Nephron Physiology: Filtration, Reabsorption, and Secretion

Detailed Notes

22.1 Overview of the Three Renal Processes

The kidneys process approximately 180 liters of plasma per day yet excrete only about 1–1.5 liters of urine. This dramatic volume reduction is accomplished by three sequential processes:

1. Glomerular filtration: Blood plasma is filtered across the glomerular capillary wall into Bowman's capsule, producing a protein-free filtrate that contains water, ions, glucose, amino acids, and waste products at essentially the same concentrations as plasma.
2. Tubular reabsorption: As filtrate flows through the renal tubules, useful substances (water, glucose, amino acids, Na⁺, Cl⁻, HCO₃⁻, and many others) are selectively transported out of the tubular lumen and returned to the peritubular capillaries.
3. Tubular secretion: Selected substances (H⁺, K⁺, NH₄⁺, organic acids, organic bases, and certain drugs) are actively transported from the peritubular capillaries into the tubular lumen for excretion.

The net urinary excretion of any substance is described by:

Excretion = Filtration − Reabsorption + Secretion

22.2 Glomerular Filtration

22.2.1 The Filtration Membrane

The glomerular filtration membrane is a three-layered barrier between the blood in the glomerular capillaries and the fluid in Bowman's capsule. It filters approximately 125 mL/min, producing about 180 L of filtrate per day:

1. Fenestrated glomerular capillary endothelium: The endothelial cells have large pores (fenestrations, ~70–100 nm) that allow plasma and dissolved solutes to pass but prevent the passage of blood cells (RBCs, WBCs, platelets).

1. Glomerular basement membrane (GBM): A fused basal lamina rich in negatively charged glycoproteins (heparan sulfate proteoglycans) and a collagen meshwork. The negative charges repel similarly charged plasma proteins (especially albumin), contributing to the charge selectivity of the filtration barrier. The collagen mesh provides size selectivity, restricting molecules larger than approximately 8 nm.

1. Podocyte layer (visceral layer of Bowman's capsule): Specialized epithelial cells called podocytes wrap their foot processes (pedicels) around the glomerular capillaries. Between adjacent foot processes are filtration slits (~25 nm wide), spanned by a thin diaphragm called the slit diaphragm composed of proteins including nephrin and podocin. These slit diaphragm proteins are critical for maintaining the final filtration barrier; mutations in nephrin cause congenital nephrotic syndrome.

Filtrate composition: The glomerular filtrate is essentially plasma minus proteins (and protein-bound substances). It contains water, electrolytes (Na⁺, K⁺, Cl⁻, Ca²⁺, Mg²⁺, HCO₃⁻, HPO₄²⁻), glucose, amino acids, urea, creatinine, and other small solutes at plasma concentrations.

22.2.2 Net Filtration Pressure (NFP)

Glomerular filtration rate (GFR) is proportional to the product of the net filtration pressure (NFP) and the filtration coefficient (Kf):

GFR ∝ NFP × Kf

where Kf is the product of the glomerular capillary surface area and the hydraulic conductivity (permeability) of the filtration membrane. Kf is approximately 400× greater than that of systemic capillaries, explaining the enormous daily filtrate volume.

The net filtration pressure is determined by three forces (Starling forces across the glomerular capillary):

NFP = HPg − HPc − OPg = 55 − 15 − 30 = 10 mm Hg

Unlike systemic capillaries, the glomerular capillary maintains a positive NFP throughout its entire length — there is no reabsorption of filtrate. This is because HPg declines very little along the glomerular capillary (from ~60 to ~50 mm Hg) due to the low resistance of the glomerular capillary bed and high resistance of the efferent arteriole downstream.

Table 22.1 — Glomerular vs. Systemic Capillary Filtration Forces

22.3 Regulation of GFR

GFR must be maintained within a narrow range. Too low → inadequate waste removal; too high → excessive loss of water and solutes, overwhelming tubular reabsorptive capacity.

