Glomerulus, proximal tubule, loop of Henle, distal tubule, collecting duct
Mnemonic
Follow the filtrate and the numbers fall:
- Proximal convoluted tubule - reabsorbs 65 to 70 per cent of sodium and water, and all of the glucose and amino acids. Isotonic throughout
- Thin descending limb - permeable to water only, so the filtrate concentrates
- Thick ascending limb - Na-K-2Cl transporter, impermeable to water, so the filtrate dilutes. The diluting segment, and the target of loop diuretics
- Distal convoluted tubule - Na-Cl cotransporter, target of thiazides. Calcium reabsorption under PTH
- Collecting duct - aldosterone on principal cells (sodium in, potassium out) and ADH on aquaporin-2 channels
“Descending loses water, ascending loses salt.” That pair generates the medullary gradient by the countercurrent multiplier.
The proximal tubule does the bulk work and the distal nephron does the fine tuning, which is why proximal diuretics are weak and why the collecting duct is where hormones act.
Expansion
| Segment | Main job |
|---|---|
| Glomerulus | Ultrafiltration of plasma |
| Proximal convoluted tubule | Bulk reabsorption: about 65 to 70 per cent of sodium and water, and essentially all glucose and amino acids |
| Descending limb of Henle | Water permeable, solute impermeable: concentrates the fluid |
| Ascending limb of Henle | Solute permeable, water impermeable: dilutes the fluid and builds the medullary gradient |
| Distal convoluted tubule | Fine sodium reabsorption, calcium regulation under parathyroid hormone |
| Collecting duct | Final water reabsorption under antidiuretic hormone; potassium and acid handling under aldosterone |
The pattern is bulk first, fine tuning later. The proximal tubule is isosmotic, reabsorbing solute and water together; regulation happens distally where small adjustments have large effects on the final urine.
Diuretics map onto the segments: acetazolamide proximally, loop diuretics on the ascending limb, thiazides on the distal tubule, and amiloride and spironolactone on the collecting duct.