tubular reabsorption

Tubular reabsorption is one of the three key processes of renal physiology (filtration, reabsorption, secretion) that determine final urine composition. After glomerular filtration produces an ultrafiltrate essentially free of protein but containing solutes at plasma concentrations, the tubules reclaim most filtered water and solutes to prevent their loss in urine.

Sites and mechanisms: - Proximal Convoluted Tubule (PCT): Reabsorbs ~65-70% of filtered Na+, water, glucose, amino acids, bicarbonate (via Na+/H+ exchanger and carbonic anhydrase), phosphate, and uric acid. Uses Na+/K+-ATPase-driven active transport plus secondary active transport (co-transporters like SGLT2 for glucose) and passive paracellular reabsorption of water/Cl- following osmotic gradients. - Loop of Henle: Thick ascending limb reabsorbs Na+/K+/2Cl- via NKCC2 transporter (site of loop diuretic action), critical for the countercurrent multiplier and urine concentrating ability. Descending limb is permeable to water but not solutes. - Distal Convoluted Tubule (DCT): Reabsorbs Na+/Cl- via NCC transporter (site of thiazide diuretic action); largely impermeable to water. - Collecting Duct: Principal cells reabsorb Na+ (via ENaC, regulated by aldosterone) and water (via aquaporin-2 channels, regulated by ADH/vasopressin). Intercalated cells regulate acid-base balance (H+/HCO3- secretion/reabsorption).

Transport can be active (requiring ATP, e.g., Na+/K+-ATPase) or passive (diffusion, facilitated diffusion, osmosis). Reabsorption can be transcellular (through cells) or paracellular (between cells via tight junctions).

Clinical relevance: - Glucosuria occurs when filtered glucose exceeds the transport maximum (Tm) of SGLT2, as in diabetes mellitus, or with SGLT2 inhibitors (e.g., empagliflozin) used therapeutically. - Fanconi syndrome: generalized PCT dysfunction causing failure to reabsorb glucose, amino acids, phosphate, bicarbonate. - Diuretics target specific reabsorption sites: loop diuretics (NKCC2), thiazides (NCC), K+-sparing diuretics (ENaC or aldosterone receptor). - Understanding reabsorption is key to calculating renal clearance, fractional excretion, and diagnosing tubulopathies (e.g., renal glucosuria, Bartter syndrome, Gitelman syndrome, nephrogenic diabetes insipidus).

This concept is tested extensively on USMLE Step 1 in renal physiology and pharmacology, particularly regarding diuretic mechanisms and electrolyte disorders.