5.3.3: Maintaining water and nitrogen balance in the body (biology only)
Not started yet — this one needs some love.
Water leaves the body through the lungs during exhalation.
Water, ions and urea are lost from the skin in sweat, and these losses are not controlled.
Excess water, ions and urea are removed by the kidneys in urine.
If body cells lose or gain too much water by osmosis, they do not function efficiently.
Higher tier: excess amino acids from protein digestion are deaminated in the liver to form ammonia, which is toxic and immediately converted to urea for safe excretion.
Kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water.
Knowledge of kidney structure, nephron structure and other urinary-system parts is not required.
Higher tier: ADH controls water level by changing the permeability of kidney tubules.
More ADH is released by the pituitary gland when the blood is too concentrated, causing more water to be reabsorbed into the blood.
Water balance by ADH is controlled by negative feedback.
Kidney failure may be treated by organ transplant or dialysis; dialysis removes waste substances and excess water and ions from the blood.
Common exam mistakes
Do not say protein is filtered by the kidney; protein molecules are too large to pass through the filter.
Do not say the kidney deaminates protein or stores glucose as glycogen; deamination and glycogen storage are liver functions.
Say reabsorption, not just absorption, when substances move back from kidney tubules into the blood.
More ADH makes kidney tubules more permeable to water, so more water is reabsorbed and urine becomes lower in volume and more concentrated.
Do not mix up water concentration with urea concentration when describing concentrated urine.