Osmoreceptors are cells which are sensitized to osmotic pressure. Osmotic pressure changes with the concentration of solutes in the body. Using osmoreceptors, the body regulates its fluid balance to keep the balance of fluids stable and safe so that the body's numerous interconnected systems will retain functionality. Even with these receptors active, it is still possible for fluid levels to become imbalanced as a result of not intaking enough fluid, intaking too much fluid, or intaking imbalanced amounts of salts which cause variations in osmotic pressure.
Many of the key functions of the body revolve around osmotic pressure. The semipermeable membranes of cells are designed to permit osmosis so that cells can receive nutrients and express wastes, for example. The body regulates osmotic pressure through the diet and also through the release or retention of fluids to ensure that solutes in the body remain consistent. Imbalances in osmotic pressure can lead to severe health complications including death.
When osmotic pressure changes, osmoreceptors expand and contract in response. When there are low levels of solutes in the blood, these sensory receptors swell, because water moves from the blood into the cells. When the blood has a high concentration of solutes, water moves out of the osmoreceptors and into the blood. Some solutes are carried across the membrane in both cases because they travel with the water.
One area in which osmoreceptors are found is in the brain, specifically inside the hypothalamus. They regulate the amount of vasopressin, a hormone which tells the kidneys to retain water, which is expressed by the hypothalamus. Osmoreceptors can also be found in the liver and kidneys, where they play similar roles in regulating the release of chemicals which are used to regulate fluid balances in the body. These cells essentially act like little sensors to alert the body to imbalances in fluid levels so that action can be taken.
When the level of solutes in fluids like the blood is becoming too high, the body starts to retain water in an attempt to regulate osmotic pressure. People may also experience thirst because the body wants to encourage water consumption. People can develop imbalances in their fluid levels as a result of dietary habits, kidney failure, disease processes in the body, exercise, and other events. It is important to stay adequately hydrated to give the body plenty of water to work with when it comes to regulating fluid levels. People in kidney failure may be encouraged to eat a special diet which will support proper kidney function.