22.3.1 Autoregulation (Intrinsic Mechanisms)

The kidneys can maintain a nearly constant GFR over a mean arterial pressure (MAP) range of approximately 80–180 mm Hg. This is accomplished by two complementary intrinsic mechanisms:

1. Myogenic Mechanism

Vascular smooth muscle in the afferent arteriole responds directly to changes in stretch:

• ↑ MAP → ↑ stretch of afferent arteriole → stretch-activated Ca²⁺ channels open → Ca²⁺ influx → smooth muscle contraction → afferent arteriole constricts → ↓ glomerular capillary hydrostatic pressure (HPg) → GFR maintained.
• ↓ MAP → ↓ stretch → afferent arteriole dilates → ↑ HPg → GFR maintained.

This is an intrinsic property of the vascular smooth muscle — it does not require neural or hormonal input.

2. Tubuloglomerular Feedback (TGF)

The juxtaglomerular apparatus (JGA) provides a feedback loop that links tubular fluid composition to afferent arteriole tone:

• The macula densa cells (specialized epithelial cells in the wall of the distal tubule at the point where it contacts the afferent and efferent arterioles of the same nephron) sense the NaCl concentration of tubular fluid flowing past them.
• ↑ GFR → ↑ NaCl delivery to macula densa → macula densa releases adenosine (and ATP) → adenosine binds to A₁ receptors on afferent arteriole → afferent arteriole constricts → ↓ GFR back to normal.
• ↓ GFR → ↓ NaCl delivery to macula densa → macula densa releases nitric oxide (NO) and prostaglandins (PGE₂, PGI₂) → afferent arteriole dilates → ↑ GFR back to normal.
• Additionally, the macula densa signals juxtaglomerular (JG) granular cells in the afferent arteriole: ↓ NaCl delivery stimulates renin release, while ↑ NaCl delivery inhibits it.

The TGF mechanism fine-tunes GFR nephron-by-nephron, ensuring that the filtered load does not exceed the tubule's reabsorptive capacity.

22.3.2 Extrinsic Regulation (Neural and Hormonal)

When systemic blood pressure requires correction, extrinsic mechanisms override autoregulation:

Sympathetic Nervous System

• During exercise, hemorrhage, or severe stress, increased sympathetic outflow releases norepinephrine onto α₁-adrenergic receptors of both afferent and efferent arterioles → constriction.
• Strong sympathetic activation constricts both arterioles, but afferent constriction predominates → ↓ HPg → ↓ GFR. This redirects blood flow away from the kidneys and conserves blood volume during hypotensive emergencies.
• Moderate sympathetic activation primarily constricts the efferent arteriole, which may actually preserve GFR by maintaining HPg.

Angiotensin II

• Angiotensin II is a potent vasoconstrictor produced via the renin-angiotensin-aldosterone system (RAAS).
• It constricts both afferent and efferent arterioles, but the efferent arteriole is more sensitive → preferential efferent constriction → ↑ HPg → GFR maintained or slightly increased despite reduced renal blood flow.
• This is physiologically important: when systemic blood pressure falls and renal blood flow decreases, angiotensin II preserves GFR so that waste excretion can continue.
• At very high concentrations, angiotensin II constricts the afferent arteriole as well → ↓ GFR.

Atrial Natriuretic Peptide (ANP)

• Released from cardiac atrial myocytes in response to increased atrial stretch (hypervolemia).
• ANP dilates the afferent arteriole and constricts the efferent arteriole → ↑ HPg → ↑ GFR.
• This promotes Na⁺ and water excretion (natriuresis and diuresis), reducing blood volume.
• ANP also inhibits renin release and blocks aldosterone secretion.

Other Modulators

• Prostaglandins (PGE₂, PGI₂): Locally produced vasodilators that buffer excessive vasoconstriction; important when sympathetic tone is high (e.g., in heart failure or hemorrhage, where NSAIDs can precipitate acute kidney injury by blocking this protective effect).
• Endothelin: A potent vasoconstrictor produced by damaged endothelial cells; contributes to renal vasoconstriction in pathological states.
• Dopamine: At low doses, dilates renal blood vessels via D₁ receptors, increasing renal blood flow and GFR.

Table 22.2 — Hormonal Regulators of GFR

22.4 Tubular Reabsorption

Tubular reabsorption returns approximately 99% of the filtered water and solutes to the bloodstream. It occurs via two routes:

• Transcellular route: Solutes cross the apical (luminal) membrane of the tubular epithelial cell, traverse the cytoplasm, and exit across the basolateral membrane into the interstitial fluid, then into the peritubular capillary.
• Paracellular route: Solutes pass through the tight junctions between adjacent tubular cells. This route is driven by concentration and electrochemical gradients and varies in permeability ("leakiness") among different nephron segments.

