Osmoregulation is the control of the water potential of the body fluids, by regulating the amount of water and the concentration of solutes (such as sodium and chloride ions) in the blood and tissue fluid. It is one of the main examples of homeostasis.Keeping the water potential of the blood and tissue fluid constant matters because water moves by osmosis from a higher (less negative) water potential to a lower (more negative) water potential across partially permeable cell membranes.• If tissue fluid has a lower water potential than the cytoplasm, water leaves the cells by osmosis and they shrink (animal cells become crenated). Metabolic reactions are disrupted.
• If tissue fluid has a higher water potential than the cytoplasm, water enters the cells and they swell. Animal cells have no cell wall, so they may burst (lyse).
The water content of the blood also affects blood volume, and so blood pressure.Water balance depends on gains and losses. Mammals gain water from drink, food and metabolic water produced by respiration. They lose water in urine, faeces, sweat and by evaporation from the lungs in exhaled air.Worked example: a person gains 1500 cm3 of water from drink, 750 cm3 from food and 250 cm3 of metabolic water per day, 2500 cm3 in total. They lose 400 cm3 in exhaled air, 500 cm3 in sweat and 100 cm3 in faeces. To stay in balance, their urine volume must be 2500 − 1000 = 1500 cm3 per day.Most of these losses cannot be controlled, so the body regulates water balance mainly by altering the volume and concentration of urine.
• If tissue fluid has a higher water potential than the cytoplasm, water enters the cells and they swell. Animal cells have no cell wall, so they may burst (lyse).
The water content of the blood also affects blood volume, and so blood pressure.Water balance depends on gains and losses. Mammals gain water from drink, food and metabolic water produced by respiration. They lose water in urine, faeces, sweat and by evaporation from the lungs in exhaled air.Worked example: a person gains 1500 cm3 of water from drink, 750 cm3 from food and 250 cm3 of metabolic water per day, 2500 cm3 in total. They lose 400 cm3 in exhaled air, 500 cm3 in sweat and 100 cm3 in faeces. To stay in balance, their urine volume must be 2500 − 1000 = 1500 cm3 per day.Most of these losses cannot be controlled, so the body regulates water balance mainly by altering the volume and concentration of urine.