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.
Plasma is about 90% water, with the remaining 10% being composed of dissolved substances, some of which help with the formation of blood clots that seal wounds.
Blood characteristics
Specific proteins, known as antigens, on the surface of the red blood cells and in the plasma differentiate the types of blood. There are two main categories of the special proteins, known as Type A and Type B. Thus, individuals fall into one of four main blood type groups based on combinations of these special proteins. The table illustrates the special protein composition of each blood type. Another feature of blood is the Rh factor. The Rh special protein is either present or absent in blood, leading to the labels of Rh positive (protein present) and Rh negative (protein absent). When a pregnant woman and her fetus are of different Rh types, problems in fetal development may occur. The Rh factor is also matched when selecting blood for transfusions.
Blood is collected and stored according to type and Rh factor for use in transfusions. The blood categorized as Type O (neither Type A nor B proteins present) and Rh negative is generally considered to be a universal donor, that is, blood of this type may be accepted by anyone, no matter what their blood type. Because of increased concern over the possibility of contracting a disease through a blood transfusion, individuals may elect to donate their own blood weeks before an elective surgical procedure to ensure compatibility and freedom from new diseases. In many hospital settings, donors may give blood designated for use by a specific patient.
Blood disorders
There are a number of disorders that affect the health and functioning of the blood. Causes of blood disorders range from genetic abnormalities (as in sickle-cell anemia) to environmental factors (as in hemorrhage associated with vitamin-C deficiency). Among the most common blood disorders are anemia, a condition in which there are too few red blood cells; polycythemia, the counterpart to anemia, where there are too many red blood cells; leukemia, a type of cancer in which the blood has too many white blood cells; and thrombosis, a condition where the blood clots too readily. Blood poisoning is a layperson's term for any infection (from mild to life-threatening) affecting the blood or its components. For example, when bacteria invade the blood (often through an injury) and multiply, the condition is known as septicemia. When excessive amounts of toxins are produced, the condition is called toxemia.
Books
Lauer, Ronald M, Richard B. Shekelle, eds. Childhood Prevention of Atherosclerosis and Hypertension. New York: Raven Press, 1980.
Audiovisual Recordings
Digestion: Blood and Circulation. Elk Grove, IL: Disney Educational Production, 1995. (One 52-minute videotape. Twenty-six minutes are devoted to blood and circulation, with experiments and activities.)
Hemo the Magnificent. Santa Monica, CA: Rhino Home Video, 1991. (One 54-minute videotape featuring live and animated characters to tell the story of blood and circulation.)
Organizations
National Heart, Lung, and Blood Institute Information Center Address: P.O. Box 30105
Bethesda, MD 20874-0105 Telephone: (301) 251-1222 Website:www.nhlbi.nih.gov/nhlbi/nhlbi.html
