Hypothalamus
The hypothalamus is a tiny part of the brain of vertebrate animals; in humans it weighs about four grams in a brain that weighs on average 1,400 grams (49 ounces). Despite its small size, the hypothalamus plays a pivotal role in an astounding number of functional and behavioral activities that are essential for day-to-day survival of the individual animal (or person) and for continuing survival of its species. Its overall role is to collect and integrate a huge variety of information from the body and to organize neural and endocrine responses that maintain homeostasis (constant internal environment).
Carrying out this single overriding task requires coordinating the activity of the autonomic nervous system and the endocrine system, and ultimately influences several important behaviors. Thus energy metabolism is regulated by control of feeding, drinking, and digestion. Body temperature is monitored and maintained at a constant level (37 to 38°C [98.6 to 100.4°F] in humans) by a complex interplay of behavior and activity in several body systems, and reproductive behavior is coordinated with endocrine regulation of the reproductive organs. Blood pressure and composition of the blood plasma are regulated by hypothalamic mechanisms. The expression of emotions such as fear, rage, and anger are partly controlled by the hypothalamus, and it even helps regulate sleep and levels of consciousness.