Physiology is the study of how living things work. Physiologists can study how organs of an organism work together to make things happen. In human beings, for example, the digestion of food hormones and other chemicals are made by the stomach, liver, and pancreas. Muscle contraction happens because of chemical messages made by nerves of that muscle. By learning how the body functions normally, physiologists and physicians can better understand what happens when organs do not function normally. For example, an understanding of how the thyroid gland functions has helped in treating goitre. Studies of the circulatory system and the nervous system have helped physicians understand and treat such illnesses like heart disease, stroke, and high blood pressure.