What is the vascular system?

Structure and Functions

The vascular system is faced with the enormous task of supplying every cell of the human body with a constant supply of the oxygen and nutrients needed to sustain life. This elaborate system circulates more than 2,000 gallons of blood per day through more than 12,000 miles of arteries, veins, and capillaries. Moreover, the job of the vascular system is not finished when the nutrients arrive at the cell. After the cell uses the nutrients, waste products that are left over from metabolism must be carried away and disposed of before they damage the cell. For this reason, several kinds of vessels exist within the human body that differ in structure and function. They can be broken into three categories: arteries, veins, and capillaries.

The term “artery” comes from the Greek word arteria, meaning "windpipe," because arteries were thought to be filled with air. (This misconception evolved because after death, much of the blood usually pumped through the arteries had been pumped out, leading observers to conclude that air, rather than blood, was circulated within them.) Arteries are thick-walled blood vessels that carry oxygen-rich blood from the left side of the heart to all parts of the body. The blood circulating within them is usually moving at high velocities and exerts pressure against the artery walls, creating an expansion of the artery during the contraction of the heart. This expansion, called a pulse, can be palpated in areas where the arteries are large and close to the surface of the skin: in the neck (the carotid artery) or in the wrist (the radial artery). The pressure being exerted against the artery varies greatly. A doctor taking a blood pressure reading is measuring this variation in pressure. A blood pressure of 120/80, for example, would mean that the force from the heart is exerting 120 millimeters of mercury pressure against the artery wall while the heart is contracting (the systolic pressure) and 80 millimeters of mercury when the heart is at rest (the diastolic pressure). Clearly, the artery has to be a very strong structure.

Veins, on the other hand, are thin-walled, almost transparent vessels that return blood to the heart after it has visited the cells. There are far more veins in the body than there are arteries. The blood moving in the veins is under very little pressure and usually is moving quite slowly in comparison to the flow in the arteries. The flow in the veins is slow because much of the force from the contraction of the heart is dissipated by the time the blood passes by the cells of the body. For this reason, the flow in the veins must be helped along by contraction of the muscles around these blood vessels. For example, with each step, the muscles in the calves of the legs propel the blood in the calf veins upward with great force. For this reason, the calf of the leg is sometimes referred to as the “venous heart.” If this pump is not active, blood flow in the veins can stagnate and life-threatening clots can form in the veins.

Another problem for the venous circulation is that its blood is often moving against gravity. If veins were built like arteries (simple hollow tubes), the blood would flow upward toward the heart with the contraction of the muscles but would fall back down as soon as the contraction stopped. Fortunately, the veins are equipped with one-way valves not found in arteries. These valves open when blood is moving toward the heart and close when blood starts to fall backward. Veins are also different from arteries in that they can expand to several times their normal size. This allows the veins to be used as a storage area for blood. When the body’s need for blood is low, such as during a resting state, the veins enlarge and fill with the blood that is not being actively circulated. When the need for blood increases, as during strenuous activity, the stored blood is forced back into active circulation. Because veins have such thin walls and stretch so easily, one might think they are not as strong as arteries. In reality, veins are strong enough to be used as surgical substitutes for failed arteries and hold up quite well under arterial pressure.

Capillaries are extremely small vessels with very thin walls. These vessels connect the smallest arteries (arterioles) with the smallest veins (venules). Although their size can vary, the average diameter of a capillary is about 8 microns (0.008 millimeter), which is about the size of a single red blood cell. The nutrients carried in the blood pass through tiny pores in the vessel wall directly into the body's other cells, which use the nutrients to produce energy and heat. During this process, waste products are created that are poisonous to the cells; they must be removed quickly or the cells will die. The waste products therefore pass from the cells into the capillaries and then into tiny veins that will carry the waste products away.

A trip through the system of arteries, capillaries, and veins—to deliver nutrients to one cell in a calf muscle, for example—would begin in the left ventricle of the heart, where blood is pumped through the aorta (the largest artery) with great force. The aorta has branches that serve the structures of the head and neck (which includes the most important organ—the brain), the upper extremities, the abdomen, and the lower extremities. On this imaginary trip, one passes through the aortic arch and travels down the main artery in the abdomen, called the abdominal aorta. This artery eventually branches into two arteries (at about the level of the navel) that send blood to each leg. This artery continues to branch into smaller and smaller arteries until one reaches the capillaries serving the particular cell of the calf muscle in the leg. Here the nutrients are delivered to the cell. The waste products are dumped back into the capillaries. From the capillaries, one travels into venules. These tiny veins become continually larger until one is finally moving up through a large vein just behind the knee called the popliteal. Soon one is back in the abdomen in the large vein called the vena cava, which enters the heart at the right atrium. The blood then travels through the right ventricle and eventually into another large vessel called the pulmonary artery (the only artery that carries blood that is not oxygenated). This artery leads into the lungs, where the waste products of metabolism are released and exchanged for oxygen. With a new load of oxygen, one travels through the pulmonary veins (the only veins that carry oxygenated blood), into the left atrium of the heart, and into the left ventricle, where the journey began. In a normal person, this entire voyage takes only eighteen to twenty-four seconds.

