Structures of the Heart

Structures of the Heart

The heart lies inside the thoracic cavity, resting on the diaphragm (FIGURE 18-2). It is hollow and cone-shaped, varying in size. The heart is roughly the size of a man’s clenched fist. The heart is within the mediastinum, which is the medial cavity of the thorax, in between the lungs. Its posterior border is near the vertebral column, and its anterior border is near the sternum. It is not situated in the exact center of the thorax, and approximately two-thirds of the heart lies to the left of the midsternal line. It is partially obscured, laterally, by the lungs.

The average adult heart is about 14 cm (5 inches) long by 9 cm (3.5 inches) wide and weighs approx-imately 300 g. The base of the heart is actually the upper portion, where it is attached to several large blood vessels­. This wide portion of the heart lies beneath the second rib. The distal end of the heart extends downward, to the left, ending in a blunt point called the apex, which is even with the fifth intercostal space. The apex points toward the left hip. Just below the left nipple, between the fifth and sixth ribs, theapi-cal impulse can easily be felt. This is caused by the apextouching the chest wall.

Coverings of the Heart

The heart is enclosed in the pericardium, a ­double-walled sac. Its superficial portion, the fibrouspericardium, is loose-fitting, and consists of tough,dense connective tissue. It functions to protect the heart, and anchor it to surrounding structures. The pericardium also prevents the heart from becoming overfilled with blood. The serous pericardium lies deep to fibrous pericardium. It is a thin and slippery membrane with two layers, and forms a closed sac enclosing the heart. The parietal layer of the serous pericardium lines the internal surfaces of the fibrous pericardium.

At the heart’s superior margin, the parietal layer is attached to the large arteries that exit the heart. The parietal layer then turns inferiorly, continuing over the external surface of the heart as the visceral layer or epicardium, which is an important part of the heart wall. A slit-like pericardial cavity lies between the parietal and visceral layers. It contains a film of serous fluid. The serous membranes are lubricated by this fluid. They move smoothly past each other and allow the heart to work with very little friction.

Heart Layers

The three layers composing the wall of the heart are the outer epicardium, middle myocardium, and inner endocardium (FIGURE 18-3). The epicardium­ protects the heart by reducing friction and is the ­visceral por-tion of the pericardium on the surface of the heart. It consists of connective tissue and some deep adipose tissue. The middle layer, or ­myocardium, is made mostly of cardiac muscle tissue that is organized in planes and richly supplied by blood capillaries, lymph capillaries, and nerve fibers. It pumps blood out of the chambers of the heart. The endocardium is made up of squamous epithelium and connective tissue with many elastic and collagenous fibers. The endocar-dium rests on the inner myocardial surface, lining the chambers of the heart and covering the fibrous tissue that forms the heart valves. It is continuous with endo-thelial linings of the heart’s blood vessels.

Heart Chambers and Great Vessels

The four chambers of the heart are the rightatrium, right ventricle , left atrium, and left ventricle (Figure 18-3). Each chamber receives blood in ­different ways. The upper chambers are called atria and receive blood returning to the heart. The lower chambers are called ventricles and receive blood from the atria, which they pump out into the arteries.

The left atrium and ventricle are separated from the right atrium and ventricle by a solid wall-like struc-ture called septum. This keeps blood from one side of the heart from mixing with blood from the other side, except in a developing fetus. There are smooth sur-faces on the right atrium’s posterior wall and the inter-atrial septum. However, prominent ridges of muscles known as pectinate muscles or musculi pectinati are found on the anterior atrial wall and inner surface of the auricle.

The left atrium makes up most of thebase of the heart. Four pulmonary veins enter the left atrium, carrying blood from the lungs. The pulmonary veins are most easily seen in a posterior view of the heart. The blood passes from the left atrium into the left ventricle through the left atrioventricular (A-V)valve ormitral valve, which prevents it from flow-ing back into the left atrium from the ventricle. Like the tricuspid valve, the papillary muscles and chordae tendineae prevent the mitral valve’s cuspsfrom swinging back into the left atrium when the ventricle contracts. The mitral valve closes passively, directing blood through the large artery known as the aorta.

The left ventricle forms most of the apex and inferoposterior area of the heart. The left ventricle is thicker because it forces blood through the aorta to all body parts, which have a much higher resistance to blood flow. The cavity of the left ventricle is almost circular. The area where the septum separates the right and left ventricles is marked on the heart’s surface by the anterior interventricular sulcus. This groove cradles the anterior interventricular artery and con-tinues to become the posterior interventricularsulcus. This sulcus is visible on the posteroinferiorsurface of the heart.

