvis, and the ureter.

The functional microscopic unit of the kidney is the nephron, with roughly $1.0 \times 10^6$ nephrons per human kidney. Nephrons fall into two major anatomical classes:

1. Cortical Nephrons (~85%): Possess glomeruli situated in the outer cortex with short loops of Henle that penetrate only into the superficial outer medulla. They are encased by a capillary network known as the peritubular capillaries and primarily execute bulk tubular reabsorption and secretion.
2. Juxtamedullary Nephrons (~15%): Glomeruli sit deep in the renal cortex adjacent to the corticomedullary junction. They feature long loops of Henle plunging deep into the inner medulla, paired with specialized hairpin vascular loops called the vasa recta. These nephrons are indispensable for establishing the hyperosmotic medullary interstitium necessary for maximal urine concentration.

Renal Microvascular Circuit:
Renal Artery -> Interlobar Arteries -> Arcuate Arteries -> Interlobular (Cortical Radiate) Arteries
  -> Afferent Arteriole -> Glomerular Capillaries (Filtration, High Hydrostatic Pressure)
  -> Efferent Arteriole -> Peritubular Capillaries (Cortex) / Vasa Recta (Medulla)
  -> Interlobular Veins -> Arcuate Veins -> Interlobar Veins -> Renal Vein

This portal-like arrangement—two capillary beds connected in series by an efferent arteriole—allows independent vascular resistance adjustments upstream and downstream of the glomerulus, providing fine-tuned control over filtration and reabsorption.

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Glomerular Ultrafiltration: Starling Forces & Filtration Barrier

Ultrafiltration occurs across the glomerular filtration barrier separating the capillary lumen from Bowman's space. This three-layered sieve exhibits precise size and charge selectivity:

1. Fenestrated Capillary Endothelium: Pores measuring 70 to 100 nm prevent the passage of cellular elements (erythrocytes, leukocytes, platelets) but permit plasma solutes and fluid to cross.
2. Glomerular Basement Membrane (GBM): An acellular meshwork of type IV collagen, laminin, and polyanionic heparan sulfate proteoglycans. The dense negative charges electrostatically repel polyanionic plasma proteins, most notably serum albumin (~66.5 kDa; effective molecular radius ~3.6 nm).
3. Visceral Epithelial Podocytes: Interdigitating foot processes (pedicels) wrap around the capillaries, leaving narrow filtration slits (~25–30 nm) bridged by a specialized protein diaphragm containing nephrin and podocin. Genetic or inflammatory disruption of slit diaphragms results in severe nephrotic-range proteinuria.

Glomerular Filtration Barrier:
Capillary Lumen -> [1. Fenestrated Endothelium] -> [2. Negatively Charged GBM] -> [3. Podocyte Slit Diaphragms] -> Bowman's Space

Filtration is driven by physical pressures summarized by the Starling equation for glomerular filtration rate (GFR):

$$\text{GFR} = K_f \cdot \big[ (P_{GC} - P_{BS}) - (\Pi_{GC} - \Pi_{BS}) \big]$$

Where:

• $K_f$ is the ultrafiltration coefficient (product of surface area and hydraulic permeability).
• $P_{GC}$ is the glomerular capillary hydrostatic pressure (~60 mmHg), promoting filtration. This value is substantially higher than in systemic capillaries due to high efferent arteriolar resistance.
• $P_{BS}$ is the Bowman's space hydrostatic pressure (~15 mmHg), opposing filtration.
• $\Pi_{GC}$ is the glomerular capillary oncotic pressure (~28–32 mmHg), opposing filtration. As protein-free fluid filters out along the length of the capillary, $\Pi_{GC}$ progressively rises.
• $\Pi_{BS}$ is the Bowman's space oncotic pressure (~0 mmHg), because healthy filtrate is protein-free.

