Renal Tubular Reabsorption and Secretion

Overview of Urinary Excretion and Basic Renal Processes

Urine composition is the algebraic sum of three fundamental renal operations—glomerular filtration, tubular reabsorption, and tubular secretion—which can be expressed as (\text{Urinary excretion}=\text{Glomerular filtration}-\text{Tubular reabsorption}+\text{Tubular secretion}). Filtrate originating in Bowman’s space travels sequentially through the proximal tubule, loop of Henle, distal convoluted tubule, cortical collecting tubule, and medullary collecting duct before leaving the kidney. Along this path, selected solutes and water are reclaimed and others are added, allowing the nephron to fine-tune extracellular fluid volume, osmolarity, electrolyte balance, acid–base status, and excretion of metabolic waste products. While secretion is essential for potassium, hydrogen ions, organic acids/bases, and certain drugs, the quantitatively dominant determinant of final urine composition is reabsorption, which is both large in magnitude and highly selective.

Quantitative Significance and Selectivity of Tubular Reabsorption

Table 28-1 (values for a 70-kg adult with a glomerular filtration rate (GFR) of 180\,\text{L day}^{-1}) demonstrates that the filtered loads of many freely-filtered substances dwarf their urinary excretion:

• Glucose: 180\,\text{g day}^{-1} filtered, \approx0 excreted (≈100\% reabsorbed).
• \text{HCO}_3^-: 4320\,\text{mEq day}^{-1} filtered, \approx2\,\text{mEq} excreted ((>99.9\%) reabsorbed).
• Na⁺: 25{,}560\,\text{mEq day}^{-1} filtered, 150\,\text{mEq} excreted ((99.4\%) reabsorbed).
• Creatinine: 1.8\,\text{g day}^{-1} filtered, 1.8\,\text{g} excreted ((0\%) reabsorbed, minor secretion).

Because reabsorption is so massive, even a modest (≈10\%) fall in tubular reabsorption, unopposed, would propel urine flow from 1.5\,\text{L day}^{-1} to (\sim19\,\text{L day}^{-1}). Tight co-ordination between filtration and reabsorption—termed glomerulo-tubular balance—prevents such swings.

Cellular and Biophysical Mechanisms of Tubular Transport

Tubular reabsorption requires that solute/water traverse two serial barriers: the epithelial layer (luminal/apical entry, cytosolic passage, basolateral exit) and the peritubular capillary wall. Movement across epithelium may be transcellular (through the cell) or paracellular (between cells through tight-junctional clefts). Peritubular uptake occurs by bulk flow (ultrafiltration) driven by Starling forces.

Active versus Passive Processes

Primary active transport uses metabolic energy (ATP hydrolysis) to move solute uphill. Examples include Na⁺-K⁺-ATPase, H⁺-ATPase, H⁺-K⁺-ATPase, and Ca²⁺-ATPase. Secondary active transport couples downhill movement of one ion (usually Na⁺) to uphill transport of another solute (e.g., Na⁺–glucose, Na⁺–amino acid co-transport; Na⁺/H⁺ counter-transport). Purely passive events include diffusion of uncharged or lipid-soluble species down electrochemical gradients and osmosis of water.

Transcellular versus Paracellular Routes

Renal epithelial cells are joined by tight junctions; nevertheless, permeability of these junctions varies by segment. In the proximal tubule, leaky junctions permit significant paracellular water and solute flow (notably K⁺, Mg²⁺, Cl⁻). By contrast, distal nephron junctions are tight, confining most transport to transcellular pathways.

Primary Active Transport Example: Na⁺-K⁺-ATPase

Located basolaterally in virtually every tubular segment, this pump extrudes three Na⁺ in exchange for two K⁺, hydrolyzing one ATP and leaving the cell interior \approx−70\,\text{mV}. The resulting low intracellular Na⁺ (≈12\,\text{mEq L}^{-1}) and negative Vm favor passive Na⁺ entry across the luminal membrane.

