Osmoregulation is the process of regulating osmotic pressure (a measurement directly related to the concentration of a solution) of body fluids by controlling the transport of salt and water across cellular membranes. Osmotic pressure is the minimum amount of pressure applied to a solution that would prevent pure water from entering the solution through a semipermeable membrane. For example, if two compartments of water, Fluid A being pure water and Fluid B being a solution (fluid containing dissolved substances called solutes), were separated by a membrane that is permeable to water but not solutes, water from Fluid A will have a net movement toward Fluid B as water moves down its gradient (from an area with more water in a given volume to an area with less water in a given volume). If a physical force is applied to Fluid B (the solution), a pressure called hydrostatic pressure is generated against the membrane. The more solutes that are in Fluid B, the higher the osmotic pressure. If Fluid B were also pure water, it would have an osmotic pressure of zero because there would be no net movement between the two compartments.
Osmotic pressure is influenced by the concentration of solutes, such as sodium ions, in body fluids. A higher osmotic pressure involves a higher concentration of solutes. Passive transport involves solutes moving from an area of higher concentration to an area of lower concentration. Active transport involves particles moving from an area of lower concentration to an area of higher concentration and requires energy. Water always passes through membranes by passive transport, specifically osmosis. Water will have a net movement to a compartment with a higher osmotic pressure until equilibrium is reached. A lower osmotic pressure, or lower solute concentration, will tend to lose water to equalize the concentration on both sides of the semipermeable membrane.
If the concentration of the extracellular fluid increases, osmoreceptors in the region of the brain called the hypothalamus detect the change and trigger the thirst mechanism. This mechanism is also triggered by a dry mouth or by a detection of low blood volume by baroreceptors in the atria and blood vessels. Antidiuretic hormone (ADH) is stimulated by low blood volume and high osmotic pressure. It is produced by the hypothalamus and released from the posterior pituitary gland. ADH acts directly on the kidney, affecting the production of urine and elimination of fluids. When ADH levels are high, aquaporins (water channels) are inserted into the collecting duct, allowing water to pass through the membrane. As the urine travels down the collecting duct, water is reabsorbed as it continuously encounters a higher osmotic pressure. This is because the collecting tubule runs through the renal medulla, whose extracellular fluids become more and more concentrated as a result of the concentration gradient set up by the nephron loop. This allows the kidney to produce hypertonic urine. This means the urine is at a higher concentration than the body fluids. Humans can concentrate urine up to 1,200 mOsm (milliosmoles per liter) (blood is at ~300 mOsm). When an individual has a higher water intake, aquaporins are removed from the collecting duct, and the collecting duct remains impermeable to water. As the urine passes through, water remains in the collecting duct, and the final output is copious, hypotonic urine. Hypotonic urine has a lower concentration than the blood. Urine concentration can range between 100 and 1,200 mOsm. When the osmotic concentration of blood decreases, the thirst mechanism turns off, and ADH production is ceased.