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Access Excellence Classic Collection

The Heart and the Circulatory System

by

Roger E. Phillips, Jr.
Assistant Director, Preserved Materials
Carolina Biological Supply Company
2700 York Road
Burlington, NC 27215-3398

The Anatomy of the Heart



The Anatomy of the Heart

From this point forward, all discussions about the heart and circulation refer to human circulation. The human heart is a muscular pump. While most of the hollow organs of the body do have muscular layers, the heart is almost entirely muscle. Unlike most of the other hollow organs, whose muscle layers are composed of smooth muscle, the heart is composed of cardiac muscle. All muscle types function by contraction, which causes the muscle cells to shorten. Skeletal muscle cells, which make up most of the mass of the body, are voluntary and contract when the brain sends signals telling them to react. The smooth muscle surrounding the other hollow organs is involuntary, meaning it does not need to be told to contract. Cardiac muscle is also involuntary. So functionally, cardiac muscle and smooth muscle are similar. Anatomically though, cardiac muscle more closely resembles skeletal muscle. Both skeletal muscle and cardiac muscle are striated. Under medium to high power magnification through the microscope, you can see small stripes running crosswise in both types. Smooth muscle is nonstriated. Cardiac muscle could almost be said to be a hybrid between skeletal and smooth muscle. Cardiac muscle does have several unique features. Present in cardiac muscle are intercalated discs, which are connections between two adjacent cardiac cells. Intercalated discs help multiple cardiac muscle cells contract rapidly as a unit. This is important for the heart to function properly. Cardiac muscle also can contract more powerfully when it is stretched slightly. When the ventricles are filled, they are stretched beyond their normal resting capacity. The result is a more powerful contraction, ensuring that the maximum amount of blood can be forced from the ventricles and into the arteries with each stroke. This is most noticeable during exercise, when the heart beats rapidly.

There are four chambers in the heart - two atria and two ventricles. The atria (one is called an atrium) are responsible for receiving blood from the veins leading to the heart. When they contract, they pump blood into the ventricles. However, the atria do not really have to work that hard. Most of the blood in the atria will flow into the ventricles even if the atria fail to contract. It is the ventricles that are the real workhorses, for they must force the blood away from the heart with sufficient power to push the blood all the way back to the heart (this is where the property of contracting with more force when stretched comes into play). The muscle in the walls of the ventricles is much thicker than the atria. The walls of the heart are really several spirally wrapped muscle layers. This spiral arrangement results in the blood being wrung from the ventricles during contraction. Between the atria and the ventricles are valves, overlapping layers of tissue that allow blood to flow only in one direction. Valves are also present between the ventricles and the vessels leading from it.

Though the brain can cause the heart to speed up or slow drain, it does not control the regular beating of the heart. As noted earlier, the heart is composed of involuntary muscle. The muscle fibers of the heart are also self-excitatory. This means they can initiate contraction themselves without receiving signals from the brain. This has been demonstrated many times in high school classes of the past by removing the heart of a frog or turtle, and then stimulating it to contract. The heart continues to beat with no further outside stimulus, sometimes for hours if bathed in the proper solution. In addition, cardiac muscle fibers also contract for a longer period of time than do skeletal muscles. This longer period of contraction gives the blood time to flow out of the heart chambers.

heart
Opened heart
The heart has two areas that initiate impulses, the SA or sinoatrial node, and the AV or atrioventricular node. The heart also has special muscle fibers called Purkinje fibers that conduct impulses five times more rapidly than surrounding cells. The Purkinje fibers form a pathway for conduction of the impulse that ensures that the heart muscle cells contract in the most efficient pattern. The SA node is located in the wall of the right atrium, near the junction of the atrium and the superior vena cava. This special region of cardiac muscle contracts on its own about 72 times per minute. In contrast, the muscle in the rest of the atrium contracts on its own only 40 or so times per minute. The muscle in the ventricles contracts on its own only 20 or so times per minute. Since the cells in the SA node contract the most times per minute, and because cardiac muscle cells are connected to each other by intercalated discs, the SA node is the pacemaker of the heart. When the SA node initiates a contraction, Purkinje fibers rapidly conduct the impulse to another site near the bottom of the right atrium and near the center of the heart. This region is the AV node, and slows the impulse briefly. The impulse then travels to a large bundle of Purkinje fibers called the Bundle of His, where they move quickly to the septum that divides the two ventricles. Here, the Purkinje fibers run in two pathways toward the posterior apex of the heart. At the apex, the paths turn in opposite directions, one running to the right ventricle, and one running to the left. The result is that while the atria are contracting, the impulse is carried quickly to the ventricles. With the AV node holding up the impulse just enough to let the atria finish their contraction before the ventricles begin to contract, blood can fill the ventricles. And, since the Purkinje fibers have carried the impulse to the apex of the ventricles first, the contraction proceeds from the bottom of the ventricles to the top where the blood leaves the ventricles through the pulmonary arteries and the aorta.