Secondary Active Transport Examples

• Na⁺–glucose co-transport (SGLT2 in S1/S2, SGLT1 in S3; basolateral exit via GLUT2/GLUT1).
• Na⁺–amino acid co-transport.
• Na⁺/H⁺ exchange (NHE3) in proximal tubule; couples Na⁺ reabsorption to H⁺ secretion.

Pinocytosis

Filtered low-molecular-weight proteins bind the brush-border, are internalised, hydrolyzed to amino acids, and reabsorbed. Because vesicular trafficking requires ATP, pinocytosis counts as active transport.

Transport Maximum (Tₘ)

Carrier-mediated systems saturate when tubular load exceeds capacity. For glucose Tₘ ≈ 375\,\text{mg min}^{-1} in adults. When filtered load >Tm (threshold ≈ P{\text{glucose}} 200\,\text{mg dL}^{-1}), excess glucose spills into urine (glucosuria in diabetes mellitus). Actively secreted solutes (e.g., PAH T_m≈80\,\text{mg min}^{-1}) show analogous saturation.

Gradient–Time Transport

Some extensively-leaky segments (e.g., proximal tubule Na⁺ reabsorption) seldom reach carrier saturation; instead, reabsorptive rate is proportional to electrochemical gradient and contact time—hence sensitive to flow rate.

Coupling of Water Reabsorption to Solute Movement

Active solute reabsorption decreases luminal osmolarity and raises interstitial osmolarity, creating an osmotic gradient that drags water transcellularly (through aquaporins) and paracellularly. Aquaporin-1 (AQP-1) confers high water permeability to proximal tubule and descending thin limb, whereas distal segments require antidiuretic hormone (ADH) to insert AQP-2 in the apical membrane; basolateral AQP-3/4 provide exit routes.

Water flow through junctions sweeps dissolved solute—“solvent drag.” Because Na⁺ reabsorption is large, water and many electrolytes mirror Na⁺ flux.

Passive Reabsorption of Cl⁻, Urea, and Other Solutes

Removal of NaHCO₃, glucose, etc., in early proximal tubule leaves Cl⁻ enriched, generating an electrical (lumen-negative) and chemical gradient that drives paracellular Cl⁻ diffusion in late proximal segments. As water exits, luminal [urea] doubles, favoring passive urea reabsorption (≈50\% overall). Creatinine, by contrast, is virtually impermeant and becomes highly concentrated.

Segmental Profiles of Nephron Function

Proximal Convoluted Tubule (PCT)

Highly metabolic epithelium with dense mitochondria, abundant Na⁺-K⁺-ATPase, and a brush border that amplifies surface area ≈20-fold. About 65\% of filtered Na⁺, Cl⁻, K⁺, and H₂O and almost 100\% of glucose and amino acids are reclaimed. Early PCT favors Na⁺ co-transport with organic solutes; late PCT switches to NaCl coupling. The PCT also secretes H⁺ (via NHE3), organic acids/bases (urate, oxalate, catecholamines), and xenobiotics (penicillin, PAH).

Loop of Henle

• Thin descending limb: very permeable to H₂O, modestly to solutes; passive equilibration removes ≈20\% of filtered water.
• Thin ascending limb: impermeable to H₂O; limited solute reabsorption.
• Thick ascending limb (TAL): vigorous active reabsorption of ≈25\% of filtered NaCl and K⁺ via NKCC2 (1 Na⁺-2 Cl⁻-1 K⁺). Potassium back-leak renders lumen +8\,\text{mV}, driving paracellular uptake of Mg²⁺ and Ca²⁺. Segment is water-impermeable, producing hypo-osmotic tubular fluid (“diluting segment”). Loop diuretics (furosemide, bumetanide, ethacrynic acid) block NKCC2.

Distal Convoluted Tubule (DCT)

Early DCT continues dilution; reabsorbs ≈5\% of filtered NaCl via NCC (Na⁺-Cl⁻ co-transporter) inhibited by thiazide diuretics. Impermeable to urea and, without ADH, to water. Contributes to Mg²⁺ and Ca²⁺ handling (PTH-sensitive).

