What does it mean to be thirsty?

Summary

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 a...