Eli5 what exactly causes the sensation of feeling thirsty when we are dehydrated?
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When you’re dehydrated, you have less water in your blood so the blood becomes more concentrated and blood volume decreases. Your kidney is used to filtering all the blood, so it can feel when you have less blood to filter, and it releases hormones that help increase your blood pressure and decrease the amount of water in your pee. A special part of your brain samples your blood and can sense both the kidney hormones and the increased blood concentration. This brain part sends signals to the area responsible for thirst, turning the thirsty feeling on.
The body has specialized cells (called “osmoreceptors”) in certain areas that measure the osmolality of blood. (An example is the organum vasculosum of the lamina terminalis, or OVLT, if you’re curious). Losing water increases blood osmolality (usually around 290 mOsm/L), which is sensed by the osmoreceptors. The osmoreceptors in turn activate two mechanisms:
1. They activate the thirst center (in the hypothelamus)
2. They cause the secretion of vasopressin (antidiuretic hormone / ADH) from the pituitary gland, which increases the retention of free water (ie water without solutes) in the kidneys
These mechanisms increase the free water concentration of blood, causing blood osmolality to return to normal.
Please note that different mechanisms exist to address a decrease in blood volume, and if both blood volume decreases and osmolality increases, both categories of mechanisms will activate. There’s considerable overlap between these mechanisms.
The sensation of feeling thirsty is caused by a variety of factors, but one of the main causes is dehydration. When our bodies lose fluids, such as through sweating, urination, or vomiting, we can become dehydrated. As the level of fluids in our bodies decreases, our bodies send signals to our brains to let us know that we need to replenish the fluids.
One of the main ways that our bodies send these signals is through a hormone called vasopressin, also known as antidiuretic hormone (ADH). When our bodies are dehydrated, the level of ADH in our bodies increases. This hormone helps to regulate the amount of water in our bodies by signaling to our kidneys to conserve water and reduce the amount of urine produced. As a result, we may feel thirsty as our bodies try to conserve water and maintain proper fluid balance.
In addition to the release of ADH, other factors that can contribute to feeling thirsty include an increase in body temperature, an increase in the concentration of electrolytes in our bodies, and certain medications or medical conditions.
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