Tubular Reabsorption

Core Concepts

Transport routes and mechanisms

Solutes leave the tubule by two routes. Transcellular transport crosses the cells — in across the apical (luminal) membrane, through the cytoplasm, out across the basolateral membrane. Paracellular transport passes between cells through tight junctions, driven by concentration and electrical gradients. The engine of most reabsorption is the basolateral Na⁺/K⁺-ATPase, which pumps Na⁺ out of the cell, keeping intracellular Na⁺ low. That gradient powers apical entry: channels let Na⁺ diffuse in, cotransporters (symporters) move Na⁺ with another solute (glucose, amino acids, Cl⁻), and antiporters exchange Na⁺ for another ion (such as H⁺). Transport may be primary active (ATP), secondary active (using the Na⁺ gradient), or passive — diffusion, osmosis, facilitated diffusion, and solvent drag (solutes riding along with water).

PCT: the workhorse

The PCT reabsorbs about 65% of filtered Na⁺ and water, essentially all filtered glucose and amino acids, most HCO₃⁻, and much of the filtered K⁺, Ca²⁺, and Mg²⁺ (commonly taught). Glucose and amino acids enter via Na⁺-coupled cotransporters (for example, SGLT) and exit basolaterally via facilitated transporters (GLUT). HCO₃⁻ is reclaimed through carbonic-anhydrase-dependent chemistry driven by Na⁺/H⁺ exchange. Water follows Na⁺ osmotically (through aquaporins and between cells), carrying solutes by solvent drag. The PCT also secretes H⁺, organic acids and bases, and some drugs, participating in both reabsorption and secretion.

Loop of Henle: building the medullary gradient

The thin descending limb is permeable to water but not solutes: water leaves, concentrating the filtrate. The ascending limb is impermeable to water; its thick portion actively transports Na⁺, K⁺, and Cl⁻ out through the Na-K-2Cl cotransporter (NKCC2), diluting the fluid and adding salt to the medullary interstitium. Because the two limbs carry fluid in opposite directions, this "countercurrent multiplier" builds a gradient of increasing osmolarity toward the papilla — the gradient the collecting duct later taps to concentrate urine. Loop diuretics block NKCC2, abolishing the gradient and producing copious dilute urine (commonly taught). About 25% of filtered Na⁺ is reabsorbed in the loop.

DCT and collecting duct: hormonal fine-tuning

The DCT reabsorbs about 5% of filtered Na⁺ via the Na-Cl cotransporter (NCC) — the thiazide target — and reabsorbs Ca²⁺ in response to PTH. In the collecting duct, principal cells reabsorb Na⁺ through ENaC channels (increased by aldosterone) and water through aquaporin-2 (inserted by ADH), and secrete K⁺ — which is why aldosterone excess tends to lower blood K⁺ (commonly taught). Intercalated cells secrete H⁺ and reabsorb HCO₃⁻. Collecting-duct water reabsorption is facultative (switched by ADH); PCT and loop water reabsorption is obligatory, always following solute transport. Urea is partially recycled in the medullary collecting duct, helping the osmotic gradient.

Transport maximums and saturation

Every carrier can handle only so much solute per minute — its transport maximum (Tm). Filtered glucose normally sits far below Tm and is fully reabsorbed. When blood glucose is very high, filtered glucose exceeds Tm: carriers saturate, and the excess spills into the urine (glucosuria). The leftover glucose osmotically drags water along, contributing to the large urine volumes classically described in uncontrolled diabetes (commonly taught).

How It Works / Step-by-Step Process

Follow one Na⁺ ion. Filtered into Bowman's capsule → enters PCT cells down its gradient → Na⁺/K⁺-ATPase pumps it into blood, water follows → NKCC2 moves it out of the water-impermeable ascending limb → NCC in the DCT → ENaC in the collecting duct. Almost all filtered sodium stays in the body.

Follow glucose. At normal blood glucose the filtered load is far below Tm, so all is reabsorbed; at very high blood glucose the load exceeds Tm, carriers saturate, and glucose appears in urine.

Common Confusions