Function of the Tubules
நுண்குழல்களின் பணிகள்
Descending limb: water out, salts stay. Ascending limb: salts out, water stays. Get these two opposite rules right and the concentration questions solve themselves.
The idea
Proximal convoluted tubule (PCT) is lined by simple cuboidal brush-border epithelium which increases the surface area for reabsorption; nearly all essential nutrients and 70-80% of electrolytes and water are reabsorbed here. PCT also maintains the pH and ionic balance of body fluids by selective secretion of hydrogen ions and ammonia into the filtrate and absorption of HCO3- from it, and also secretes K+. In Henle's loop reabsorption is minimal, but this region maintains the high osmolarity of the medullary interstitial fluid. The descending limb of the loop of Henle is permeable to water but almost impermeable to electrolytes, so the filtrate concentrates as it moves down. The ascending limb is impermeable to water but allows transport of electrolytes actively or passively, so the filtrate gets diluted as it moves up. In the distal convoluted tubule (DCT) conditional reabsorption of Na+ and water takes place; DCT can also reabsorb HCO3- and selectively secrete hydrogen and potassium ions and NH3 to maintain pH and sodium-potassium balance. The collecting duct reabsorbs large amounts of water to produce concentrated urine, allows passage of small amounts of urea into the medullary interstitium to keep up the osmolarity, and helps maintain pH and ionic balance by selective secretion of H+ and K+.
Why it matters
Each segment is specialised: bulk recovery first (PCT), building the salt gradient next (loop), and fine-tuning at the end (DCT, collecting duct) under hormonal control.
NEET lens
Classic statements: PCT reabsorbs nearly all nutrients and 70-80% of electrolytes and water (brush border). Descending limb — permeable to water, almost impermeable to electrolytes (concentrates filtrate). Ascending limb — impermeable to water, transports electrolytes (dilutes filtrate). DCT — conditional Na+ and water reabsorption. Collecting duct — large water reabsorption, passes urea into medulla.
Clinical significance
Many diuretic drugs used for high blood pressure and oedema act by blocking salt reabsorption in the ascending limb or DCT, so more water stays in the tubule and leaves as urine.
Introduction
After filtration, each part of the renal tubule handles the filtrate differently — together they decide how much water and salt leave in urine.
Core concept
PCT: simple cuboidal brush-border epithelium; reabsorbs nearly all essential nutrients, 70-80% of electrolytes and water; secretes H+, NH3, K+; absorbs HCO3-. Henle's loop: minimal reabsorption; maintains high osmolarity of medullary interstitium. Descending limb — permeable to water, almost impermeable to electrolytes → filtrate concentrated. Ascending limb — impermeable to water, transports electrolytes → filtrate diluted. DCT: conditional reabsorption of Na+ and water; reabsorbs HCO3-; secretes H+, K+, NH3. Collecting duct: extends cortex → inner medulla; reabsorbs large amounts of water (concentrated urine); passes small amounts of urea into medullary interstitium; secretes H+, K+.
Go deeper
'Conditional' reabsorption in DCT and collecting duct means it depends on hormones — aldosterone for Na+ and ADH for water.
From Class 10
The tubule is where useful things are taken back. Each part of it has a special job, and NEET asks which part does what.