Maintaining water and nitrogen balance (HT)
The kidneys are the body's filtering and water-balancing organ. They produce urine, get rid of urea, and adjust how much water is reabsorbed depending on how hydrated you are.
What the kidneys remove
The blood contains substances that must be removed:
- Urea — produced in the liver from the deamination of excess amino acids (B4.3).
- Excess water and salts — must not build up because they would change blood concentration.
- Other unwanted substances — drugs, toxins, etc.
How the kidneys work — filtering and selective reabsorption
- Ultrafiltration — high blood pressure forces small molecules (water, glucose, ions, urea) out of the blood at a knot of capillaries (the glomerulus) into the kidney tubule. Big molecules (proteins, blood cells) stay in the blood.
- Selective reabsorption — useful substances are taken back into the blood:
- All the glucose is reabsorbed (active transport).
- The mineral ions needed by the body are reabsorbed.
- As much water as the body needs is reabsorbed.
- What's left — urea, excess salts and excess water — flows down the tubule, into the bladder, and is excreted as urine.
Controlling water by ADH — the classic feedback loop
When you sweat or don't drink enough water, the blood becomes more concentrated. The hypothalamus monitors this.
- Concentrated blood detected → hypothalamus signals the pituitary.
- Pituitary releases antidiuretic hormone (ADH).
- ADH makes the kidney tubules more permeable to water.
- More water is reabsorbed back into the blood, less is lost in urine.
- Urine becomes concentrated and small in volume.
- Blood becomes less concentrated → ADH release reduced (negative feedback).
The reverse happens after drinking lots of water: less ADH → kidney tubules less permeable → little water reabsorbed → lots of dilute urine.
The opposite of antidiuretic ("preventing urination") is diuretic — drinks like coffee and alcohol can act as diuretics by suppressing ADH.
Kidney failure: dialysis vs transplant
If the kidneys fail (e.g. due to disease or injury), urea and excess fluid build up in the blood and the patient will die without treatment.
Dialysis (mechanical kidney)
- Patient's blood is passed through a machine, separated from a dialysis fluid by a partially permeable membrane.
- The dialysis fluid contains the same concentration of glucose and ions as healthy blood, so these are not lost. It contains no urea, so urea diffuses out.
- Done several times a week, several hours each time.
Pros: available immediately, no need for a donor, no immune rejection. Cons: time-consuming, expensive long-term, restrictive diet, can cause infections, no permanent cure.
Kidney transplant
- A healthy kidney is donated by a living relative or someone who has died.
- One transplant can replace the function of both failed kidneys.
- The patient must take immunosuppressant drugs to stop their immune system rejecting the foreign organ.
Pros: permanent cure (10–15 years lifespan typical); cheaper long-term; better quality of life. Cons: requires major surgery; donors are scarce; lifelong immunosuppression has side effects (more vulnerable to infection); risk of rejection.
⚠Common mistakes
- Saying "all water is reabsorbed". No — only as much as the body needs.
- Saying glucose is in normal urine. It is not — it's all reabsorbed in healthy kidneys.
- Saying ADH "produces water". It changes the kidney tubule's permeability to water; the water comes from filtrate.
- Mixing up "diuretic" and "antidiuretic". ADH = anti = stops urination = retains water.
Links
Builds on B4.3 (deamination → urea) and B5.6 (the endocrine system, ADH from the pituitary). Connects to B5.1 (negative feedback).
AI-generated · claude-opus-4-7 · v3-deep-biology