Transport mechanisms include primary active transport (Na⁺/K⁺-ATPase on the basolateral membrane), secondary active transport (cotransport and antiport on the apical membrane), facilitated diffusion (channels and carriers), and osmosis (water following solutes).

22.4.1 Proximal Convoluted Tubule (PCT)

The PCT is the workhorse of reabsorption, reclaiming 65–70% of filtered Na⁺ and water, nearly 100% of filtered glucose, amino acids, and HCO₃⁻, and significant fractions of other solutes. The PCT cells are characterized by:

• Brush border (microvilli) on the apical surface → enormous surface area.
• Numerous mitochondria → abundant ATP for active transport.
• Leaky tight junctions → significant paracellular reabsorption of water and ions.

Sodium Reabsorption (65–70% of filtered Na⁺)

Na⁺ reabsorption in the PCT is powered by the Na⁺/K⁺-ATPase pump on the basolateral membrane (3 Na⁺ out, 2 K⁺ in, per ATP hydrolyzed). This creates a low intracellular Na⁺ concentration and a negative intracellular potential, which drive several apical entry mechanisms:

1. Na⁺/H⁺ antiporter (NHE3): Na⁺ enters the cell in exchange for H⁺ secretion into the lumen. This is the dominant Na⁺ entry mechanism in the early PCT.
2. Na⁺-glucose cotransporter (SGLT2): Couples Na⁺ entry (down its gradient) to glucose entry (against its gradient). This is the main glucose reabsorption mechanism in the early PCT — see "Glucose Reabsorption" below.
3. Na⁺-amino acid cotransporters: Multiple specific transporters couple Na⁺ entry to the reabsorption of various amino acids.
4. Na⁺-phosphate cotransporter (NaPi-IIa): Reabsorbs filtered phosphate, regulated by parathyroid hormone (PTH) — PTH inhibits this transporter, causing phosphaturia.
5. Na⁺-lactate/citrate cotransporters: Reabsorb filtered organic anions.

Water Reabsorption

• Water follows the osmotic gradient created by solute (primarily Na⁺) reabsorption → obligatory water reabsorption.
• Water moves through aquaporin-1 (AQP1) channels, which are constitutively expressed on both the apical and basolateral membranes of PCT cells (unlike AQP2 in the collecting duct, AQP1 is not regulated by ADH).
• Water also moves through the paracellular route (leaky tight junctions).
• Because water follows solute reabsorption proportionally, the tubular fluid remains isosmotic (~300 mOsm/L) throughout the PCT.

Glucose Reabsorption

Filtered glucose is virtually 100% reabsorbed in the PCT under normal conditions. The process uses two families of transporters:

1. Apical entry: SGLT2 (high-capacity, low-affinity, 1 Na⁺:1 glucose) in the early PCT and SGLT1 (low-capacity, high-affinity, 2 Na⁺:1 glucose) in the late PCT couple Na⁺ movement down its gradient to glucose movement against its gradient.
2. Basolateral exit: Glucose exits the cell into the interstitium via GLUT2 (early PCT) and GLUT1 (late PCT) facilitated diffusion carriers.

The transport maximum (Tm) for glucose is approximately 375 mg/min in adults. When the filtered load exceeds Tm (plasma glucose > ~180–200 mg/dL, the renal threshold), glucose begins to appear in the urine (glucosuria). This is a hallmark of uncontrolled diabetes mellitus.

Bicarbonate Reabsorption

Approximately 85–90% of filtered HCO₃⁻ is reabsorbed in the PCT, though the mechanism is indirect — HCO₃⁻ does not cross the apical membrane directly:

1. H⁺ is secreted into the lumen via the Na⁺/H⁺ antiporter (NHE3) and an H⁺-ATPase.
2. In the lumen, H⁺ combines with filtered HCO₃⁻ to form H₂CO₃ (carbonic acid), catalyzed by carbonic anhydrase IV (CA-IV) on the apical membrane.
3. H₂CO₃ dissociates into CO₂ and H₂O. CO₂ is lipid-soluble and diffuses freely into the PCT cell.
4. Inside the cell, carbonic anhydrase II (CA-II) catalyzes the reverse reaction: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻.
5. The regenerated H⁺ is secreted back into the lumen (recycling), and the regenerated HCO₃⁻ is transported across the basolateral membrane into the blood via a Na⁺-3HCO₃⁻ cotransporter (NBCe1).

