Thirst is the craving for fluids, which results in the basic instinct of animals to drink. Drinking is an essential mechanism that is involved in fluid balance. Thirst comes from the lack of fluids or an increase in the concentration of certain osmolytes, like salt (sodium, potassium, and chloride). Dehydration can cause many problems and is associated with disorders including but not limited to renal (kidney) problems and neurological problems, such as blurry vision and seizures. Excessive thirst, or polydipsia, could be an indication of diabetes. Receptors in the body are able to detect a decreased volume of osmolytes or an increased concentration of osmolytes. The central nervous system is then signaled to identify this change in the body, resulting in producing the desire to drink to remedy the problem.
There are multiple detection methods of thirst including but not limited to (1) decreased blood volume, (2) the renin-angiotensin system, and (3) arterial baroreceptors. Hypovolemia (decreased blood volume) results from excessive blood loss, vomiting, and/or diarrhea. If the blood volume falls too low, then the heart will not be able to pump and circulate blood effectively. This will eventually result in heart failure. Loss of blood volume is detected by the kidneys and triggers the renin-angiotensin system. This system secretes an enzyme called renin into the blood stream when the kidneys detect low blood volume. Renin enters the blood and catalyzes angiotensinogen to angiotensin I, which is almost immediately converted to angiotensin II. Traveling through the blood until it reaches the posterior pituitary gland and the adrenal cortex, angiotensin II causes a cascade of hormones, causing the kidneys to retain more water and sodium to increase blood pressure and therefore blood volume.
The major cause of thirst is dehydration. When your body does not have enough water to carry out normal tasks, and thirst is the main symptom, you are already dehydrated. Dehydration can be caused by a number of factors including but not limited to exercise, diarrhea, vomiting, and too much sweating. Signs of dehydration include dry mouth, thirst, dry skin, and headaches.
Another cause of thirst is diabetes. Diabetes is a disease in which your body does not make enough insulin or is not able to use insulin properly. This results in increased sugar in the bloodstream. The kidneys excrete some of the excess sugar into the bladder for release with urination. However, due to the increased concentration of sugar in the bladder, more water is drawn in to dilute the sugar. Thus, persons with diabetes urinate more often, resulting in losing more water.
Thirst will cause dry mouth, a condition called xerostomia. With dry mouth, the salivatory glands are not able to produce as much saliva and therefore a person will feel thirsty all the time. Severe decreases in salivation can result in tooth decay and gum disease. Anemia is another condition in which your body does not have enough healthy red blood cells. Severe anemia can result in thirst.
The first and foremost treatment for thirst is to drink more water. In the past, it was suggested that each person drink eight cups or 64 ounces of water daily. However, new research shows that to maintain hydration, a person should drink half of his or her weight in ounces of water. For example, a 150-pound person should drink at least 75 ounces of water each day. If that person is also exercising, then he or she should drink even more water each day. If drinking more water does not satisfy the craving of thirst, then the next step is to seek medical care. There could be an underlying condition or a side effect of medication causing thirst.
See also: Baroreceptors; Hunger; Visceral Sensation
Institute of Medicine. (2005). Dietary reference intakes for water, potassium, sodium, chloride, and sulfate. Washington, DC: National Academies Press. http://dx.doi.org/10.17226/10925.
Mayo Clinic. (2015). Disease and conditions: Dry mouth. Retrieved from http://www.mayoclinic.org/diseases-conditions/dry-mouth/basics/complications/con-20035499
Stanhewicz, Anna E., & W. Larry Kenney. (2015). Determinants of water and sodium intake and output. Nutrition reviews, 73(2), 73—82. http://dx.doi.org/10.1093/nutrit/nuv033