Late DCT and Cortical Collecting Tubule (CCT)

Two principal cell types:

• Principal cells: Aldosterone-sensitive Na⁺ reabsorption (ENaC) and K⁺ secretion; water reabsorption regulated by ADH. Potassium-sparing diuretics act here—spironolactone/eplerenone (mineralocorticoid receptor antagonists) and amiloride/triamterene (ENaC blockers).

• Intercalated cells: Type A secrete H⁺ (H⁺-ATPase, H⁺-K⁺-ATPase) and reabsorb HCO₃⁻/K⁺ (acidosis defense). Type B exhibit reversed polarity, secreting HCO₃⁻ via pendrin and reclaiming H⁺ (alkalosis defense). Cell population adapts to chronic acid–base disorders.

Both cell types are nearly impermeable to urea; thus urea becomes concentrated.

Medullary Collecting Duct (MCD)

Final arbiter of water and solute excretion. ADH-dependent water permeability; expresses urea transporters (UT-A1/3) allowing facilitated urea reabsorption—critical for medullary hypertonicity and urine concentration. Actively secretes H⁺, contributing to final urine acidity (pH \approx4.5 possible). Only ≈5\% of filtered Na⁺ and water reach the MCD, but fine control here strongly influences final balances.

Evolution of Tubular Fluid Composition

Because proximal water removal parallels Na⁺, [Na⁺] and osmolarity remain ≈isosmotic throughout PCT (TF/P ≈ 1.0). Glucose, amino acids, and HCO₃⁻ fall steeply (TF/P < 0.1), whereas creatinine and PAH rise dramatically. By the end of the collecting duct, inulin TF/P ≈ 125, indicating >99\% of filtered water has been reabsorbed.

Regulation of Tubular Reabsorption

Glomerulotubular Balance (GTB)

An intrinsic property whereby a constant fraction (≈65\%) of filtered Na⁺ and water is reabsorbed in the proximal tubule despite fluctuations in GFR. GTB prevents distal overload and operates independent of hormones, relying on changes in oncotic/hydrostatic forces and flow-dependent dynamics.

Peritubular Capillary & Interstitial Starling Forces

Reabsorption (Jv) from interstitium into peritubular capillaries obeys Jv = Kf[(\pic-\pi{if})-(Pc-P_{if})].

Typical values:
Pc≈13\,\text{mmHg},\;P{if}≈6\,\text{mmHg},\;\pic≈32\,\text{mmHg},\;\pi{if}≈15\,\text{mmHg} giving net reabsorptive force ≈10\,\text{mmHg} and, with Kf≈12.4\,\text{ml min}^{-1}\text{mmHg}^{-1}, fluid uptake ≈124\,\text{ml min}^{-1} (matches proximal delivery). Alterations of Pc or \pi_c (via changes in arterial pressure, afferent/efferent tone, plasma proteins, or filtration fraction) proportionally modify both peritubular and tubular reabsorption because interstitial pressure/oncotic pressure secondarily shift, affecting back-leak.

Pressure Natriuresis & Diuresis

Raising renal arterial pressure (75–160 mmHg range) modestly increases GFR but markedly reduces fractional reabsorption, partly by elevating P_c and renal interstitial pressure, diminishing angiotensin II, and triggering internalisation of Na⁺ transporters. The resulting natriuresis/diuresis serves as a long-term blood-pressure stabiliser.

Hormonal Modulators

• Aldosterone (zona glomerulosa): up-regulates ENaC/Na⁺-K⁺-ATPase in principal cells, enhancing Na⁺/water retention and K⁺/H⁺ secretion; stimulated by \uparrowK⁺ and \uparrowAng II.

• Angiotensin II: potent Na⁺-retaining agent—induces aldosterone, constricts efferent arterioles (↑FF, ↓P_c), and directly stimulates Na⁺ transporters (NHE3, Na⁺-HCO₃⁻ cotransport, Na⁺-K⁺-ATPase).

• ADH (arginine vasopressin): binds V₂ receptors ➔ cAMP/PKA ➔ AQP-2 insertion, escalating water (and urea in MCD) permeability.

