This week, the I Spy Physiology blog answers a reader question: Why do we get dizzy when getting out of a hot tub?
There may not be a better way to chase away the winter “blahs” than soaking in the hot tub or standing under a steaming shower. However, sometimes, after a lengthy soak or steam you may feel lightheaded when you stand up. You may start to feel woozy and your balance may waver. You may even see stars for a moment or faint. Why does this happen?
When body temperature rises due to hot water, hot weather or fever, the body activates mechanisms to cool down toward normal temperature (97–99°F). The blood coursing through the body must rise to the surface of the skin, releasing heat, to reduce core temperature. This happens through a process called vasodilation. During vasodilation, blood vessels carrying blood away from the heart (arteries) open up to allow a larger volume of blood to flow through them. Blood flowing through the arteries near the skin’s surface releases heat into the environment and cools the body. However, when many arteries open up at the same time (while a person is standing up), gravity pulls the blood into the legs and away from the brain and heart. Pooling of blood in the legs can cause dizziness or lightheadedness because the brain and heart aren’t getting enough blood.
Lightheadedness and even fainting are the body’s way of “fixing” the lack of blood in the brain and heart. Once the brain, heart and legs are at the same level (when a person is lying down), blood flows into each organ more easily because gravity no longer pulls so much blood into the legs. Flexing the muscles is another way to return more blood to the brain and heart. Muscle contraction works against gravity and forces blood back to the heart and ultimately the brain. This provides the brain with enough blood volume to eliminate wooziness.
The next time you are enjoying a dip in a hot tub or a steaming shower, pause, flex your muscles and steady yourself before stepping out.
Jessica C. Taylor, PhD, is the Senior Manager of Higher Education Programs at the American Physiological Society. She is a cardiovascular physiologist, exercise enthusiast and firm believer in warming up in hot water.