Other Reabsorbed Substances

• Amino acids: Nearly 100% reabsorbed via multiple Na⁺-dependent cotransporters (specific for acidic, basic, and neutral amino acids). Defects in specific transporters cause aminoacidurias (e.g., cystinuria).
• Urea: Approximately 50% of filtered urea is passively reabsorbed in the PCT (paracellularly, driven by the concentration gradient created by water reabsorption). This urea recycling is important for the medullary osmotic gradient.
• Proteins and peptides: Small filtered proteins (e.g., peptide hormones, small amounts of albumin) are taken up by receptor-mediated endocytosis in the PCT and degraded to amino acids within lysosomes.
• Phosphate: ~80% reabsorbed in the PCT via NaPi-IIa cotransporters; the remaining 20% is reabsorbed in the distal tubule, regulated by PTH.

Table 22.3 — PCT Reabsorption Summary

22.4.2 Loop of Henle

The loop of Henle reabsorbs approximately 25% of filtered Na⁺ and 15% of filtered water. Critically, it creates the medullary osmotic gradient that enables the kidney to concentrate urine (see Section 22.6). The loop consists of two limbs with fundamentally different permeabilities:

Descending Limb (Thin)

• Highly permeable to water via constitutive aquaporin-1 (AQP1) channels.
• Impermeable to NaCl — essentially no active or passive Na⁺/Cl⁻ transport.
• As tubular fluid descends into the increasingly hyperosmotic medullary interstitium, water leaves the tubule by osmosis → the tubular fluid becomes progressively more concentrated (up to ~1200 mOsm/L at the hairpin turn in the inner medulla).
• Very permeable to urea (via urea transporters UT-A2 in the thin descending limb), allowing urea from the interstitium to enter the tubule (urea recycling).

Thin Ascending Limb

• Impermeable to water (no aquaporins).
• Permeable to NaCl: NaCl diffuses passively out of the tubule down its concentration gradient into the medullary interstitium (the tubular fluid is now highly concentrated in NaCl because water left in the descending limb).
• Permeable to urea (UT-A2), facilitating urea recycling.

Thick Ascending Limb (TAL)

• Impermeable to water (no aquaporins) — this is absolutely critical for generating the medullary osmotic gradient.
• Active reabsorption of Na⁺, K⁺, and Cl⁻ via the Na⁺/K⁺/2Cl⁻ cotransporter (NKCC2) on the apical membrane:
  • 1 Na⁺, 1 K⁺, and 2 Cl⁻ ions enter the cell together. Energy is provided by the Na⁺ gradient maintained by the basolateral Na⁺/K⁺-ATPase.
  • This is the target of loop diuretics such as furosemide and bumetanide.
• Na⁺ exits the basolateral membrane via Na⁺/K⁺-ATPase; Cl⁻ exits via Cl⁻ channels (ClC-Kb) and K⁺/Cl⁻ cotransporters (KCC4); K⁺ recycles back to the lumen via apical ROMK channels (important for maintaining the positive luminal potential).
• The positive luminal potential (+5 to +10 mV) — generated by K⁺ recycling and Cl⁻ exit — drives paracellular reabsorption of Ca²⁺ and Mg²⁺.
• The TAL reabsorbs approximately 25% of filtered Ca²⁺ and 50–60% of filtered Mg²⁺ via this paracellular route, regulated by parathyroid hormone (PTH) (↑ Ca²⁺ reabsorption) and the Ca²⁺/Mg²⁺-sensing receptor (CaSR) on the basolateral membrane.

Because the TAL pumps NaCl out while remaining water-impermeable, the tubular fluid becomes increasingly dilute (hypo-osmotic, ~100 mOsm/L) as it ascends. This is why the TAL is also called the diluting segment, and why the fluid entering the distal tubule is always hypo-osmotic regardless of hydration status.

