Blood circulates throughout the entire human body and is pumped by the heart to flow thousands of miles through blood vessels. The blood is dependent on the circulatory system to manage wastes by using the excretory system, and to carry nutrients, water and oxygen to the cells.
Although the human body starts off with around a gallon of blood circulating through the veins, it is reduced to about 5 quarts of blood by the time the body reaches adulthood. The blood is typically composed of red blood cells, white blood cells, platelets, and plasma. The body produces red blood cells, white blood cells and platelets inside of the bone marrow.
Red blood cells are responsible for carrying oxygen to the cells and removing carbon dioxide from the body. White blood cells allow the body to fight invading pathogens, viruses, bacteria, and germs, which may otherwise cause harm to the body. Platelets allow for scabs to coagulate and patch up bleeding wounds to stop injury. Plasma accounts for the liquid portion of the blood, located outside of the bone marrow.