Thirst and dehydration: physiological mechanisms and clinical implications

Thirst and Hydration in the Human Body

It is a fairly well-known fact the water is the most common molecule in the human body, and one of the most essential to sustaining life in almost all (if not absolutely all) organisms. This is the underlying cause for the feeling of thirst; when levels of water in the body fall too low, the body and brain create a thirst response that encourages the dehydrated individual to drink -- that is, to take in fluids and correct the hydration issue. But as simple as this might seem on the surface, the physical process that occurs in the body is somewhat more complex.

The most essential organs and glands in the regulation of thirst homeostasis are the kidneys and the hypothalamus. Dehydration can occur for many different reasons, but the overall effect is generally the same -- there can be a decrease of blood volume and/or blood plasma in extreme cases, but no matter what the salt concentration of in blood goes up as the water level decreases. Certain receptors in the hypothalamus known as osmoreceptors detect this rise in salt concentration, and cause the hypothalamus to release antidiuretic hormone, also known as ADH, which travels to the kidneys and causes a reduction in the production of urine. When this situation continues, the kidneys themselves release the hormone restin, which in turn creates angiotensin in the blood, causing further retention of fluids by the kidney.

These processes simply stop the body from eliminating fluids, however, or at least slows the kidneys' release of fluids through the production of urine. It is, of course, impossible for the body to create more fluids itself, but other mechanisms create the thirst impulse in the brain that an individual consciously feels and -- hopefully -- responds to by ingesting fluids. The osmoreceptors in the hypothalamus are the origins for this brain impulse.

From Water to Urine

Though the skin and even the lungs can also absorb water into the body, the bulk of our fluid intake occurs (rather obviously) in the mouth. Likewise, the skin and lungs represent two of the ways that water is eliminated from the body (as sweat and vapor, respectively), but the bulk of fluid elimination takes place from the urethra in the form of urine. How it gets there is a fairly straightforward process, but it does require some basic explanation.

After being absorbed into the bloodstream by the intestines, water travels through the bloodstream acting largely as a transport agent, carrying nutrients and other things in the blood to cells, and carrying wastes away. Individual water molecules also play an important role in the Krebbs cycle and glycolysis, but the bulk of the water we use each day is needed for transport within the bloodstream. Water is especially important for the removal of wastes; as certain byproducts and toxins build up in the cells of an individual, they need to be eliminated, and the way that the body does the bulk of this elimination is through the process of urination.