22.4.3 Distal Convoluted Tubule (DCT)

The DCT reabsorbs approximately 5–8% of filtered Na⁺ and Cl⁻ and is the primary site of regulated Ca²⁺ reabsorption:

• Na⁺/Cl⁻ cotransporter (NCC) on the apical membrane: Electroneutral cotransport of Na⁺ and Cl⁻. This is the target of thiazide diuretics (e.g., hydrochlorothiazide).
• Na⁺ exits basolaterally via Na⁺/K⁺-ATPase; Cl⁻ exits via Cl⁻ channels (ClC-Kb).
• Ca²⁺ reabsorption: Active transcellular Ca²⁺ transport via apical TRPV5 Ca²⁺ channels (regulated by PTH and vitamin D/calcitriol) and basolateral NCX1 (Na⁺/Ca²⁺ exchanger) and PMCA1b (Ca²⁺-ATPase).
• The DCT is impermeable to water (no aquaporins constitutively expressed; AQP2 only appears under ADH stimulation if present in the late DCT/connecting tubule).
• The DCT continues the dilution begun in the TAL → the tubular fluid remains hypo-osmotic (~100 mOsm/L).

22.4.4 Collecting Duct System

The collecting duct is the final site of urine modification and is the primary locus of hormonal fine-tuning of water, Na⁺, K⁺, and acid-base balance. It consists of two principal cell types:

Principal Cells (Majority)

Principal cells are responsible for Na⁺ reabsorption, K⁺ secretion, and ADH-regulated water reabsorption:

1. Na⁺ Reabsorption (ENaC + Aldosterone)
   • Na⁺ enters the principal cell across the apical membrane through epithelial sodium channels (ENaC) , driven by the negative intracellular potential and low intracellular Na⁺ concentration.
   • Na⁺ is pumped out across the basolateral membrane by the Na⁺/K⁺-ATPase.
   • Aldosterone (from the adrenal cortex zona glomerulosa) binds to intracellular mineralocorticoid receptors → increases transcription and insertion of ENaC channels in the apical membrane and Na⁺/K⁺-ATPase pumps in the basolateral membrane → ↑ Na⁺ reabsorption and ↑ K⁺ secretion.
   • Aldosterone release is stimulated by angiotensin II, hyperkalemia, and (to a lesser extent) ACTH. ANP inhibits aldosterone release.

1. K⁺ Secretion (ROMK + Aldosterone)
   • K⁺ is secreted into the tubular lumen through apical ROMK (renal outer medullary potassium) channels and BK (maxi-K) channels.
   • The driving forces: (a) the Na⁺/K⁺-ATPase actively pumps K⁺ into the cell, maintaining a high intracellular K⁺ concentration; (b) Na⁺ entry via ENaC depolarizes the apical membrane, creating a favorable electrochemical gradient for K⁺ exit; (c) the negative luminal potential (created by Na⁺ reabsorption without accompanying Cl⁻).
   • Aldosterone stimulates K⁺ secretion by increasing Na⁺/K⁺-ATPase activity and ENaC expression.

1. Water Reabsorption (AQP2 + ADH)
   • Antidiuretic hormone (ADH, also called arginine vasopressin, AVP) , released from the posterior pituitary in response to increased plasma osmolality or decreased blood volume, binds to V₂ receptors on the basolateral membrane of principal cells.
   • V₂ receptor activation triggers a Gs-adenylyl cyclase-cAMP-PKA signaling cascade → phosphorylation and translocation of intracellular vesicles containing aquaporin-2 (AQP2) to the apical membrane.
   • With AQP2 inserted in the apical membrane, water can enter the cell (driven by the hyperosmotic medullary interstitium), then exit through constitutively expressed aquaporin-3 (AQP3) and aquaporin-4 (AQP4) on the basolateral membrane.
   • In the absence of ADH, AQP2 is removed from the apical membrane by endocytosis and returned to intracellular vesicles → the collecting duct becomes virtually impermeable to water → dilute urine is excreted.

Intercalated Cells (Minority)

Intercalated cells regulate acid-base balance. There are two subtypes:

• Type A (α) Intercalated Cells: Secrete H⁺ and reabsorb HCO₃⁻.
  • Apical H⁺-ATPase and H⁺/K⁺-ATPase pump H⁺ into the lumen.
  • Basolateral Cl⁻/HCO₃⁻ exchanger (AE1) transports HCO₃⁻ into the blood.
  • Active during acidosis → net H⁺ secretion and new HCO₃⁻ generation.

