How Does the Body Maintain Water and Nitrogen Balance?

The kidneys maintain water balance by adjusting how much water is reabsorbed into the blood. They help maintain nitrogen balance by removing urea from the blood and excreting it in urine.

Assumption: this answer is for AQA GCSE separate Biology at Higher Tier. Kidney function is biology-only content, while detailed deamination and ADH control include Higher Tier content.

How the kidneys make urine

Blood is filtered in the kidneys. Useful substances are then selectively reabsorbed, meaning they are taken back into the blood as needed. These include:

• all glucose
• some ions
• different amounts of water

Excess water, ions and urea remain and form urine. The kidneys can control water, ion and urea loss, unlike losses through the lungs and skin.

Water balance and ADH

ADH stands for antidiuretic hormone. It is released by the pituitary gland when the blood becomes too concentrated.

ADH makes the kidney tubules more permeable to water. More water is therefore reabsorbed into the blood, producing a smaller volume of concentrated urine. When the blood contains too much water, less ADH is released, less water is reabsorbed and a larger volume of dilute urine is produced.

This is controlled by negative feedback: a change is detected and the response reverses that change, returning conditions towards the optimum level.

Example: dehydration

After heavy sweating, the blood becomes more concentrated. More ADH is released, so the kidneys reabsorb more water. Less water is lost in urine.

Nitrogen balance

Digestion of proteins produces amino acids. Excess amino acids cannot be stored, so the liver removes their amino group in a process called deamination. This produces toxic ammonia, which is immediately converted into the safer substance urea. Urea travels in the blood to the kidneys and is excreted in urine.

Common exam mistake: do not say that the kidneys make urea. Urea is made in the liver and removed from the blood by the kidneys.