Blood transports many different molecules and cells throughout the body. Since many of the body's processes use oxygen as fuel, blood must be able to transport oxygen and carbon dioxide to and from the lungs.
Oxygen enters the body through the lungs, where it attaches to hemoglobin proteins. Hemoglobin in blood cells is the main method of oxygen transportation, flowing from the heart in blood through the lungs and to the rest of the body by arteries. When oxygen is removed from the blood by body processes, the blood cells are considered deoxygenated, and they flow back through the heart by veins. While passing through the body, blood picks up carbon dioxide as waste, which attaches to blood plasma as bicarbonate ions. At the end of the cycle, carbon dioxide passes through the lungs and out of the mouth and nose, while new oxygen in air -- which is eighty percent inert nitrogen and other molecules -- is newly absorbed. The inert molecules exit the body with the waste carbon dioxide.