We're all familiar with the feeling of thirst. When we feel this need, a sip of water can seem especially delicious and satisfying. Recent research by scientists has revealed why this happens, explaining how our bodies respond to the need for fluids.
It's no secret that water makes up over 60% of an adult's body weight, making it literally essential for survival. But why does it taste so good to us when we're thirsty and take that first sip on the hottest day? Scientists think they know the answer, writes Live Science.
According to Patricia DiLorenzo, professor emeritus of psychology at Binghamton University in New York, people feel thirsty when they exercise vigorously or on a particularly hot day. This is because when we sweat, our blood volume decreases.
Most parts of the brain are separated by the blood-brain barrier, a layer of cells that keeps harmful toxins and pathogens from entering the brain. However, some parts of the human brain extend beyond this barrier, allowing the body to quickly detect changes in our blood. When we lose blood volume through exercise, heat, or salty foods, neurons in these parts of the brain send a signal to trigger the sensation of thirst.
According to Yuki Oka, a professor of biology at the California Institute of Technology, this rapid response is actually important for survival, and if it took longer, we would simply feel dehydrated.
Previous studies have already shown that three areas of our brain are responsible for processing thirst:
- subfornical organ (SFO);
- organum vasculosum terminal plate (OVLT);
- medial preoptic nucleus (MnPO).
The SFO and OVLT are located outside the blood-brain barrier. Back in 2018, scientists conducted a study in mice and found that all three areas have neurons that control thirst, but when the nerve cells are excited, only the MnPO is in the middle of this process. It is this area that transmits thirst signals to the other two areas, as well as other parts of the brain, which prompts us to seek water.
After we take a sip, water is absorbed and circulated through our bodies. However, our bodies start sending signals to our brains that we are hydrated long before we are fully hydrated. In fact, after the first sip, the human brain releases a flood of the neurotransmitter dopamine, and most scientists agree that dopamine is involved in reward seeking, movement, and motivation. In addition, it is dopamine that motivates animals, including humans, to expend energy on activities that give us rewards or help us stay alive—this includes consuming food and water.
According to Oka, if dopamine is released during a certain action, people and animals tend to repeat those actions, receiving a “positive” signal from the brain. Unfortunately, scientists still don’t know exactly how drinking water causes dopamine to be released, but a 2019 study showed that mice that felt thirsty and drank water produced dopamine. At the same time, mice that felt thirsty and received water directly into their intestines did not produce dopamine. This indicates that the process of drinking itself plays an important role in dopamine production.
The researchers also found that the act of swallowing itself also sends a message to neurons in the MnPO that the body is getting water. The MnPO then deactivates thirst neurons in the SFO, creating a feeling of fullness. However, swallowing is not the only process that helps quench thirst.
Once water enters the intestines, the body detects a drop in the salt-to-water ratio in the blood. This leads to an increase in a hormone that helps activate neurons that signal the brain that the body's demand has been met. However, how exactly this works, unfortunately, also remains a mystery to scientists.
