A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
The proximal tubule selectively reabsorbs glucose, amino acids, and ions via active transport and recovers water by osmosis. The loop of Henle creates an osmotic gradient through countercurrent multiplication, allowing water reabsorption in the distal tubule and collecting duct. The distal tubule and collecting duct regulate sodium and water excretion via hormonal control, determining final urine composition.
The kidneys filter about 180 liters of plasma per day at the glomerulus, yet you excrete only 1-2 liters of urine. The difference — over 99% of the filtrate — is reclaimed by tubular reabsorption as fluid travels through the nephron. From your study of proximal tubule function and the loop of Henle, you understand the individual segments; this topic integrates them into a complete picture of how the nephron transforms a massive, indiscriminate filtrate into precisely composed urine.
The proximal tubule does the bulk work, reabsorbing approximately 65% of filtered sodium, water, bicarbonate, glucose, and amino acids. Its strategy is straightforward: Na+/K+-ATPase on the basolateral membrane creates a low intracellular sodium concentration, and sodium-coupled cotransporters on the apical membrane harness this gradient to pull glucose, amino acids, and phosphate into the cell. Water follows osmotically through aquaporin-1 channels, and solutes like urea and chloride are dragged along by solvent drag. The proximal tubule is obligatory and unregulated — it reabsorbs a fixed fraction of whatever is filtered, regardless of whether the body needs to conserve or excrete more water. Think of it as a first-pass recovery system that grabs everything valuable before the filtrate moves on.
The loop of Henle serves a fundamentally different purpose: it builds the medullary osmotic gradient that makes concentrated urine possible. The descending limb is permeable to water but not solutes, so water leaves as the tubular fluid descends into the increasingly hyperosmotic medulla. The ascending limb is impermeable to water but actively pumps out NaCl via the Na+/K+/2Cl− cotransporter (NKCC2), diluting the tubular fluid while adding solute to the medullary interstitium. This countercurrent multiplication creates a gradient from about 300 mOsm/kg at the cortex to 1200 mOsm/kg at the papilla — a standing osmotic "hill" that the collecting duct can later exploit.
The distal tubule and collecting duct are where hormonal fine-tuning occurs, making this the regulated portion of the nephron. Aldosterone (from the adrenal cortex) increases sodium reabsorption and potassium secretion in the distal tubule and cortical collecting duct by upregulating ENaC sodium channels and Na+/K+-ATPase. Antidiuretic hormone (ADH, or vasopressin) controls water permeability of the collecting duct by inserting aquaporin-2 channels into the apical membrane. When ADH is high (dehydration), aquaporins are inserted, water flows out of the collecting duct into the hyperosmotic medulla, and urine becomes concentrated. When ADH is low (overhydration), aquaporins are removed, the collecting duct is impermeable to water, and dilute urine is excreted. This is the final decision point — the body's last chance to adjust water and solute balance before fluid exits as urine.