Physiology
Kidney function: the nephron explained
The kidneys filter your entire blood volume many times a day, then reclaim almost everything they filtered — the reabsorption is the impressive part.
Three processes
- Filtration — at the glomerulus, pressure forces water and small solutes into Bowman's capsule. Blood cells and most proteins are too large to pass, so their presence in urine is a sign of damage.
- Reabsorption — useful substances are returned to the blood. Around 180 litres are filtered per day but only 1–2 litres leave as urine, meaning over 99% of filtrate is reabsorbed.
- Secretion — additional wastes, drugs, and excess H+ or K+ are actively added to the filtrate.
Along the nephron
| Segment | Main job |
|---|---|
| Proximal convoluted tubule | Bulk reabsorption — all glucose and amino acids, most Na+ and water |
| Descending loop of Henle | Permeable to water only — water leaves, filtrate concentrates |
| Ascending loop of Henle | Impermeable to water — Na+/K+/Cl− pumped out, building the medullary gradient |
| Distal convoluted tubule | Fine-tuning of Na+, K+ and pH, under hormonal control |
| Collecting duct | Final water reabsorption, controlled by ADH |
Hormonal control
- ADH (vasopressin) — released when blood is too concentrated. It inserts aquaporins into the collecting duct, increasing water reabsorption, producing smaller volumes of more concentrated urine. Alcohol suppresses ADH, which is why it is a diuretic — see alcohol and macros.
- Aldosterone — released via the renin-angiotensin system when blood pressure or sodium falls. It increases Na+ reabsorption in the distal tubule, and water follows osmotically.
Countercurrent multiplication
The loop of Henle establishes a salt gradient in the medulla, becoming saltier with depth. The collecting duct passes back down through that gradient, so water can be drawn out along its whole length. Animals needing highly concentrated urine have notably longer loops.
These mechanisms are what make electrolyte balance and hydration physiologically self-correcting for most people under normal conditions.
Frequently asked questions
What are the three main processes of the nephron?
Filtration at the glomerulus, reabsorption of useful substances back into the blood, and secretion of additional wastes into the filtrate.
How much filtrate is reabsorbed?
Around 180 litres are filtered per day but only 1 to 2 litres are excreted as urine, meaning over 99% of the filtrate is reabsorbed.
What does ADH do?
Antidiuretic hormone inserts aquaporins into the collecting duct, increasing water reabsorption and producing a smaller volume of more concentrated urine. Alcohol suppresses ADH, which is why it acts as a diuretic.
Keep reading: Electrolytes explained · Hydration and performance · Homeostasis explained