Three types of blood vessels form a complex network of tubes
throughout the body. Arteries carry blood away from the heart, and veins carry
it toward the heart. Capillaries are the tiny links between the arteries and
the veins where oxygen and nutrients diffuse to body tissues. The inner layer
of blood vessels is lined with endothelial cells that create a smooth passage
for the transit of blood. This inner layer is surrounded by connective tissue
and smooth muscle that enable the blood vessel to expand or contract. Blood
vessels expand during exercise to meet the increased demand for blood and to
cool the body. Blood vessels contract after an injury to reduce bleeding and
also to conserve body heat.
Arteries have thicker walls than veins to withstand the
pressure of blood being pumped from the heart. Blood in the veins is at a lower
pressure, so veins have one-way valves to prevent blood from flowing backwards
away from the heart. Capillaries, the smallest of blood vessels, are only
visible by microscope—ten capillaries lying side by side are barely as thick as
a human hair. If all the arteries, veins, and capillaries in the human body
were placed end to end, the total length would equal more than 100,000 km (more
than 60,000 miles—they could stretch around the earth nearly two and a half
times.
The arteries, veins, and capillaries are divided into two
systems of circulation: systemic and pulmonary. The systemic circulation
carries oxygenated blood from the heart to all the tissues in the body except
the lungs and returns deoxygenated blood carrying waste products, such as
carbon dioxide, back to the heart. The pulmonary circulation carries this spent
blood from the heart to the lungs. In the lungs, the blood releases its carbon
dioxide and absorbs oxygen. The oxygenated blood then returns to the heart
before transferring to the systemic circulation.
(Encarta Encyclopedia)