graphic
The cardiac cycle
The contraction of the heart and its anatomy cause the distinctive sounds heard when listening to the heart with a stethoscope. The "lub-dub" sound is the sound of the valves in the heart closing. When the atria end their contraction and the ventricles begin to contract, the blood is forced back against the valves between the atria and the ventricles, causing the valves to close. This is the "lub" sound, and signals the beginning of ventricular contraction , known as systole. The "dub" is the sound of the valves closing between the ventricles and their arteries, and signals the beginning of ventricular relaxation, known as diastole.
Stethoscope placements (shade areas)
for hearing heart sounds
A physician listening carefully to the heart can detect if the valves are closing completely or not. Instead of a distinctive valve sound, the physician may hear a swishing sound if they are letting blood flow backward. When the swishing is heard tells the physician where the leaky valve is located.



The Pulmonary and Systemic Circuits and the Blood Supply to the Heart.

The heart is responsible for pumping the blood to every cell in the body. It is also responsible for pumping blood to the lungs, where the blood gives up carbon dioxide and takes on oxygen. The heart is able to pump blood to both regions efficiently because there are really two separate circulatory circuits with the heart as the common link. Some authors even refer to the heart as two separate hearts--a right heart in the pulmonary circuit and left heart in the systemic circuit. In the pulmonary circuit, blood leaves the heart through the pulmonary arteries, goes to the lungs, and returns to the heart through the pulmonary veins.
blood and vein flow diagram
Arterial and Venous Systems
In the systemic circuit, blood leaves the heart through the aorta, goes to all the organs of the body through the systemic arteries, and then returns to the heart through the systemic veins. Thus there are two circuits. Arteries always carry blood away from the heart and veins always carry blood toward the heart. Most of the time, arteries carry oxygenated blood and veins carry deoxygenated blood. There are exceptions. The pulmonary arteries leaving the right ventricle for the lungs carry deoxygenated blood and the pulmonary veins carry oxygenated blood. If you are confused as to which way the blood flows through the heart, try this saying "When it leaves the right, it comes right back, but when it leaves the left, it's left." The blood does not have to travel as far when going from the heart to the lungs as it does from the heart to the toes. It makes sense that the heart would be larger on one side than on the other. When you look at a heart, you see that the right side of the heart is distinctly smaller than the left side, and the left ventricle is the largest of the four chambers.

While you might think the heart would have no problem getting enough oxygen-rich blood, the heart is no different from any other organ. It must have its own source of oxygenated blood. The heart is supplied by its own set of blood vessels. These are the coronary arteries. There are two main ones with two major branches each. They arise from the aorta right after it leaves the heart. The coronary arteries eventually branch into capillary beds that course throughout the heart walls and supply the heart muscle with oxygenated blood. The coronary veins return blood from the heart muscle, but instead of emptying into another larger vein, they empty directly into the right atrium.

The Blood Vessels

cross section of blood vessels
Blood vessel anatomy
We need to briefly discuss the anatomy of the vessels. There are three types of vessels - arteries, veins, and capillaries. Arteries, veins, and capillaries are not anatomically the same. They are not just tubes through which the blood flows. Both arteries and veins have layers of smooth muscle surrounding them. Arteries have a much thicker layer, and many more elastic fibers as well. The largest artery, the aorta leaving the heart, also has cardiac muscle fibers in its walls for the first few inches of its length immediately leaving the heart. Arteries have to expand to accept the blood being forced into them from the heart, and then squeeze this blood on to the veins when the heart relaxes. Arteries have the property of elasticity, meaning that they can expand to accept a volume of blood, then contract and squeeze back to their original size after the pressure is released. A good way to think of them is like a balloon. When you blow into the balloon, it inflates to hold the air. When you release the opening, the balloon squeezes the air back out. It is the elasticity of the arteries that maintains the pressure on the blood when the heart relaxes, and keeps it flowing forward. if the arteries did not have this property, your blood pressure would be more like 120/0, instead of the 120/80 that is more normal. Arteries branch into arterioles as they get smaller. Arterioles eventually become capillaries, which are very thin and branching.
graphic of capillaries
Capillary Bed
Capillaries are really more like a web than a branched tube. It is in the capillaries that the exchange between the blood and the cells of the body takes place. Here the blood releases its oxygen and takes on carbon dioxide, except in the lungs, where the blood picks up oxygen and releases carbon dioxide. In the special capillaries of the kidneys, the blood gives up many waste products in the formation of urine. Capillary beds are also the sites where white blood cells are able to leave the blood and defend the body against harmful invaders. Capillaries are so small that when you look at blood flowing through them under a microscope, the cells have to pass through in single file. As the capillaries begin to thicken and merge, they become venules. Venules eventually become veins and head back to the heart. Veins do not have as many elastic fibers as arteries. Veins do have valves, which keep the blood from pooling and flowing back to the legs under the influence of gravity. When these valves break down, as often happens in older or inactive people, the blood does flow back and pool in the legs. The result is varicose veins, which often appear as large purplish tubes in the lower legs.