Under normal physiological conditions, the Net Filtration Pressure (NFP) is:

$$\text{NFP} = (60 - 15) - (30 - 0) = 45 - 30 = +15\text{ mmHg}$$

Normal GFR averages 125 mL/min (approximately 180 L/day). The filtration fraction (FF) denotes the portion of renal plasma flow (RPF, ~625 mL/min) filtered across the glomerulus:

$$\text{FF} = \frac{\text{GFR}}{\text{RPF}} = \frac{125\text{ mL/min}}{625\text{ mL/min}} = 0.20\text{ (20\%)}$$

Arteriolar Hemodynamics and GFR Regulation

• Afferent Arteriolar Constriction (e.g., via high sympathetic tone, high endothelin, NSAID-induced prostaglandin inhibition): Decreases $P_{GC}$, thereby decreasing GFR and decreasing RPF; FF remains relatively unchanged.
• Efferent Arteriolar Constriction (e.g., via low-dose Angiotensin II): Restricts blood exit from the glomerulus, increasing $P_{GC}$. This increases GFR while decreasing RPF, markedly increasing the filtration fraction (FF).
• Afferent Arteriolar Dilation (e.g., via prostaglandins, nitric oxide, ANP): Increases $P_{GC}$, increasing GFR and increasing RPF.
• Efferent Arteriolar Dilation (e.g., via ACE inhibitors, ARBs): Decreases $P_{GC}$, decreasing GFR and increasing RPF, thereby decreasing the filtration fraction (FF).

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Tubular Transport: Reabsorption and Secretion Along the Nephron

Tubular Mass Balance Equation:
\text{Excreted Amount} = \text{Filtered Amount} - \text{Reabsorbed Amount} + \text{Secreted Amount}

1. Proximal Convoluted Tubule (PCT): Bulk Obligatory Processing

The PCT reabsorbs ~65% to 70% of filtered water, $\text{Na}^+$, and $\text{Cl}^-$, and virtually 100% of filtered glucose and amino acids:

• Apical Secondary Active Transport: The basolateral $\text{Na}^+/\text{K}^+$ ATPase pumps $3\text{ Na}^+$ out and $2\text{ K}^+$ into the cell, creating a low intracellular $\text{Na}^+$ concentration (10–15 mM) and negative membrane potential (-70 mV). This electrochemical gradient powers apical cotransporters:
  • SGLT2 (early PCT) and SGLT1 (late PCT) cotransport glucose with $\text{Na}^+$. Glucose exits basolaterally into peritubular capillaries via GLUT2 or GLUT1 facilitated diffusion. The Transport Maximum ($T_m$) for glucose is ~375 mg/min; when plasma glucose exceeds the renal threshold (~200–250 mg/dL), SGLT transporters saturate, resulting in glucosuria and osmotic diuresis (classic hallmark of uncontrolled diabetes mellitus).
  • Amino acid cotransporters, $\text{Na}^+/\text{phosphate}$ cotransporters (NPT2, inhibited by PTH), and $\text{Na}^+/\text{lactate}$ symporters.
• Bicarbonate Reabsorption (~85%): Apical $\text{Na}^+/\text{H}^+$ antiporters (NHE3) secrete $\text{H}^+$ into the lumen. Filtered $\text{HCO}_3^-$ combines with $\text{H}^+$ to form $\text{H}_2\text{CO}_3$. Membrane-bound brush-border carbonic anhydrase (CA IV) rapidly converts it to $\text{CO}_2$ and $\text{H}_2\text{O}$, which diffuse into the enterocyte. Intracellular carbonic anhydrase (CA II) regenerates $\text{H}^+$ and $\text{HCO}_3^-$; the $\text{HCO}_3^-$ exits basolaterally via a $\text{Na}^+/3\text{HCO}_3^-$ cotransporter (NBCe1).
• Obligatory Water Reabsorption: Transcellular water transport via constitutively active Aquaporin-1 (AQP1) channels and paracellular transport follow solute reabsorption isosmotically. The tubular fluid entering the descending loop of Henle remains isosmotic to plasma (~300 mOsm/L).
• Active Secretion ("DUMP the HUNK"): The PCT actively secretes $\text{H}^+$, Urea, $\text{NH}_3/\text{NH}_4^+$ (ammoniagenesis from glutamine deamination), and $\text{K}^+$, alongside organic anions (urate, penicillin, probenecid, PAH) and organic cations (creatinine, cimetidine) via basolateral OAT/OCT transporters.