The right atrium receives blood from two large veins called the superior vena cava and the inferiorvena cava and from a smaller vein called thecoro-nary sinus, which drains blood into the right atriumfrom the heart’s myocardium (FIGURE 18-4). The supe-rior vena cava returns blood from areas of the body that are superior to the diaphragm, whereas the infe-rior vena cava returns blood from areas of the body that are inferior to the diaphragm. The right atrium sends blood to the right ventricle through the tricus-pid valve. When each atrium is not filled with blood, its outer portion deflates to resemble a wrinkled flap. This expandable atrial appendage is also called an auricle. The auricle slightly increases blood volumein the atria.

The right ventricle forms most of the anterior sur-face of the heart. Its muscular wall is about three times thinner than that of the left ventricle, because it only pumps blood to the lungs, which have a low resistance to blood flow. The inner surface of the right ventricle contains a series of muscular ridges called trabeculaecarnae. The cavity of the right ventricle is flatter thanthat of the left ventricle, with a crescent shape and par-tially enclosing the left ventricle. As the right ventricle contracts, its blood increases in pressure to passively close the tricuspid valve. Therefore, this blood can only exit through the pulmonary trunk, which divides into the left and right pulmonary arteries that supply the lungs. At the trunk’s base is a pulmonary valve that allows blood to leave the right ventricle while preventing­ backflow into the ventricular chamber. The pulmonary valve contains three cusps. Before the pul-monary valve, the superior end of the right ventricle becomes tapered, forming a cone-shaped pouch called the conus arteriosus.

1. Explain the systemic and pulmonary circuits.

2. Where is the heart located?

3. Briefly explain the layers of the heart wall.

4. Why are the walls of the left ventricle thicker than

those of the right ventricle?

5. Distinguish between the visceral pericardium and the parietal pericardium.

Heart Valves

The blood flow through the heart is in one direc-tion because of the actions of the heart valves. The four heart valves open and close because of pressure ­differences in each chamber. The four heart valves are the two A-V valves and the two semilunar valves.

A-V Valves

The tricuspid valve and bicuspid or mitral valve are known as A-V valves because they lie between the atria and ventricles. They prevent backflow or regurgitation of blood from the ventricles to theatria while the ventricles contract. The tricuspidvalve, also known as the right A-V valve, has threeflexible projections called cusps and lies between the right atrium and ventricle. The cusps are actually flaps of endothelium that are reinforced by cores of connective tissue. This valve allows blood to move from the right atrium into the right ventricle while preventing backflow. The cusps of the tricuspid valve are attached to strong fibers called chordae tend-ineae, which originate from smallpapillary musclesthat project inward from the ventricle walls. These muscles contract as the ventricle contracts. When the tricuspid valve closes, the papillary muscles pull on the chordae tendineae to prevent the cusps from swinging back into the atrium.

The mitral valve is located between the left atrium and left ventricle. It is also called the bicuspid valve because it has two cusps. When the left atrium is filled with oxygenated blood, it pushes the mitral valve open, sending the blood into the left ventricle. ­Figure 18-4 shows various views of the heart valves.

Semilunar Valves

The pulmonary and aortic valves have half-moon shapes and are therefore referred to as semilunar valves. The pulmonary and aortic valves prevent backflow of blood from the aorta and pulmonary trunk back into their associated ventricles. They respond to pressure differ-ences, similar to the A-V valves. The semilunar valves are forced open, causing their cusps to flatten against the artery walls as blood moves past them. This occurs when the ventricles contract, increasing intraventricu-lar pressure above the pressure in the aorta and pulmo-nary trunk. When the ventricles are relaxed, the blood can then flow back toward the heart, filling the cusps and closing the valves.

At the base of the aorta is the aortic valve, which has three cusps. This valve opens to allow blood to leave the left ventricle during contraction and flow into the ascending aorta. Blood then flows through the aortic arch and into the descending aorta . When the left ventricle relaxes, the valve closes to prevent blood from backing up into the ventricle. The pulmo-nary valve is located in the right ventricle and opensto the pulmonary trunk to send deoxygenated blood to the lungs. Near each cusp of the aortic valve are sac-like structures called aortic sinuses, which prevent the cusps from sticking to the aortic wall as the valve opens. TABLE 18-1 summarizes the various heart valves.

Connective tissue arranged in “rings” surrounds the proximal ends of the pulmonary trunk and aorta, providing firm attachments for heart valves and mus-cle fibers. They prevent the outlets of the atria and ven-tricles from dilating during contraction. These rings, as well as other dense connective tissue masses, form the heart’s “skeleton.”

