Blood is fluid that passes through the heart, arteries, capillaries, and veins, bringing nutrients and oxygen to, and waste products away from, all parts of the body. Blood performs several major functions in the body, including supporting immunological functions, forming blood clots, and regulating body temperature.
During a process called respiration, blood carries oxygen from the lungs to various tissues and organs in the body, and then it carries carbon dioxide waste away from body tissues to the lungs, where it is exhaled. Respiration is an important part of the metabolic process whereby cells break down nutrients such as carbohydrates into glucose, proteins into amino acids, and fats into fatty acids. These nutrients provide energy and build tissues in the body. Blood also transports hormones from endocrine glands to target tissues.
Blood also carries specialized cells and antibodies that detect foreign substances and perform immunological functions to fight infection. These cells can protect the body from bacteria, viruses, and parasites. Additionally, they trigger allergic responses and inflammation in order to fight off invasions by foreign substances. Blood has the ability to form clots through the process of coagulation in response to an injury, in order to protect the body from excessive bleeding.
Finally, the blood can aid in regulating body temperature by dilating the blood vessels when a person is too warm in order to radiate excess heat and constricting to prevent heat loss when the person is too cold. In extreme situations, when a person is ill, the body can initiate an above-average rise in temperature to fight off infections.