Osmoregulation
Osmoregulation is the control of osmosis. The tissue fluid (highlighted in yellow) surrounding the cells must be isotonic with the cytoplasm of the cells. Blood going in and out of the fluid must be equal so that the cells maintain their size. If the blood circulating into the tissue is too concentrated, causing a hypertonic fluid, too much water will be removed from the cells. In this case, the kidneys will remove less water and more salts to maintain the balance. Too diluted fluid will cause a hypotonic fluid, where too little water will be removed from the cells. In this case, the kidneys will have to excrete more water adn less salts. Both are undesirable. We want the cytoplasm to be isotonic.
This can be controlled by the kidney controlling the composition of the blood circulating through the kidneys. Excess water and salts can be removed and excreted down through the ureter. By controlling the content of water and salts in the blood, the kidney can keep the tissue fluid isotonic, maintaining the function of the cells.
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