What does it mean to be thirsty?

Because these brain areas are difficult to study — due not only to their location, but also to their composition, with many different cell types and crisscrossed circuitry — it’s only in the last decade or so that neuroscientists have begun to understand how thirst fundamentally works. The body, researchers have found, is filled with sensors that feed clues to the brain about how much water or salt an organism needs to consume. How those sensors work, or what they even are, continues to elude scientists. Their existence offers a tantalizing insight: Water may be fundamental to life, but thirst is an educated guess. Environmental Sensing To understand thirst in mammals, think of it less as the body stating a fact to the brain — “I need water” — and more as the brain monitoring its environment, the body. Like an ecologist sampling a river, the brain examines blood’s chemical composition to learn what the body needs. In nearly all cases, the so-called blood-brain barrier protects the brain from bacteria, viruses or other dangers circulating in the blood. But there are a few exceptions where the brain directly interfaces with blood, including in the circumventricular organs, deep in the brain near the hypothalamus. Like an ecologist bending down to sample a river, the brain examines the chemical composition of the blood to learn what the body needs. Two of these organs — the vascular organ of lamina terminalis (OVLT) and the subfornical organ (SFO) — are sensory organs not unlike a nose or an ear. They act like scientists dipping a bucket into the body’s river of blood to test its health. The brain infers the body’s salt and water needs from that data and funnels the information to neural circuits in even deeper regions, which then can trigger what we experience as thirst — the scratchy throat, dry mouth and foggy brain that accompany desire for water. The blood-testing organs don’t measure water levels but rather the concentration of salt, whose healthy range lies at almost exactly the same concentration as that of the brackish intertidal water in which vertebrates first evolved (which is about one-third as salty as seawater). When the water-salt ratio is too low, we get thirsty. The human body is about 60% water, although that number varies from tissue to tissue (bone is 31%, the brain 73%, the lungs 83%). A change of between 1% and 3% in the blood’s water content, which is normally around 60%, is sufficient for the OVLT and SFO to initiate familiar, unpleasant feelings that motivate a behavior. If salt levels are high, the animal drinks. But there is a disconnect between drinking water and correcting the water-salt balance. It takes 30 to 60 minutes for water to enter the bloodstream once it’s consumed, and the brain cannot wait that long to figure out if the body has the water it needs. It must make a decision more or less immediately; an animal can’t sit around and do nothing but drink water for half an hour. So, the brain guesses. More of those mysterious sensors kick in. One roughly estimates the volume of water passing through the mouth and throat and sends an initial signal to the brain. A second signal comes from the gut — from specific cell types that respond to water, and even to the mechanical stretching of the stomach as it takes water in. Within a minute, these signals reach the brain and block the neurons in the OVLT and SFO that were activated to trigger thirst. The thirst response shuts off; the throat cools, and the mouth becomes moist again.