1. Identify the locations of the A-V and semilunar valves.

2. Explain the location and functions of the mitral valve.

3. Discuss the components that make up the “skeleton” of the heart.

4. Explain the function of the chordae tendineae and the papillary muscles.

Heart Circulation

The heart is nourished via the coronary circulation. This is the shortest circulation in the entire body and the functional blood supply of the heart. It is import-ant to remember that blood flows through the heart in only one direction, controlled by the four heart valves. The valves open and close as a result of differences in pressure on their two sides.

Coronary Arteries

The first two aortic branches are called the right and left coronary arteries. They supply blood to the heart tissues, with openings lying just beyond the aortic valve. The coronary arteries deliver blood when the heart is relaxed and have less function while the ven-tricles are contracting because they are compressed by the myocardium.

Both of these coronary arteries enclose the heart in the coronary sulcus and provide the arterial supply of the coronary circulation. The left coronary artery runs toward the left side of the heart, whereas therightcoronary artery runs toward the right side. The leftcoronary artery is divided into the anterior interven-tricular artery and thecircumflex artery. The anteriorinterventricular artery is also known as the left anteriordescending artery. It supplies blood to the anterior wallsof both ventricles and to the interventricular septum. This artery follows the anterior interventricular sulcus. The circumflex artery supplies the posterior walls of the left ventricle and the left atrium FIGURE( 18-5).

Coronary artery branches supply many capillaries in the myocardium. These arteries have smaller branches with connections called anastomoses between vessels that provide alternate blood path-ways. Known as collateral circulation, these pathways may supply oxygen and nutrients to the myocardium when blockage of a coronary artery occurs. The rightcoronary artery is divided into the right marginal artery and the posterior interventricular artery. The right mar-ginal artery serves the myocardium of the heart’s lateral right side. There also may be more than one marginal artery. The posterior interventricular artery supplies the posterior ventricular walls and runs to the heart apex. It merges oranastomosesnear the apex of theheart with the anterior interventricular artery.

Coronary Veins

The coronary veins are basically located along the same paths as the coronary arteries. They join to form the enlarged coronary sinus, emptying into the right atrium. On the posterior aspect of the heart, the coronary­ sinus can easily be seen. It empties into the great cardiac vein, middle cardiac vein,and small cardiac vein. Also, several anterior cardiac veins empty into the right atrium’s anterior portion. The pos-terior cardiac vein also empties into the great cardiacvein or coronary sinus. The coronary sinus empties directly into the right atrium. FIGURE 18-6 illustrates the pathway of blood to the heart tissues.

Heart Contraction

Some cardiac muscle cells are self-excitable, whereas each skeletal muscle fiber requires stimulation by a nerve ending to contract. Certain cardiac muscle cells are interconnected by intercalated discs, at which point interlocking membranes of nearby cells are linked by gap junctions and held together by desmosomes. The intercalated discs transfer the force of contractions from cell to cell and propagate action potentials. 

There are three major differences between the ways in which cardiac muscle and skeletal muscle contract. First, self -excitable cardiac muscle cells can also ­initiate the depolarization of the rest of the heart, spontaneously and with rhythm. This property is known as automaticity or autorhythmicity. Sec-ond, cardiac muscle has a property in which either all heart fibers contract as a unit or not at all. This is coordinated because all cardiac muscle cells are electrically linked together into one contractile unit by gap junctions. As a result, the wave of depolar-ization moves across the heart’s cells via ion passage along the gap junctions. All this differs from skel-etal muscle, in which impulses do not spread from cell to cell. Contraction of muscle fibers occurs from individual stimulation by nerve fibers. Contraction is usually with just some of the motor units being activated. Third, in cardiac muscle cells, the absolute refractory period lasts over 200 milliseconds (ms), which is almost as long as the contraction. This nor-mally prevents tetanic contractions from occurring, which would stop the heart from pumping. In skeletal muscle cells, contractions last only 15 to 100 ms and have brief refractory periods of 1 to 2 ms. The abso-lute refractory period is the period of no excitation,in which sodium ion channels remain open or are inactivated. As a result of the long refractory period, cardiac muscle cannot exhibit tetany.

Energy Requirements

Cardiac muscle needs much more oxygen for energy metabolism than skeletal muscle and also has more mitochondria. The heart needs aerobic respiration primarily. Therefore, without oxygen, cardiac muscle cannot operate very well for very long. Skeletal muscle differs in that it can contract for a long time via anaer-obic respiration and restore oxygen and fuel reserves via excessive oxygen consumption after exercise.

Glucose, fatty acids, and other fuel molecules are used by cardiac and skeletal muscle. However, because it is more adaptable, cardiac muscle easily changes metabolic pathways so it can use whatever nutrients are available. Such nutrients include lactic acid, which is generated by skeletal muscle activity. Therefore, a lack of oxygen is much more dangerous to the myo-cardium than any lack of nutrients.