The hypothalamus (a structure in the midbrain) is the control center of all autonomic regulatory activities of the body. The main function of the hypothalamus is homeostasis, or maintaining the body's status quo. It modulates the autonomic nervous system, either directly or indirectly (through pituitary hormones). It also influences growth and basic metabolism, including core temperature, appetite, energy level, and reproductive function, among other things. For more information check out: www.biology.about.com/library/organs/brain/blhypothal.htm

This 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. Certain parts of the hypothalamus regulate body temperature, breathing, sleep, hunger, thirst, urination, sexual drives, and emotions. Other parts produce hormones called releasing hormones or releasing factors. The releasing hormones travel in the bloodstream to the anterior lobe (front part) of the pituitary gland and control secretion of individual pituitary hormones. The pituitary hormones, in turn, affect many of the endocrine glands. These glands secrete hormones that influence growth, sexual development, and the rate the body changes food into energy and living tissue. In addition to the releasing hormones, the hypothalamus produces the hormones vasopressin, oxytocin, and somatostatin. These hormones are stored in, and released by, the posterior lobe (rear part) of the pituitary gland. Vasopressin, which is also known as antidiuretic hormone, reduces the amount of water lost from the body in urine. Oxytocin helps regulate the birth process and milk production in females. Somatostatin slows the secretion of growth hormone. • Richard Restak, M.D., Clinical Professor of Neurology, George Washington University Medical School. Restak, Richard. "Hypothalamus." World Book Online Reference Center. 2005. World Book, Inc. 27 Feb. 2005. <http://www.aolsvc.worldbook.aol.com/wb/Article?id=ar270220>.

From what I've read about the hypothalamus, it is also the decision making center of the brain. It communicates what it needs to keep going such as thirst, hunger, emotional needs etc. All humans have 2 separate brains, the new brain and the old brain. The new brain or cortex is the area of thoughts, decision making and evaluation. The old brain, of which the hypothalamus is a major part, is the seat of instinctual feelings. The hypothalamus works by sending messages to the new brain (cortex), where decisions are made as to what to do about human needs, especially human needs upon which survival depends.