Water makes up about 70% of body weight in a typical adult. Humans can’t easily adapt to a lack of water, and you may begin to experience the effects of dehydration with as little water loss as 1% of your body weight. Humans are unique because, unlike other animals, we respond not only to physiological signs (such as a dry mouth), but also to multiple non-physiological cues and social rituals that affect our drinking habits.
You lose the majority of your fluids through urine. Your kidneys regulate fluid balance through a process of filtration, secretion of hormones and reabsorption of water, salt and other chemicals. Sweating is another source of fluid and electrolyte loss that largely depends on the climate you live in and your activity levels. Physiological thirst is a sensation that helps you maintain fluid volume and the right amount of electrolytes and sugar in your body.
Scientists recognize two types of thirst: hypovolemic (not having enough water and minerals in your body) and osmotic (not having enough water).
Hypovolemic thirst may result from excessive sweating, bleeding or vomiting. If you go for a long run on a hot day, it’s important to replenish both your water and electrolyte loss. Staying hydrated will help you avoid fainting due to a low blood volume and a resulting drop in blood pressure. The dramatic decrease in blood volume that occurs during dehydration makes your heart unable to pump blood to your organs, most importantly the brain.
Osmotic thirst has to do with what you eat. Salty foods lead to an increased blood osmolarity, which means you don’t have as much water as other substances in your blood. Your body senses this and produces an osmotic thirst. Ever get really thirsty after eating salty food? Your body responds to this kind of thirst by decreasing how much saliva you produce. It also releases more of the antidiuretic hormone (ADH) from the pituitary gland in your brain. That helps your body reabsorb water while also getting rid of the excess sodium through urine.
Your sympathetic nervous system helps raise your blood pressure when you’re dehydrated by constricting blood vessels and decreasing the kidneys’ filtration rate. It also activates a hormonal pathway called the renin-angiotensin-aldosterone system. Angiotensin is a hormone that causes your blood vessels to constrict, stimulates thirst, causes release of ADH and makes the adrenal glands that sit above the kidneys secrete aldosterone. Aldosterone is a hormone that helps restore blood volume by reabsorbing sodium and water in the kidneys so you don’t lose too much when you urinate.
If just reading this blog post made you reach for a glass of your favorite non-alcoholic beverage, listen to your body and take a sip. Your body’s thirsty!
Natalya Zinkevich, PhD, teaches anatomy and physiology courses at Carroll University in Waukesha, Wisconsin. She studies vascular biology with a focus on human health and disease at the Cardiovascular Center of the Medical College of Wisconsin.