Blood consists of two basic components: the cellular components or the red and white blood cells. They are made in the bone marrow(a tissue in the central cavity inside almost all of the bones in the body) and some white cells in the lymph nodes. The bone marrow releases about 10-15 million red blood cells per second. In children, the marrow of most of the bones produces blood. But in adults, only the marrow of certain bones -- the spine, ribs, pelvis, and some others -- continues to make blood. The other component is the watery part is the blood plasma and contains all the hormones, the water, the sugars, dissolved blood gasses, plasma proteins made in the bone marrow and liver and a host of other substances.
Red blood cells carry oxygen from the lungs to the tissues and carbon dioxide from the tissue to the lungs. Blood plasma transports antibodies, which protect the body from infection, carries food substances absorbed from the intestines and takes waste products to the kidneys for excretion. It also transports hormones, secreted by endocrine glands, to their sites of action.