The circulatory system is the body's transportation superhighway. As the blood circulates through the body, it brings oxygen and nutrients to the body tissues and picks up the waste products, delivering them to the kidneys and lungs to be expelled. The heart is the pump of this closed-circuit system. Three types of blood vessels comprise the circulatory system: arteries, veins and capillaries.
Arteries carry oxygen-rich blood away from the heart to the organs and tissues of the body. The arterial system is under high pressure as it receives blood from the pumping heart. Therefore, the walls of arteries are thick. They contain elastic fibers and muscle cells, which help propel the blood through them. Commonly known arteries of the human body include the aorta, the femoral arteries, the carotid arteries and the coronary arteries.
Capillaries are tiny blood vessels that connect the arterial and venous sides of circulation. The walls of these vessels are only one-cell thick to allow for easy exchange of substances in the tissues. Oxygen is transported out of the capillaries into the tissues; carbon dioxide flows in the other direction, from the tissues into the capillaries. Similarly, nutrients and waste products cross back and forth as needed, across the capillary walls.
Capillaries also have a role in regulating body temperature. When excess heat is present in the body, the capillaries release the heat into the tissues. You have surely noticed how your skin turns pink on a hot summer day or after you have taken a jog--that flush is the result of your capillaries working to rid your body of excess heat.
The venous side of circulation carries deoxygenated blood that has picked up carbon dioxide back toward the heart and lungs. Venous circulation is under relatively low pressure compared to arterial circulation. Therefore, the walls of the veins are much thinner than those of the arteries. Veins have a feature not found in arteries--valves. Keeping in mind that there is not high pressure to force venous blood flow toward the heart, the valves are an aid to help prevent backflow and keep circulation a one-way system. Commonly known large veins of the body include the superior vena cava, the inferior vena cava, the jugular veins and the great saphenous veins of the legs.