• Type B (β) Intercalated Cells: Secrete HCO₃⁻ and reabsorb H⁺.
  • Apical Cl⁻/HCO₃⁻ exchanger (pendrin) secretes HCO₃⁻ into the lumen.
  • Basolateral H⁺-ATPase pumps H⁺ into the blood.
  • Active during alkalosis → net HCO₃⁻ secretion.

Table 22.4 — Nephron Segment Summary: Permeability and Transport

22.5 Tubular Secretion

Tubular secretion is the active transport of substances from the peritubular capillaries into the tubular lumen. It provides a second route (beyond filtration) for substances to enter the urine and is critical for:

1. Eliminating substances that are tightly protein-bound (and therefore poorly filtered): e.g., many drugs, toxins, and organic anions.
2. Rapidly clearing potentially toxic metabolites: e.g., endogenous organic acids.
3. Regulating acid-base balance: via secretion of H⁺, NH₄⁺.
4. Regulating K⁺ homeostasis: via K⁺ secretion in the collecting duct.

Secreted Substances

PAH Clearance and Renal Plasma Flow

Para-aminohippuric acid (PAH) is an organic anion that is both freely filtered at the glomerulus and avidly secreted by the PCT. At low plasma concentrations, virtually all PAH entering the kidney is removed from the blood in a single pass (extraction ratio ~0.9). Therefore, PAH clearance provides a clinical estimate of effective renal plasma flow (ERPF) :

ERPF = CPAH = (UPAH × V̇) / PPAH

where UPAH is urine PAH concentration, V̇ is urine flow rate, and PPAH is plasma PAH concentration. Renal blood flow (RBF) can then be calculated as:

RBF = ERPF / (1 − hematocrit)

22.6 The Countercurrent Multiplier System

The kidney's ability to excrete urine that is either more dilute or more concentrated than plasma depends on the medullary osmotic gradient — a progressive increase in interstitial osmolality from the cortex (~300 mOsm/L) to the deep inner medulla (~1200 mOsm/L). This gradient is generated and maintained by the countercurrent multiplier (the loop of Henle) and preserved by the countercurrent exchanger (the vasa recta).

22.6.1 The Countercurrent Multiplier: Loop of Henle

The countercurrent multiplication mechanism operates as follows:

1. Active step — solute pumping in the thick ascending limb (TAL) : The NKCC2 cotransporter actively transports NaCl from the tubular lumen into the medullary interstitium. Because the TAL is impermeable to water, the interstitial osmolality rises while the tubular fluid becomes hypo-osmotic (~200 mOsm/L below the surrounding interstitium — the single effect). A single pass through the TAL generates approximately a 200 mOsm/L gradient.

1. Countercurrent flow amplification: The descending and ascending limbs run parallel and in opposite directions (countercurrent). Fluid entering the descending limb equilibrates with the now-slightly-higher interstitial osmolality by losing water and gaining solutes (urea). As this more-concentrated fluid rounds the hairpin turn and enters the ascending limb, it presents a higher solute load to the NaCl pumps. With each cycle, the gradient is multiplied along the length of the loop — from 300 to 1200 mOsm/L.

1. The single effect × length = large axial gradient: The 200 mOsm/L single effect, multiplied over the length of the loop, creates the final corticomedullary gradient of 300 → 1200 mOsm/L.

The corticomedullary osmotic gradient composition varies by depth:

• In the outer medulla: NaCl is the dominant solute (~50–60% of osmolality).
• In the inner medulla: urea contributes approximately 50% of osmolality (via urea recycling), with NaCl contributing the remainder.

22.6.2 The Countercurrent Exchanger: Vasa Recta

The vasa recta are the peritubular capillaries of the juxtamedullary nephrons. They run parallel to the loops of Henle as long, hairpin-shaped vessels that descend into and ascend out of the medulla. Without a special mechanism, blood flowing through the medullary capillaries would rapidly wash out the osmotic gradient. Instead, the vasa recta function as a countercurrent exchanger:

1. As blood descends into the medulla, water diffuses out of the capillaries (into the hyperosmotic interstitium) while solutes (NaCl, urea) diffuse in. By the hairpin turn, the blood is at near-interstitial osmolality.

