No discussion of the circulatory system would be complete without mentioning some of the problems that can occur. As mentioned earlier, several problems can occur with the valves of the heart. Valvular stenosis is the result of diseases such as rheumatic fever, which causes the opening through the valve to become so narrow that blood can flow through only with difficulty. The result can be blood damming up behind the valve. Valvular regurgitation occurs when the valves become so worn that they cannot close completely, and blood flows back into the atria or the ventricles. If the blood can flow backward, the efficiency of the cardiac stroke is drastically reduced.
The coronary arteries are also subject to problems. Atherosclerosis is a degenerative disease that results in narrowing of the coronary arteries. This is caused by fatty deposits, most notably cholesterol, on the interior walls of the coronary arteries. When the walls become narrowed or occluded, they reduce the blood flow to the heart muscle. If the artery remains open to some degree, the reduced blood flow is noticed when the heart is under stress during periods of rapid heartbeat. The resulting pain is called angina. When the artery is completely closed or occluded, a section of the heart muscle can no longer get oxygenated blood, and begins to die. This is called a heart attack. Only quickly restoring the blood flow can reduce the amount of heart muscle that will die. At times, the walls of the systemic arteries become weakened. When this occurs, the wall may balloon outward, much like a weak spot in the radiator hose. This called an aneurysm, and is an extremely dangerous condition. Like a radiator hose under pressure, the wall can rupture. Blood can then spill out of the circulatory system into the body cavity. If an aneurysm ruptures in the aorta, death is almost certain.
The systemic veins also can have problems. When the valves in the veins break down, blood can pool in the lower legs, causing varicose veins. Clots can also form in veins of the legs. These clots can break loose and flow to the lungs, causing a pulmonary embolism and possible death.
The capillary beds are not without their problems. True capillaries do not have any smooth muscle in their walls. They have no way to control excess pressure other than a small muscle, the precapillary sphincter. A precapillary sphincter encircles each capillary branch at the point where it branches from the arteriole. Contraction of the precapillary sphincter can close the branches off to blood flow. If the sphincter is damaged or can not contract, blood can flow into the capillary bed at high pressures. When capillary pressures are high (and this can be the result of gravity), fluid passes out of the capillaries into the interstitial space, and edema or fluid swelling is the result. This can be seen in people who have to stand all day. Their feet and ankles often swell from the excess fluid accumulating there. Capillaries are fragile and can be damaged easily. It is often ruptured capillaries in the skin that cause bruises when one falls or sustains a blow.
Source: Carolina Biological Supply /Access Excellence