American Medical Association Family Medical Guide - American Medical Association [353]
Blood does not flow at a constant rate to all parts of the body. The rate varies according to the amount of blood the tissues need at a given time. For example, when you are running, your leg muscles receive more blood than they do when you are resting. When you feel cold, less blood flows in superficial blood vessels near the chilled skin, and more blood flows in deeper vessels, to conserve heat. This pattern reverses when you feel hot, making you perspire to avoid becoming overheated.
The circulatory system is complex, and it can stop working if the heart malfunctions or if problems develop inside the blood vessels. For example, artery walls may weaken or harden, causing circulation problems or high blood pressure. Blood clots can form and block blood vessels, and a number of other disorders, some mild and some severe, can affect circulation.
The circulatory system
The circulatory system, which consists of the heart and blood vessels, continuously transports oxygen and other nutrients to all the cells of the body and removes carbon dioxide and other waste products. Arteries carry oxygen-rich blood away from the heart; veins return oxygen-depleted blood to the heart to receive a fresh supply of oxygen.
Blood flow to and from heart
Oxygen-depleted blood returns from the tissues to the heart, which pumps it to the lungs for a fresh supply of oxygen. The oxygen-rich blood returns from the lungs to the heart, which pumps it out to tissues throughout the body.
Aneurysms
An aneurysm is an abnormal ballooning of a damaged or weakened area in an artery wall. If part of an artery wall is damaged or weakened, the force of blood flowing through the artery may cause the affected area to balloon outward. Although aneurysms can form in any artery, they usually form in the aorta (the main artery in the body) or in arteries in the brain. An aneurysm can develop for any of the following reasons:
• Defective artery wall An artery wall has an outer, middle, and inner layer of tissue. In some people, the muscular middle layer, which provides support and strength to the artery, has a congenital (present at birth) defect that makes it thin and weak. As blood flows through the defective artery, it may produce a saclike swelling (called a saccular aneurysm) at the weakened area. This type of aneurysm occurs most often in men and in people who have close relatives (especially parents or siblings) who have had an aneurysm, indicating a genetic component. Aneurysms caused by congenital defects usually form in the arteries at the base of the brain.
• Inflamed artery Inflammation caused by disorders such as infective endocarditis (see page 593), polyarteritis nodosa (see page 927), or syphilis (see page 483) may weaken part of an artery wall. If the cause of the inflammation is unknown, the condition is called arteritis. Normal blood flow through the inflamed artery can cause the weakened section of the artery wall to bulge.
• Damaged artery wall A portion of the muscular middle layer of an artery wall may slowly weaken as a result of a chronic (long-lasting) condition such as atherosclerosis (see page 557). An aneurysm caused by atherosclerosis is usually a ballooning of both artery walls along a short length of the artery (called a fusiform aneurysm). High blood pressure can accelerate damage to artery walls and is often a factor in the formation of aneurysms. Increased blood pressure in an artery can stretch the artery wall in various ways. In some cases, the layers of the artery wall split and blood flows between them, causing a bulge (called a dissecting aneurysm) in the artery wall.