Disorders and Diseases

When the vascular system is functioning correctly, all the cells of the body are receiving the right amount of blood at all times. Many problems can arise, however, in the complex functioning of the human organism, and the vasculature must have ways of meeting these challenges. Such problems include the obstruction of vital vessels by plaque formation (a buildup of fatty deposits called atherosclerosis), thrombus (blood clot) formation, and vasospasm (a closing down of a blood vessel in response to cold or trauma). Moreover, some organs in the body cannot survive for more than a few minutes without oxygen before damage occurs. For example, the brain can survive for only a few minutes without oxygen, while the cells in the arms and legs can be deprived of oxygen for a matter of hours without irreversible damage. For this reason, whenever there is a problem the vascular system must be able to set priorities about which systems receive blood flow and which systems do not. When there is a life-threatening
problem, the vessels in the arms and legs contract, forcing blood out of the extremities; this allows more flow to reach the brain, where it is most urgently needed. When an artery is narrowed by plaque, the vascular system will compensate by enlarging smaller vessels in the area to help maintain flow. If the artery is totally obstructed, this system of collateral vessels takes over.

While these and other mechanisms work quite well, sudden obstruction or other disease processes involving an artery or vein can result in major problems. The major problems that can result from arterial disease include stroke, myocardial infarction (heart attack), and peripheral artery disease. Problems involving the veins may include deep vein thrombosis, pulmonary embolism, and varicose veins.

A stroke is a condition in which part of the brain is deprived of oxygen long enough to cause permanent damage. The medical term for such an event is a cerebrovascular accident, or CVA. The symptoms may include one-sided weakness or numbness, headache, difficulty in speaking, or transient blindness in one eye. If these symptoms completely resolve within twenty-four hours, the event is referred to as a transient ischemic attack, or TIA. The difference between a TIA and a CVA is that the damage done by the TIA is not permanent. TIAs, however, are often precursors of impending full-blown strokes. Therefore, patients who experience them should see a doctor immediately so that steps can be taken to prevent another, perhaps more severe, episode. The treatment for patients who experience a TIA may include surgery to remove plaque buildup from the carotid artery, bypass surgery (in which another vessel is used to bypass a narrowed area), the use of blood-thinning
drugs, or the use of antiplatelet drugs (such as aspirin). Rehabilitation, the use of blood-thinning or antiplatelet drugs, and lifestyle modification are often prescribed for those who have already suffered major strokes.

Myocardial infarction is one of the leading killers in Western societies. A
heart attack occurs when blood flow is inadequate to the heart muscle and part of the heart muscle dies. The symptoms include pain in the chest (especially pain that is brought on by exertion), shortness of breath, sweating, nausea, and fatigue. Similar, although usually less severe, symptoms may be present with a condition called angina, in which blood flow to the heart muscle is impaired but there is no permanent damage. Acute treatment for heart attacks can include rest (to reduce additional damage to the heart muscle), treatment with blood-thinning drugs, treatment with drugs that dissolve blood clots, balloon catheters (to help open narrowed arteries), or coronary bypass surgery.

Peripheral artery disease—the narrowing or blockage of arteries in the arms or legs—is also quite common. Symptoms may include pain in the limb, loss of feeling, coolness, and discoloration; in severe cases, tissue loss may result. This disease process is usually progressive. A patient may first notice pain in the calf of the leg that comes on only with walking and goes away as soon as the exercise stops. This condition, called intermittent claudication, indicates that there is minimal narrowing of the arteries in the leg. As more of the artery narrows, the pain occurs even without exercise. Finally, blood flow to the limb is not sufficient to maintain the cells, and tissue begins to die. Treatment of peripheral artery disease may include medication and exercise (during the early stages) and progress to the surgical bypass of narrowed arteries (in later stages). Sometimes arteries in the extremities become clogged by a thrombus instead of plaque. If this is the case, drugs that dissolve blood clots or surgical operations to remove the clot may be used. If treatment for severe peripheral disease is
unsuccessful,
amputation may be necessary.

