The fluid that carries substances such as hormones, oxygen, and glucose to the tissues of the body and carries carbon dioxide away from the tissues as waste.
Blood is the red-colored fluid that flows through the arteries and veins of the body. Vital to the survival of the body, blood travels through the arteries carrying oxygen from the lungs and glucose from the liver to every cell in the organs and tissues of the body. It returns to the lungs via the network of veins, having exchanged oxygen for carbon dioxide, a waste product. Adults have about sixand-one-half pints (four liters) of blood, which is composed of blood cells (representing slightly less than half of the blood's volume) and plasma.
There are three basic types of blood cells.
Red blood cells (RBC) contain hemoglobin which carries the oxygen from the lungs. Hemoglobin, made up of protein and iron, is bright red when carrying oxygen. When the oxygen has been delivered to the cells of the body, hemoglobin loses its red pigmentation and takes on a bluish purple color. It then carries the waste product, carbon dioxide, back to the lungs. Red blood cells outnumber white blood cells by around 500:1.
White blood cells (WBC), also known as leukocytes, are larger but fewer in number than red blood cells. The white blood cells' role is to fight infection or invasion from foreign substances outside the body, such as bacteria or a virus, or a splinter in one's finger.
Platelets or thrombocytes are cell fragments responsible for the blood's ability to clot. When a body tissue is cut or injured, the platelets begin to join together to form a sticky mass to seal the injured area and stop the flow of blood.