What is circulation? |

Structure and Functions

The cardiovascular system is made up of the heart, arteries, veins, capillaries, and lungs. The heart serves as a pump to deliver blood to the arteries for distribution throughout the body. The veins bring the blood back to the heart, and the lungs oxygenate the blood before returning it to the arterial system.

Contraction of the heart muscle forces blood out of the heart. This period of contraction is known as systole. The heart muscle relaxes after each contraction, which allows blood flow into the heart. This period of relaxation is known as diastole. A typical blood pressure taken at the upper arm provides a pressure reading during two phases of the cardiac cycle. The first number is known as the systolic pressure and represents the pressure of the heart during peak contraction. The second number is known as the diastolic pressure and represents the pressure while the heart is at rest. A typical pressure reading for a young adult would be 120/80. When blood pressure is abnormally elevated, it is commonly referred to as high blood pressure, or hypertension.

The heart is separated into two halves by a wall of muscle known as the septum. The two halves are known as the left and right heart. The left side of the heart is responsible for high-pressure arterial distribution and is larger and stronger than the right side. The right side of the heart is responsible for accepting low-pressure venous return and redirecting it to the lungs.

Because of these pressure differences from one side of the heart to the other, the vessel wall constructions of the arteries and the veins differ. Strong construction of the arterial wall allows tolerance of significant pressure elevations from the left heart. The arterial wall is made up of three major tissue layers, known as tunics. Secondary layers of tissue that provide strength and elasticity to the artery are known as elastic and connective tissues. As with the artery, the wall of the vein is made up of three distinct tissue layers. Compared to that of an artery, the wall of a vein is thinner and less elastic, which allows the wall to be easily compressed by surrounding muscle during contraction.

While the heart is at rest, between contractions, newly oxygenated arterial blood passes from the lungs and enters the left heart. Each time the heart contracts, blood is forced from the left heart into a major artery known as the aorta. From the aorta, blood is distributed throughout the body. Once depleted of nutrients and oxygen, arterial blood passes through an extensive array of minute vessels known as capillaries. A significant pressure drop occurs as blood is dispersed throughout the immense network of capillaries. The capillaries empty into the venous system, which carries the blood back to the heart.

The primary responsibility of the venous system is to return deoxygenated blood to the lungs and heart. Much more energy is required from the body to move venous flow compared to arterial flow. Unlike the artery, the vein does not depend on the heart or gravity for energy to move blood. The venous system has a unique means of blood transportation known as the “venous pump,” which moves blood toward the heart.

The components making up the venous pump include muscle contraction against the venous wall, intra-abdominal pressure changes, and one-way venous valves. Compression against the walls of a vein induces movement of blood. Muscle contraction against a vein wall occurs throughout the body during periods of activity. Activity includes every movement, from breathing to running. Variations in respiration cause fluctuations in the pressure within the abdomen, which produces a siphonlike effect on the veins, pulling venous blood upward. Valves are located within the veins of the extremities and pelvis. A venous valve has two leaflets, which protrude inward from opposite sides of the vein wall and meet one another in the center. Valves are necessary to prevent blood from flowing backward, away from the heart.

The venous system is divided into two groups known as the deep and superficial veins. The deep veins are located parallel to the arteries, while the superficial veins are located just beneath the skin surface and are often visible through the skin.

Disorders and Diseases

Numerous variables may affect the flow of blood. The autonomic nervous system is connected to muscle within the wall of the artery by way of neurological pathways known as sympathetic branches. Various drugs and/or conditions can trigger responses in the sympathetic branches and produce constriction of the smooth muscle in the arterial wall (vasoconstriction) or relaxation of the arterial wall (vasodilation). Alcohol consumption and a hot bath are examples of conditions that produce vasodilation. Exposure to cold and cigarette smoking are examples of conditions that produce vasoconstriction. Various drugs used in the medical environment are capable of producing similar effects. The diameter of the lumen of an artery influences the pressure and the flow of blood through it.

Another condition that alters the arterial diameter is atherosclerosis, a disease primarily of the large arteries, which allows the formation of fat (lipid) deposits to build on the inner layer of the artery. Lipid deposits are more commonly known as atherosclerotic plaque. Plaque accumulation reduces the diameter of the arterial lumen, causing various degrees of flow restriction. Plaque is similar to rust accumulation within a pipe that restricts the flow of water. A restriction of flow is referred to as a stenosis. The majority of stenotic lesions occur at the places where arteries divide into branches, also known as bifurcations. In advanced stages of plaque development, plaque may become calcified. Calcified plaque is hard and may become irregular, ulcerate, or hemorrhage, providing an environment for new clot formation and/or release of small pieces of plaque debris downstream. When pieces of plaque break off, they may move downstream into smaller blood vessels, causing a blockage and restricting the flow of oxygenated blood; this can cause tissue death, stroke, and heart attack.

An arterial wall may become very hard and rigid, a condition commonly known as hardening of the arteries. Hardened arteries may eventually become twisted, kinked, or dilated as a result of the hardening process of the arterial wall. A hardened artery which has become dilated is known as an aneurysm.

Normal arterial flow is undisturbed. When blood cells travel freely, they move together at a similar speed with very little variance. This is known as laminar flow. Nonlaminar (turbulent) flow is seen when irregular plaque or kinks in the arterial wall disrupt the smooth flow of cells. Plaque with an irregular surface may produce mild turbulence, while a narrow stenosis produces significant turbulence immediately downstream from the stenosis.

