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.
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.
