Blood, a liquid made up of plasma and cells, has several functions including controlling body temperature, supplying oxygen and nutrients, removing waste, assisting the immune system and transporting hormones. Blood cells, red and white, are made in bone marrow. Plasma is 90 percent water but contains dissolved substances such as hormones, electrolytes, vitamins and proteins.
Blood can cool the body by carrying warmth away from the internal organs to the capillaries near the surface of the skin. Additionally, it can carry warmth to extremities that have been over-cooled by the environment. Blood is necessary to transport waste products from the organs and tissues where they are produced to the appropriate area for disposal. For example, blood moves carbon dioxide from the tissues and organs to the lungs to be exhaled. Blood supplies oxygen to the brain, muscles, organs and tissues and carries away by-products of metabolism. Blood transports nutrients, such as sugars, fats and proteins, and cell-building blocks, such as amino acids, from the organs where they are created, processed and stored to the organs and tissues that are in need of them. Blood transports white blood cells to sites of infection so they can attack bacteria and viruses. Blood also transports clotting agents to the site of a hemorrhage. Blood allows for hormonal control of organs and other parts of the body by the glands. Without blood, hormonal cycles like ovulation would not occur, because the glands and organs are too far away from each other to communicate.