Many moderate or severe stenoses can be heard with the use of a standard stethoscope over the vessel of interest. A high-pitched sound can be heard consequent to the increased velocity of the blood cells moving through a narrow space. (A similar effect is produced when a standard garden hose is kinked to create a spray and a hissing sound is heard.) Medically, this sound is often referred to as a bruit. Bruit (pronounced “broo-ee”) is a French word meaning noise.

Patients with significant lower extremity arterial disease will consistently experience calf pain and occasionally experience thigh discomfort with exercise. The discomfort is relieved when the patient stands still for a few moments. This is known as vascular claudication and occurs from a pressure drop as a consequence of a severely stenotic (reduced in diameter by greater than 75 percent) or occluded artery. If the muscle cannot get enough oxygen as a result of reduced blood flow, it will cramp, forcing the patient to stop and rest until blood supply has caught up to muscle demand. Alternate pathways around an obstruction prevent pain at rest, when muscle demand is low. Alternate pathways are also referred to as collateral pathways. Small, otherwise insignificant branches from a main artery become important vessels when the body uses them as collateral pathways around an obstruction. Time and exercise help to collateralize arterial branches into larger, more prominent arterial pathways. If collateral pathways do not provide enough flow to prevent the patient from experiencing painful muscle cramps while performing a daily exercise routine or to heal a wound on the foot, it may be necessary to perform either a surgical bypass around the obstruction or another interventional procedure such as angioplasty, atherectomy, or laser surgery.

Claudication may also occur in the heart. The main coronary arteries lie on the surface of the heart and distribute blood to the heart muscle. Patients suffering from coronary artery disease (CAD) may experience tightness, heaviness, or pain in the chest subsequent to flow restriction to the heart muscle as a result of atherosclerotic plaque within the coronary arteries. These symptoms are known as angina pectoris, or simply angina, usually occurring with exercise and relieved by rest. Intensity of the symptoms is relative to the extent of disease. A myocardial infarction (heart attack) is the result of a coronary artery occlusion.

Unlike the arteries, the venous system is not affected by atherosclerosis. The primary diseases of the veins include blood clot formation and varicose veins. A varicose vein is an enlarged and meandering vein with poorly functioning valves. A varicosity typically involves the veins near the skin surface, the superficial veins, and is often visualized as an irregular and/or raised segment through the skin surface. Varicosities are most common in the lower legs.

Valve leaflets are common sites for development of a thrombus. Thrombosis is the formation of a clot within a vein, which occurs when blood flow is delayed or obstructed for many hours. Several conditions that may induce venous clotting include prolonged bed rest (postoperative patients), prolonged sitting (long airplane or automobile rides), and the use of oral contraceptives. Cancer patients are at high risk of clot formation secondary to a metabolic disorder that affects the natural blood-thinning process.

Because numerous tributaries are connected to the superficial system, it is easy for the body to compensate for a clot in this system by rerouting blood through other branches. The deep venous system, however, has fewer branches, which promotes the progression of a thrombus toward the heart. A thrombus in the deep venous system is more serious because the risk of pulmonary emboli, commonly known as blood clots in the lungs, is much higher than superficial vein thrombosis. The further a thrombus propagates, the higher the risk to the patient.

Lower extremity venous return must take an alternate route via the superficial venous system when the deep system is obstructed by a thrombus. This is known as compensatory flow around an obstruction.

Perspective and Prospects

Historically, the vasculature of the human body was evaluated by placing one’s fingers on the skin, palpating for the presence or absence of a pulse, and making note of the patient’s symptoms. Prior to the 1960s, treatment of the circulatory system was very limited or nonexistent, resulting in a high death rate and large numbers of amputations, strokes, and heart attacks. The development of arteriography (the angiogram), a procedure in which dye is injected into the vessels while x-rays are obtained, revealed more about the vasculature and the nature of disease involving it. In conjunction with arteriography came corrective bypass surgery.

This period of development was followed by vast improvements in diagnostics, treatment, and knowledge of preventive maintenance. Today, synthetic bypass grafts are commonplace and are used to reroute flow around an obstruction. In many cases, procedures such as atherectomy and angioplasty, in which plaque or a thrombus is removed through a catheter inserted into the vessel, are often performed as outpatient procedures. In cases where an artery been seriously narrowed or weakened, a stent, a small mesh tube, may be inserted into the blood vessel to keep it open and unobstructed following the angioplasty procedure.

Diagnostic imaging of the cardiovascular system and the study of hemodynamics with the use of ultrasound have been useful for patient screening, the monitoring of disease progression, and the postoperative evaluation of surgical/interventional procedures. Ultrasound is a particularly valuable diagnostic tool because, compared to x-rays or arteriography, it is less expensive; it is also quick, painless, and noninvasive (no radiation, needle, or dye is required).

In addition to technological advances, new medications have been made available to reduce the risk of graft rejection, hypertension, and clotting, and to lower blood cholesterol. Preventive measures such as a healthy diet, weight maintenance, and regular exercise, however, constitute the most effective approach to good cardiovascular health. Much new information has been made available to improve the knowledge of the general public regarding diet, exercise, and the avoidance of unhealthy habits such as cigarette smoking as the way to create and maintain a healthier cardiovascular system.

Bibliography

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