Heart
The heart is a hollow, muscular organ that maintains blood circulation throughout the body. It lies behind the sternum (breastbone), between the lungs. Its size in most adults approximates that of the clenched fist. A heartbeat is a rhythmic contraction of the heart muscle as it pumps blood. A normal heart usually beats 60 to 90 times per minute when the person is at rest.
 The heart is a hollow, muscular organ that acts as a pump to maintain the circulation of the blood. One-way valves direct the movement of blood through the four chambers of the heart. |
The rejuvenated blood then returns to the left atrium. From there, the blood passes to the left ventricle, which forces the blood away from the heart through the aorta (the main artery of the body, which extends through the chest and abdomen) to the other arteries and thereby to all the tissues of the body.
Blood Vessels
Blood vessels form the network of passageways that transport blood throughout the body. The blood leaves the heart and passes through arteries of progressively smaller diameter. When the blood reaches the smallest blood vessels, called capillaries, the oxygen and nutrients it carries are exchanged for carbon dioxide and other waste products. The "used" blood then continues on its journey back toward the heart through veins of progressively greater diameter.
Blood
Blood is the fluid that courses through the blood vessels of the body. It consists of plasma (a yellowish liquid composed of water, proteins, salts, and other substances) and three formed elements -- red blood cells, white blood cells, and platelets -- that are visible only under a microscope. These elements are manufactured by the bone marrow (the soft tissue in the center of some bones).
Red blood cells carry oxygen from the lungs to the various body tissues. Oxygen travels attached to hemoglobin, a pigmented substance in red blood cells that contains iron. When the amount of hemoglobin in each red blood cell or the total number of red blood cells falls below a certain level, anemia is said to be present. Normally, there are about 25 billion red blood cells in one teaspoon of blood.
The job of the white blood cells is to protect the body from invasive organisms. Whenever the body becomes wounded or infected, white blood cells attack and kill disease-causing agents in the affected area. In addition, certain white blood cells produce antibodies. These substances counteract harmful agents by destroying or inactivating them. Because the body produces more white blood cells than usual in response to infection, an increase in their number signals disease. From 25 million to 45 million white blood cells are normally present in one teaspoon of blood.
Platelets are small, colorless, irregularly shaped bodies numbering approximately 1.5 billion in a single teaspoon of blood. They work to stop excessive bleeding by forming clots.
Antigens and Blood Types
An antigen is a substance that can provoke an immune response from the body. An antigen promotes the manufacture of antibodies that will interact only with that antigen. On the surface of all cells are numerous antigens, which cause production of different antibodies. The major blood groups (types A, B, AB, and O) are differentiated on the basis of the presence of two of those surface antigens: Type A blood has antigen A, type B has antigen B, type AB has both antigen A and antigen B, and type O has neither.
A person's blood type must be identified before a blood transfusion can be given because his or her body may reject transfused blood of the wrong type. For example, if a person who has type A blood were to receive a transfusion of type B blood, the body would not recognize the type B antigens and would treat the type B blood as an invader. The recipient would produce anti-B antibodies and destroy all of the blood cells that exhibit type B antibodies.
