AQA GCSE Biology: Water and Nitrogen Balance

This whole guide is Triple award only. The body has to control its water and its nitrogen waste, and the kidneys do most of this work. This guide explains why the balance matters, how the kidneys make urine, how ADH controls water levels, and how kidney failure is treated. The recurring idea is removing waste while keeping useful substances.

Why balance matters

The water and ion content of the body must be controlled to protect cells from osmosis.

• If body fluids become too concentrated (too little water), cells lose water by osmosis and shrink.
• If body fluids become too dilute (too much water), cells gain water by osmosis and may swell or burst.

Water, ions and urea are lost from the body in three ways: from the lungs in exhaled air (not controlled), from the skin in sweat (not controlled), and from the kidneys in urine (controlled).

Getting rid of excess amino acids

(Higher tier only.) Digesting proteins releases amino acids, some more than the body needs. Excess amino acids cannot be stored, so they are broken down.

• Excess amino acids are deaminated in the liver, producing ammonia.
• Ammonia is toxic, so it is immediately converted into urea in the liver.
• Urea is carried in the blood to the kidneys, filtered out, and excreted in urine.

How the kidneys make urine

The kidneys make urine in two stages.

• Filtration: blood entering the kidney is filtered. Small molecules, water, glucose, ions and urea, pass into the kidney tubules, while large molecules such as proteins and blood cells stay in the blood.
• Selective reabsorption: useful substances are taken back into the blood. All the glucose is reabsorbed, along with some ions and some water, but no urea is reabsorbed, so it passes on and is excreted in urine.

Controlling water with ADH

(Higher tier only.) The water level of the blood is controlled by the hormone ADH (antidiuretic hormone), by negative feedback.

• When the blood is too concentrated (water level low), the pituitary gland releases more ADH. This makes the kidney tubules more permeable to water, so more water is reabsorbed and a small volume of concentrated urine is produced.
• When the blood is too dilute, the pituitary releases less ADH, the tubules become less permeable, less water is reabsorbed, and a large volume of dilute urine is produced.

Kidney failure

If the kidneys fail, toxic urea builds up in the blood, and the water and ion balance is disturbed, damaging cells by osmosis. There are two treatments.

• Dialysis: the patient's blood flows past a partially permeable membrane, on the other side of which is dialysis fluid containing the same concentration of glucose and ions as healthy blood, but no urea. Urea diffuses out of the blood into the fluid, while glucose and ions are not lost because there is no concentration gradient for them.
• Kidney transplant: a healthy kidney from a donor is surgically fitted. It is a long-term solution, but the kidney may be rejected (so immunosuppressant drugs are needed) and a donor may not be available.

How this fits together

Water and nitrogen balance completes the homeostasis half of Topic 5. The topic then turns to hormones in reproduction, starting with the hormones in reproduction guide.