Hypothalamus, hy puh THAL uh muhs, is a small area at the base of the brain. The hypothalamus makes up less than 17 percent of the brain's total volume, but it plays a key role in regulating the body's general level of activity. The hypothalamus helps control the autonomic nervous system, part of the nervous system that regulates such automatic body processes as breathing, blood pressure, and heart rate. The hypothalamus also controls the pituitary gland, the so-called "master gland" of the endocrine (hormone-producing) system.