2. The Loop of Henle and the Countercurrent Multiplier

The loop of Henle establishes the hyperosmotic medullary gradient (300 mOsm/L at the corticomedullary junction up to 1200 mOsm/L at the hairpin turn in humans):

• Thin Descending Limb: Highly permeable to water via apical and basolateral Aquaporin-1 (AQP1); strictly impermeable to $\text{Na}^+$, $\text{Cl}^-$, and urea. As filtrate descends into the hypertonic medullary interstitium, water passively flows out into the interstitium and is removed by the vasa recta. Tubular fluid concentrates progressively, reaching ~1200 mOsm/L at the hairpin apex.
• Thin Ascending Limb: Impermeable to water; moderately permeable to $\text{Na}^+$ and $\text{Cl}^-$ via passive paracellular and transcellular diffusion down their concentration gradients into the interstitium.
• Thick Ascending Limb (TAL): Completely impermeable to water. Actively extracts solutes via the apical $\text{Na}^+/\text{K}^+/2\text{Cl}^-$ cotransporter (NKCC2):
  • The basolateral $\text{Na}^+/\text{K}^+$ pump provides the driving sodium gradient.
  • $\text{Cl}^-$ exits basolaterally through ClC-Kb chloride channels.
  • Apical ROMK (Renal Outer Medullary Potassium) channels recycle $\text{K}^+$ back into the tubular lumen. This continuous positive ion back-leak generates a lumen-positive transepithelial potential (~+8 to +10 mV), which drives the paracellular reabsorption of divalent cations: $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$.
  • Clinical Target: Loop diuretics (e.g., furosemide, bumetanide) reversibly inhibit NKCC2, dissipating the lumen-positive potential (wasting $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$) and washing out the medullary gradient, inducing profound diuresis.
  • Because solutes are extracted without water, the TAL functions as the "diluting segment", delivering hypotonic fluid (~100 mOsm/L) to the distal convoluted tubule.

3. Distal Convoluted Tubule (DCT) and Collecting Duct

• Early DCT: Impermeable to water; actively reabsorbs $\text{Na}^+$ and $\text{Cl}^-$ via the apical electroneutral $\text{Na}^+/\text{Cl}^-$ cotransporter (NCC) (inhibited by thiazide diuretics). Regulated parathyroid hormone (PTH) stimulates active basolateral $\text{Ca}^{2+}$ reabsorption via apical TRPV5 calcium channels.
• Late DCT and Cortical/Medullary Collecting Duct: Cell populations:
  • Principal Cells: Mediate $\text{Na}^+$ reabsorption and $\text{K}^+$ secretion. Apical ENaC (Epithelial Sodium Channels) allow electrogenic $\text{Na}^+$ entry; basolateral $\text{Na}^+/\text{K}^+$ pumps extrude $\text{Na}^+$. Negative luminal voltage pulls $\text{K}^+$ into the lumen via apical ROMK and BK channels. Regulated directly by aldosterone and ADH.
  • Type A Intercalated Cells: Mediate acid excretion during acidosis. Apical $\text{H}^+$-ATPase and $\text{H}^+/\text{K}^+$ ATPase pump protons into the lumen; basolateral $\text{HCO}_3^-/\text{Cl}^-$ exchangers (AE1) return newly synthesized bicarbonate to the blood.
  • Type B Intercalated Cells: Mediate base excretion during alkalosis. Possess apical pendrin ($\text{HCO}_3^-/\text{Cl}^-$ exchanger) to secrete bicarbonate into the lumen, and basolateral $\text{H}^+$-ATPases.

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Nephron Segment Transport Characteristics

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