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CSEC Biology: The Kidneys and Osmoregulation

Osmoregulation is the process by which the balance of salt and water (osmotic pressure or water potential) is maintained across the fluids of the body. This state of normalcy or balance of blood plasma concentration is known as homeostasis. This is necessary to prevent the unnecessary movement of water into or out of body cells.

If bodily fluids become too dilute, water enters the cells via osmosis, causing the cells to swell. They may burst or lyse as a result.

If bodily fluids become too concentrated, water leaves the cells via osmosis, causing the cells to shrink and become dehydrated. Metabolic reactions cannot occur in the absence of water, so the cells die if too much water leaves.

The body does not exist in isolation, though. There is a constant input of water and electrolytes into the system through food, drink and water produced by cells from respiration. Excess water, electrolytes, and wastes are transported to the kidneys and excreted in the form of urine, to the skin as sweat and in faeces, helping to maintain osmotic balance. Insufficient fluid intake results in fluid conservation by the kidneys.

Without a mechanism to regulate osmotic pressure, or when a disease damages this mechanism, toxic waste and water will accumulate, which can have dire consequences.


The kidneys are responsible for the regulation of the osmotic pressure/concentration of bodily fluids by controlling how much water is reabsorbed into the blood plasma during selective reabsorption. This determi