The risk factors for developing arterial disease—of the coronary, carotid, or peripheral arteries—include high blood pressure, smoking, diabetes, elevated cholesterol levels, stress, a family history of arterial disease, obesity, and advancing age.

Veins do not develop plaque as do arteries; instead, blood can stagnate and form clots that can obstruct them. When this happens, a condition called
venous thrombosis, blood stagnates in the veins behind the clot and a larger clot forms. It is not unusual for clots to fill all the major veins in the leg once this process begins. These clots cause swelling and pain in the leg but do not usually threaten the leg as obstruction of the arteries does. Instead, the danger lies in the possibility of a clot breaking loose and traveling to the lungs. This clot, called a pulmonary embolism, can be fatal. The risk factors for developing venous thrombosis include anything that can slow blood flow in the veins, such as prolonged sitting or standing, a long airplane trip or car trip, a surgical operation, or pregnancy. Injury to the vein can also trigger clots, as can an imbalance of clotting factors in the blood. The best way to prevent venous thrombosis is to keep active.

Another venous problem that strikes as many as one of every four women and one of every five men is a condition most commonly known as varicose veins, in which the veins become stretched out and elongated to the point where they bulge out when the patient is sitting or standing. Although this is mostly a cosmetic problem, severe cases can lead to blood pooling in the leg and tissue damage.

Perspective and Prospects

The vasculature of the human body has not always been understood, even in recent times. The ancient Egyptians knew about the importance of the heart and the pulse, but this knowledge was not passed on to more modern civilizations. The Greek physician Hippocrates (c. 460–c. 370 bce) had serious misconceptions about the functions of the circulatory system: he thought that the pulse was caused by movements of the blood vessels. Other great thinkers, such as Aristotle and Galen, made similar errors in their study of the vascular system, errors that influenced medicine for many years.

In 1628, a doctor in London named William Harvey published a paper introducing radical theories about how the blood circulates. He changed the way medical people thought about this system by describing it as a closed circuit, with blood being forced through it via contractions of the heart. He postulated that blood passed from the arteries into the veins at the cellular level. It was not until the 1660s, when early microscopes were developed, that this theory could be confirmed.

In 1733, a clergyman named Stephen Hales became the first person to measure blood pressure within the arterial system. He inserted a large glass tube into the neck artery of a horse. To his amazement, the blood rose 9 feet up the tube. This method of measuring blood pressure was not practical, however, and it was not until the late nineteenth century that the sphygmomanometer was developed to measure blood pressure, utilizing blood pressure cuffs and air pressure.

Another pioneer in the understanding of the vascular system was German physician and biologist Rudolf Virchow (1821–1902), who theorized about how blood clots formed in veins. He concluded that clots formed when the blood flow was slowed down, the vein wall was injured, or an imbalance of clotting factors in the blood existed. These observations were astonishingly correct considering that, at this time, many people still thought blood clots in the veins were composed of pus. Understanding of these principles makes possible modern treatments and prevention techniques.

In the late nineteenth century, modern vascular surgery began with development of techniques to repair blood vessels. By the early twentieth century, methods for connecting the ends of vessels with a watertight suture became commonplace. In 1948, a surgeon in Paris took a saphenous vein and used it to bypass a blockage in an artery in the leg. In the 1950s, the technology necessary to support sustained heart surgeries was introduced, and heart surgery has since become routine. Blood vessels can now be surgically repaired, bypassed, or cleaned out. Laser surgery and clot-dissolving drugs are also becoming routine.

Bibliography

Hershey, Falls B., Robert W. Barnes, and David S. Sumner, eds. Noninvasive Diagnosis of Vascular Disease. Pasadena, Calif.: Appleton Davies, 1984.

Loscalzo, Joseph, and Andrew I. Schafer, eds. Thrombosis and Hemorrhage. 3d ed. Philadelphia: Lippincott Williams & Wilkins, 2003.

Mohrman, David E., and Lois Jane Heller. Cardiovascular Physiology. 7th ed. New York: Lange Medical Books/McGraw-Hill, 2010.

Standring, Susan, et al., eds. Gray’s Anatomy. 40th ed. New York: Churchill Livingstone/Elsevier, 2008.

Strandness, D. Eugene, Jr. Duplex Scanning in Vascular Disorders. 4th ed. London: Lippincott Williams & Wilkins, 2009.

"Vascular Diseases." MedlinePlus, June 26, 2013.