The purpose of the cardiovascular system is to transport oxygen to all of the tissues in the body and remove, from these same tissues, metabolic waste products. The system itself consists of:

• Blood -- the medium for exchanging oxygen, nutrients, and waste products throughout the body
• Blood vessels -- the pipes through which the blood flows
• The heart -- the muscular pump that forces oxygen-rich blood through the arteries to all of the organs and tissues in the body

The components of blood

While the heart is the cardiovascular component we think of most often, the blood and blood vessels are also vital to maintaining good health.

The average adult has between 5 to 6 liters of blood or blood volume. The blood carries oxygen and essential nutrients to all of the living cells in the body, and also carries waste products to systems that eliminate them. Most of the blood is made up of a watery, protein-laden fluid called plasma. A little less than half of this blood volume is composed of red and white blood cells, and other solid elements called platelets.

Platelets are responsible for coagulation of blood at the point of an injury to a blood vessel. Without platelets and coagulation factors found in the plasma part of blood, our blood would not be able to clot and hemorrhaging or uncontrolled bleeding would result.

How blood clots

The image below, which illustrates a cut section of a small artery, shows how platelets work to form clots. The "traffic" in the blood includes red blood cells carrying oxygen; platelets; and clotting factors, which are proteins that help the blood to clot. When a blood vessel becomes damaged, as shown here, the blood cells and plasma begin oozing out into the surrounding tissue. This begins the clotting process. Platelets immediately begin to adhere to the cut edges of the artery; they release chemicals to attract even more platelets. Eventually a platelet plug is formed, and the bleeding stops.

Inside, the clotting factors take a much more active role by creating a cascade of clotting activity. The clotting factors cause strands of blood-borne material, called fibrin, to stick together and seal the inside of the wound. Eventually, the cut blood vessel heals, and the blood clot dissolves after several days.

While platelets play an important role in clotting, red blood cells carry on the important job of carrying oxygen and other nutrients to all the tissues of the body and carrying waste products to the organs, which remove them from the body.

How red blood cells carry oxygen

Red blood cells are the oxygen carriers. As they travel away from the heart, they traverse smaller and smaller arteries, finally arriving at the collections of microscopic blood vessels known as capillaries. Here, they exchange oxygen for carbon dioxide, which is removed from the body in the lungs.

The exchange of oxygen between the red blood cells and the surrounding tissues occurs through a process called diffusion. Since capillaries contain a high concentration of oxygen, while the surrounding tissues contain a lower concentration of oxygen, oxygen leaves the capillaries and enters the tissues.

Conversely, body tissues contain high concentrations of carbon dioxide and metabolic waste, while the capillaries contain a lower concentration, the waste products and carbon dioxide diffuse from the tissues into the capillaries and from there are carried by the venous system back toward the heart and lungs.

Blood pressure

Normal blood pressure is important for proper blood flow to the body's organs and tissues. Each heartbeat forces blood to the rest of the body. The force of the blood on the walls of the arteries is called blood pressure. Blood pressure moves from high pressure near the heart to low pressure away from the heart. Blood pressure depends on many factors, including the amount of blood pumped by the heart. The diameter of the arteries through which blood is pumped is also an important factor. Generally, blood pressure is higher when more blood is pumped by the heart, and the diameter of an artery is narrow.

Blood pressure is measured both as the heart contracts, which is called systole, and as it relaxes, which is called diastole. A systolic blood pressure of 120 millimeters of mercury is considered right in the middle of the range of normal blood pressures, as is a diastolic pressure of eighty. In common terms, this normal measurement would be stated as "120 over 80."

Watch an animation about blood pressure. Need Shockwave?