This is the regulation of the amount of water present in the human body. The human body is approximately 70% water, as such, a certain level of water must be maintained. If the body fluids become too diluted, water will enter cells by osmosis causing them to swell or even burst. If the body fluids become too concentrated, then water is drawn from the cell. This is similar to what happens when animal cells are placed in hypotonic and hypertonic solutions.
In mammals the kidney ensures that the correct balance is maintained by regulating the amount of water and salt re-absorbed into the blood from the fluid in the proximal and distal convoluted tubules. This process is controlled by the hormone ADH (anti-diuretic hormone). ADH affects the permeability of the walls of the kidney tubules allowing more water to be re-absorbed from the tubules into the blood.
The Hypothalamus controls the secretion of ADH. The Hypothalamus has cells sensitive to blood concentration and will send a signal to the pituitary gland for the secretion of ADH depending on the concentration of the blood. For example, if the blood is too concentrated (very little water), the pituitary gland releases ADH. ADH increases the permeability of the walls of the kidney tubules, resulting in a large amount of water being reabsorbed into circulatory system, reducing the concentration of the blood (or increase the water concentration of blood). As a result less water will be lost through urine. The opposite is true if the blood is too dilute; no ADH will be released from the pituitary and no water will be re-absorbed from the kidney tubules.