Each of us is familiar with the feeling of thirst. When we feel this need, a sip of water can seem especially delicious and exhausting. Recent research by scientists has revealed why this happens, explaining how our body reacts to the need for fluid.
It's no secret that water makes up more than 60% of the body weight of adults, and therefore it is literally necessary for a person to survive. But why does it seem so delicious to us when we feel thirsty and take the first sip on the hottest day? Scientists think they know the answer, writes Live Science.
According to Patricia Di Lorenzo, 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 areas of the brain are separated by the blood-brain barrier, a layer of cells that prevents harmful toxins and pathogens from infecting the brain. However, some parts of the human brain go beyond this barrier, which allows 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 quick reaction 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);
- median preoptic nucleus (MnPO).
The SFO and OVLT are located outside the blood-brain barrier. Back in 2018, scientists conducted a study on mice and found that all three areas have neurons that control thirst, but only the MnPO is in the middle of this process when the nerve cells are excited. It is this area that transmits thirst signals to two other areas, as well as other parts of the brain, which prompts us to seek water.
After we take a sip, the water is absorbed and circulates in our body. However, our body starts sending signals to our brain to tell us that we are getting water 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. Furthermore, it is dopamine that motivates animals, including humans, to expend energy on activities that give us rewards or help us stay alive, including eating and drinking 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 found that mice that felt thirsty and drank water produced dopamine. At the same time, mice that felt thirsty and received water directly in the intestine did not produce dopamine. This indicates that the process of drinking itself plays an important role in the production of dopamine.
The researchers also found that the act of swallowing also sends a message to neurons in the MnPO that the body is getting water. MnPO then deactivates thirst neurons in the SFO, creating a sensation of satiety. However, swallowing is not the only process that helps quench thirst.
After water enters the intestine, the body detects a drop in the ratio of salt and water in the blood. This leads to an increase in the hormone, which helps to activate the neurons that send a signal to the brain that the body's request has been satisfied. However, how exactly this works, unfortunately, also remains a mystery to scientists.
