Difference Between Arteries and Capillaries
The arteries derive toughness and elasticity from their unique three-layer composition. The innermost layer consists of a delicate, thin cellular lining. The middle, much thicker, layer is composed of smooth muscle fibers and elastic connective tissue. The outermost layer is made of connective tissue rich with nerves and blood vessels.
Generally the arteries are filled with blood, and therefore their walls are somewhat stretched. They stretch even further when the ventricle ejects blood into them. During the relaxing phase, the walls of the arteries contract. As they contract, they press upon the blood and force it toward its destinations. This elastic give and take of healthy arteries prevents large changes in blood pressure during systole and diastole.
The aorta, the largest artery and the first to receive oxygenated blood from the heart, is about 1 inch (2.5 centimeters) in diameter where it begins at the upper part of the left ventricle. Two main arteries, called the coronary arteries, lead from the aorta to supply the heart muscle itself with blood.
As they branch out farther and farther from the aorta, the arteries grow smaller and smaller. The tiniest tubes in the arterial system are not much thicker than hairs. These blood vessels are called arterioles, or little arteries.
The arterioles play an important role in distributing blood in appropriate quantities to the different parts of the body. When the skeletal muscles, for example, are used in strenuous exercise, they consume nutrients and oxygen rapidly, and their waste output increases as well. To respond to this demand, the arteriole muscles relax. This relaxation increases the blood flow through the skeletal muscles to supply the necessary nutrients and to carry off the wastes.
Arterioles branch dozens of times into a network of tiny capillaries that together form a huge surface area—about the size of a tennis court. Capillaries are found in tissues throughout the body, and also surround the alveoli, or air sacs, in the lungs. The walls of the capillaries are so thin—only one cell thick—that molecules can easily diffuse across this barrier. Fortunately, blood pressure in capillaries is very low—about one-fifth of the blood pressure in arteries—so there is no risk that capillaries might rupture.
Blood flows through a capillary much slower than it does in large arteries. This slow movement allows nutrients to cross, or diffuse through, the thin capillary walls into the tissue, and for wastes to diffuse from the tissue into the blood. This diffusion occurs through a process by which molecules move from an area where they are present in relatively high concentration into a region of lower concentration.
For example, there is more oxygen in the blood that flows into the capillaries than there is in the tissue fluid. Therefore, the oxygen will pass through the capillary walls into this fluid. To do so, the oxygen breaks loose from the hemoglobin with which it has been combined in the red blood cells. Nutrients transported by the blood will also pass into the tissue fluid.