1. As blood ascends back toward the cortex, the process reverses: solutes diffuse out of the capillaries and water diffuses in (the interstitium is now less concentrated than the ascending blood).

1. Net result: Solutes are trapped in the medullary interstitium by continuous short-circuit recycling between the descending and ascending limbs of the vasa recta. The vasa recta carry away only the small net excess of water reabsorbed from the collecting ducts. This preserves the medullary osmotic gradient while still removing reabsorbed water.

The slow blood flow through the vasa recta (low medullary blood flow, ~5–10% of total renal blood flow) is another critical factor that minimizes gradient washout.

22.6.3 Urea Recycling

Urea recycling contributes significantly to the inner medullary osmotic gradient:

1. In the collecting duct, ADH increases water reabsorption, concentrating urea in the tubular lumen.
2. ADH also increases the permeability of the inner medullary collecting duct to urea by upregulating UT-A1 and UT-A3 urea transporters on the apical and basolateral membranes.
3. Concentrated urea diffuses out of the collecting duct into the medullary interstitium → contributes ~50% of inner medullary osmolality.
4. This urea then enters the thin descending limb (via UT-A2) and recycles through the loop, continuously augmenting the gradient.

22.7 Formation of Dilute vs. Concentrated Urine

The final urine osmolality is determined by the amount of water reabsorption that occurs in the collecting duct, which is controlled entirely by ADH. The medullary gradient (300–1200 mOsm/L) is the driving force; the presence or absence of ADH determines whether water can actually move in response to that gradient.

22.7.1 Dilute Urine Formation (No ADH — Water Diuresis)

When the body is water-loaded (↓ plasma osmolality), ADH release from the posterior pituitary is suppressed:

1. The cortical and outer medullary collecting ducts remain impermeable to water (AQP2 is not inserted in the apical membrane).
2. The Na⁺/K⁺/2Cl⁻ cotransporter in the TAL and the NCC in the DCT continue to reabsorb solutes without water → the tubular fluid entering the collecting duct is already hypo-osmotic (~100 mOsm/L).
3. In the collecting duct, Na⁺ continues to be reabsorbed (via ENaC and Na⁺/K⁺-ATPase) but without accompanying water movement → the tubular fluid becomes even more dilute.
4. Final urine: Osmolality as low as ~50 mOsm/L (about one-sixth of plasma osmolality). Urine volume is large (up to ~15–20 L/day in complete absence of ADH, as in central diabetes insipidus).

Key point: Dilute urine can be formed even though the medullary interstitium is hyperosmotic, because without AQP2 pores, water cannot cross the collecting duct epithelium to reach the gradient.

22.7.2 Concentrated Urine Formation (ADH Present — Antidiuresis)

When the body is dehydrated (↑ plasma osmolality or ↓ blood volume), ADH is released:

1. ADH binds to V₂ receptors on principal cells → insertion of AQP2 channels in the apical membrane.
2. As tubular fluid descends through the cortical collecting duct, it encounters the first osmotic gradient (~300 mOsm/L) and water begins to leave → fluid approaches isotonicity.
3. As the fluid continues through the outer medullary collecting duct (interstitium ~600 mOsm/L), more water is reabsorbed.
4. In the inner medullary collecting duct (interstitium ~1200 mOsm/L), maximal water reabsorption occurs → tubular fluid osmolarity equilibrates with the interstitium.
5. Final urine: Osmolality up to ~1200 mOsm/L (about four times plasma osmolality). Urine volume is small (~0.5 L/day).

The role of urea: ADH also increases urea permeability in the inner medullary collecting duct (UT-A1 upregulation), allowing urea to contribute to the interstitial osmolality. This urea is what drives the final osmotic equilibration to 1200 mOsm/L — NaCl alone can only achieve ~600 mOsm/L in the outer medulla.

Table 22.5 — Dilute vs. Concentrated Urine

22.8 Clinical Correlations

22.8.1 Estimation of GFR

GFR is the single best indicator of kidney function. It cannot be measured directly in routine clinical practice, so it is estimated using endogenous filtration markers:

• Creatinine clearance (CrCl) : Creatinine is a waste product of muscle creatine metabolism, freely filtered at the glomerulus, and only minimally secreted. A 24-hour urine collection is used:

CrCl = (UCr × V̇) / PCr

where UCr is urine creatinine, V̇ is urine flow rate (mL/min), and PCr is plasma creatinine. Normal: ~90–140 mL/min (males), ~80–125 mL/min (females).

