Renal & Urinary

Proximal convoluted tubule → loop of Henle (thin descending, thin/thick ascending) → distal convoluted tubule → collecting duct.

About 65-67% of filtered Na+ and water, reabsorbed isosmotically. The PCT reabsorbs the bulk of filtrate.

Glucose and amino acids are nearly 100% reabsorbed via Na+-coupled cotransporters (SGLT for glucose). Bicarbonate is reclaimed via Na+/H+ exchange + carbonic anhydrase.

The Na+-K+-2Cl- (NKCC2) cotransporter on the apical membrane. It is impermeable to water, so it dilutes the filtrate while building the medullary interstitial gradient. K+ leaks back, creating a lumen-positive potential driving paracellular Ca2+/Mg2+ reabsorption.

It actively reabsorbs NaCl but is impermeable to water, so the tubular fluid becomes hypotonic (dilute) while the medullary interstitium becomes hypertonic.

The Na+-Cl- cotransporter (NCC), which is blocked by thiazide diuretics. The DCT is also water-impermeable and is a site of PTH-regulated Ca2+ reabsorption.