Maintaining water and nitrogen balance

Biology · Homeostasis and response. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Body cells work properly only if the water and ion content of the blood is right. If it is not, cells gain or lose too much water by osmosis.

Water leaves the lungs in the air we breathe out. Water, ions and urea leave the skin in sweat. The body cannot control these losses. The kidneys do the controlling: they remove excess water, excess ions and urea in the urine.

The kidneys first filter the blood and then take back what the body needs. This is selective reabsorption. The hormone ADH decides how much water is taken back. If the kidneys fail, a person needs dialysis or a transplant.

Rules to remember

Filtration: water, glucose, ions and urea pass out of the blood. Proteins are too large to be filtered.
Selective reabsorption: all of the glucose, some ions and as much water as the body needs go back into the blood. Urine is what is left.
Excess amino acids are deaminated in the liver → ammonia (toxic) → converted at once to urea, which is safer to carry in the blood.
Blood too concentrated → pituitary gland releases more ADH → kidney tubules more permeable → more water reabsorbed → small volume of concentrated urine.
Blood too dilute → less ADH → less water reabsorbed → large volume of dilute urine. This is negative feedback.
Dialysis fluid has the same glucose and ion concentrations as healthy blood and no urea, so only urea and any excess ions diffuse out of the blood.
Transplant: no regular dialysis sessions, but donors are few, the kidney may be rejected and immunosuppressant drugs are needed.

Common mistake

Thinking that more ADH means more urine. More ADH means more water is reabsorbed into the blood, so there is less urine and it is more concentrated.

Worked example

In one day a person's kidneys filter 180 dm³ of fluid from the blood and produce 1.5 dm³ of urine. What percentage of the filtered fluid is reabsorbed?

Volume reabsorbed = 180 − 1.5 = 178.5 dm³.
Percentage reabsorbed = (178.5 ÷ 180) × 100.
178.5 ÷ 180 = 0.9917, and 0.9917 × 100 = 99.2 (to 3 significant figures).

Answer: About 99.2% of the filtered fluid is reabsorbed into the blood.

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