• Estimated GFR (eGFR) : Calculated from a single serum creatinine measurement using equations (CKD-EPI or MDRD) that account for age, sex, and race. Does not require urine collection. Normal eGFR: ≥90 mL/min/1.73 m².

• Serum creatinine: Inversely related to GFR. Because creatinine production is relatively constant, a rise in serum creatinine generally reflects a fall in GFR. However, serum creatinine does not rise above the normal range until GFR has fallen by ~50%.

• Cystatin C: An alternative endogenous marker; may be more accurate than creatinine at higher GFR ranges because it is less affected by muscle mass, age, and diet.

22.8.2 Acute Kidney Injury (AKI)

AKI is a rapid decline in GFR (hours to days), resulting in azotemia (↑ BUN and creatinine), oliguria, and electrolyte disturbances. Three categories:

Key distinguishing feature: In prerenal AKI, the tubules are intact and respond appropriately to low perfusion by avidly reabsorbing Na⁺ and water → very low urine Na⁺ and concentrated urine. In ATN, the tubules are damaged and cannot reabsorb Na⁺ → high urine Na⁺ and isosthenuric urine.

22.8.3 Chronic Kidney Disease (CKD)

CKD is defined as either kidney damage or GFR <60 mL/min/1.73 m² for ≥3 months. Staged by eGFR:

• Stage 1: eGFR ≥90 with evidence of kidney damage (proteinuria, hematuria, abnormal imaging)
• Stage 2: eGFR 60–89 with evidence of kidney damage
• Stage 3: eGFR 30–59
• Stage 4: eGFR 15–29
• Stage 5 (ESRD): eGFR <15 or on dialysis

Progressive loss of nephrons leads to: inability to concentrate urine (nocturia, polyuria), hyperkalemia (↓ K⁺ secretion), metabolic acidosis (↓ H⁺ secretion), hyperphosphatemia and hypocalcemia (↓ vitamin D activation), anemia (↓ EPO production), and volume overload.

22.8.4 Diuretics

Diuretics target specific nephron transporters to increase Na⁺ and water excretion:

22.8.5 SIADH vs. Diabetes Insipidus

Syndrome of Inappropriate ADH (SIADH) :

• Pathophysiology: Excess ADH secretion (or action) despite low plasma osmolality → persistent water reabsorption in the collecting duct.
• Lab findings: Hyponatremia (dilutional), low plasma osmolality, inappropriately concentrated urine (Urine Osm >100 mOsm/L, often > Plasma Osm), low urine output, euvolemic.
• Causes: Small cell lung cancer (ectopic ADH), CNS disorders (meningitis, stroke, trauma), pulmonary disease, certain drugs (SSRIs, carbamazepine, cyclophosphamide).

Diabetes Insipidus (DI) :

• Pathophysiology: Inability to concentrate urine due to ADH deficiency (central DI) or renal resistance to ADH (nephrogenic DI).
• Lab findings: Polyuria (up to 15–20 L/day), polydipsia, hypernatremia (if water intake inadequate), inappropriately dilute urine (Urine Osm <300 mOsm/L, often <100 mOsm/L), high plasma osmolality.
• Central DI: ADH deficiency from pituitary/hypothalamic damage. Treat with desmopressin (dDAVP) , a synthetic V₂ agonist.
• Nephrogenic DI: Renal resistance to ADH — V₂ receptor mutations, lithium toxicity, hypercalcemia, hypokalemia. Does not respond to desmopressin. Treat with thiazide diuretics (paradoxical ↓ urine volume by reducing GFR and increasing PCT water reabsorption) + low-solute diet.

Water Deprivation Test: Used to differentiate causes of polyuria. After fluid deprivation, administer desmopressin:

• Normal: Urine Osm rises with dehydration; no further rise with desmopressin.
• Central DI: Urine Osm does not rise with dehydration alone; rises significantly (>50%) with desmopressin.
• Nephrogenic DI: Urine Osm does not rise with either dehydration or desmopressin.