• Atrial natriuretic peptide (ANP): released by atrial stretch; inhibits Na⁺ reabsorption (collecting duct), suppresses renin/aldosterone; augments GFR, promoting natriuresis.

• Parathyroid hormone (PTH): decreases phosphate uptake (PCT) and increases Ca²⁺ reabsorption (TAL/DCT).

Sympathetic Nervous System

α-adrenergic activation raises proximal and TAL Na⁺ reabsorption, stimulates renin, and at high levels constricts renal vasculature, lowering GFR.

Clearance Concepts for Quantifying Renal Function

Renal clearance (Cₛ): volume of plasma completely cleared of substance s per unit time Cs = \dfrac{Us\,V}{P_s}.

Glomerular Filtration Rate (GFR)

Inulin (freely filtered, neither secreted nor reabsorbed): \text{GFR}=C{\text{inulin}}. Example: U{in}=125\,\text{mg ml}^{-1},\;P_{in}=1\,\text{mg ml}^{-1},\;V=1\,\text{ml min}^{-1} ⇒ \text{GFR}=125\,\text{ml min}^{-1}.

Creatinine, an endogenous marker, provides a practical estimate: C{Cr}\approx\text{GFR}. Because creatinine production is constant, P{Cr}\propto1/\text{GFR}; a 50 % fall in GFR doubles P_{Cr}.

Renal Plasma Flow (RPF)

Para-aminohippuric acid (PAH) is ≈90\% extracted in one pass. C{PAH}=\dfrac{U{PAH}V}{P{PAH}}\approx\text{Effective RPF}; true RPF = C{PAH}/E{PAH} where E{PAH} is the extraction ratio.

Filtration Fraction

\text{FF}=\dfrac{\text{GFR}}{\text{RPF}} (normal ≈0.19).

Estimating Tubular Reabsorption or Secretion

Net reabsorption =\big(GFR\,Ps\big)-UsV; net secretion =UsV-GFR\,Ps.

Clearance Ratios

Comparing C_s to inulin clearance classifies handling:

• Cs=C{in} ➔ filtration only (e.g., inulin).
• Cs{in} ➔ net reabsorption (e.g., Na⁺, Cl⁻).
• Cs>C{in} ➔ net secretion (e.g., PAH, K⁺ under high-aldosterone states).

Key Numerical Benchmarks

• GFR: \approx125\,\text{ml min}^{-1} (180\,\text{L day}^{-1}).
• Renal blood flow: \approx1.2\,\text{L min}^{-1} (˜20\% of cardiac output).
• Average proximal fractional reabsorption: 65\% of filtered Na⁺/H₂O.
• Na⁺-K⁺-ATPase stoichiometry: 3\,\text{Na}^+!:!2\,\text{K}^+ : 1\,\text{ATP}.
• TAL NKCC2 stoichiometry: 1\,\text{Na}^+!:!2\,\text{Cl}^-!:!1\,\text{K}^+.
• Maximum H⁺ gradient achievable by Type A cells: 1000{:}1 (urine pH ≈4.5).

Clinical & Pharmacological Correlates

Loop diuretics block NKCC2, inducing potent natriuresis and Ca²⁺/Mg²⁺ loss; thiazides inhibit NCC, modestly reducing Ca²⁺ excretion; K⁺-sparing agents antagonise aldosterone or ENaC, averting hypokalaemia. Pathologic hypoaldosteronism (Addison disease) yields Na⁺ wasting and hyperkalaemia; hyperaldosteronism (Conn syndrome) produces volume expansion and hypokalaemia. Absence of ADH action (central or nephrogenic diabetes insipidus) causes excretion of large volumes of hypotonic urine. Elevated ANP in heart failure offsets renal Na⁺ retention. Measurement of PAH clearance helps estimate RPF, important in diagnosing renal vascular disorders.

In sum, the nephron’s orchestrated transport events, modulated by physical forces, hormones, and neural input, permit minute-to-minute regulation of extracellular fluid composition while efficiently clearing metabolic wastes.