Circulatory Problems

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.

Stained Cross sections through coronary artery (left) and
a coronary atery with lipid deposits in its walls (right).
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.

Since the advent of modern medical research, physicians have made quantum leaps in their understanding of the heart and in ways to treat cardiovascular disorders. When we hear of breakthroughs in cardiac medicine, we often think of radical treatments such as heart transplants or artificial hearts. The first heart transplant took place in 1967. It was performed by the South African surgeon Dr. Christiaan Barnard. The patient lived just 18 days. The first U.S. transplant took place in 1968. The rate of transplants increased in the 1970's, but most patients died within a year. The drugs given to fight rejection of the heart also lowered the body's resistance to infections. It was these infections that often killed the patients. Then, in the 1980's physicians began using the drug cyclosporine to fight rejection. Patients taking cyclosporine had a much greater rate of survival. In 1982, the first artificial heart was implanted into Barney Clark by the American surgeon Dr. William DeVries. Due to complications, Clark lived only 112 days. As of this writing, the use of the artificial heart is not approved in the United States. While these two methods both sound less than successful, you must remember that they are last resort treatments. They are not typical of the success rates that other, more common, treatments have enjoyed.

Most cardiovascular emergencies are directly caused by coronary artery disease. As noted earlier, coronary arteries can become clogged or occluded, leading to damage to the heart muscle supplied by the artery. There are three methods for treating coronary artery disease. They may be used individually or in combination with the each other. Medication can be given to control the blood flow to the heart. This is not always effective. Another method, coronary bypass surgery, involves replacing a blocked coronary artery with either a vein from the leg or with a thoracic artery from the chest wall. This method requires that the patient's chest be opened. The heart must be stopped, then restarted after the new vessels are connected. Another technique, although not new (it was first performed in 1977 by a Swiss physician), is a highly successful treatment called percutaneous transluminal coronary angioplasty, or balloon angioplasty by most laypersons. In this procedure, the patient remains awake. Under local anesthesia, tubes called catheters are inserted into an artery and vein in the groin. Next, a tiny, flexible guide wire is maneuvered through the arteries, eventually passing through the narrowed opening in the occluded coronary artery. Next, another catheter with a balloon near the end is run along the guide wire. When the balloon is in place, it is inflated and deflated several times, enlarging the opening of the artery and increasing the blood flow. When the surgeon is satisfied with the size of the opening, the catheters are removed. The patient remains in the hospital for a few days, but can resume normal activities in a matter of weeks. Other current cardiovascular research involves drugs that control the blood pressure or heart rate, artificial blood substitutes, and devices implanted in the wall of the heart that can detect changes in the rate or patterns of contraction of the ventricles and correct them before a heart attack occurs.

painting of Hale
Stephen Hales
Modem cardiovascular medicine and our understanding of the heart and circulation have certainly come a long way since the days of Pliny, Galen, and Harvey. While we jest about broken hearts in romances, or having the heart needed to work hard to win an event, we all know that the heart and the circulatory system are not related to emotions, the soul, or intellect. Without the four-chambered heart and double circuit circulatory system, mammals would not have been able to successfully evolve, for this type of circulation gave rise to the warm-bloodedness needed to out compete the slower responding reptiles. Our own circulatory system has evolved to feed large amounts of blood to our brains, letting the brain develop and evolve into the organ it is today. Modern medical research on the heart has changed the face of the future. Advances in cardiovascular surgery and cardiac care have given thousands of people the opportunity to live on after the attack of disease, often for decades. What once would have killed can now be not only survived, but even prevented. All because an English physician in the 1600's decided that maybe everything was not as he had been taught, and had the "heart" to try something different.


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