The tendency to maintain balance is called homeostasis. The concept of homeostasis was developed in 1932 by American physiologist Walter Cannon at Harvard Medical School. He used the term homeostasis---which is Greek for "similar and standing still"---to describe the body's need to maintain a relatively constant internal environment.
Homeostasis refers to the internal balance the body must maintain to ensure health. Every cell, organ and system in the body relies on a stable environment to function. Fluids and biochemicals must be supplied in specific amounts and body temperature must be kept within a limited range. Stress is anything that creates an imbalance. Whether it's a hot day, emotional distress, an injury or tumor, our ongoing health depends on the ability of internal systems to quickly adjust to imbalances and reset homeostasis.
The body uses different processes to maintain homeostasis. Receptors throughout the body sense changes in the internal and external environment and send messages to the brain; it responds by telling the appropriate organs to restore equilibrium. Hormones are often used to signal the changes that must be made to restore balance, but the body also uses other mechanisms. For example, receptors in the skin sense when temperature rises and signals the hypothalamus, which sends impulses to glands that release more perspiration to cool the skin and reduce body temperature.
Feedback systems inside the body monitor the internal environment and send information to immediately make changes to ensure homeostasis. Envision a "loop" of information, in which an imbalance causes a response to correct the imbalance, the resulting change is sensed by the body and that in turn causes another response. In that way, information is continuously looped and adjustments are constantly made. When any feedback mechanism does not work properly, or when the systems are overwhelmed by constant stress, the imbalance causes illness or disease, such as dehydration and diabetes.
Every system in the body contributes to homeostasis. During exercise, the muscles need more nutrients and oxygen, so nerve cells in the muscles signal the brain to increase the heart rate and lung contractions. When blood pressure goes up, pressure-sensitive nerve cells in the vessel walls signal the brain to adjust blood volume and heart rate. The control of blood sugar is a great illustration of homeostasis. The amount of glucose (sugar) must be maintained within strict limits in the blood. When you eat candy, blood sugar goes up (a stress), the pancreas senses the change and immediately secretes insulin to remove the excess sugar. As the sugar level drops, the pancreas receives negative feedback and secretes glucagon to increase blood sugar. The continuous loop through the pancreas ensures the correct balance of blood sugar.
Homeostasis of food intake is primarily achieved by the hormone leptin. As you eat, adipose tissue releases leptin and the brain responds with signals that say you're full. When leptin levels go down, the brain signals that you're hungry. This system becomes imbalanced when you gain too much weight because leptin is produced by the fat cells, and when extra fat produces a lot of leptin for an extended period of time, the body learns to ignore leptin's signals. Leptin can no longer play its role in homeostasis, and losing weight becomes increasingly difficult.