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The heart wall is composed of three layers of tissue (Fig. 5.11): pericardium, myocardium and endocardium.

Figure 5.11

Layers of the heart wall.

Pericardium

The pericardium is the outermost layer and is made up of two sacs. The outer sac consists of fibrous tissue and the inner of a continuous double layer of serous membrane.

The outer fibrous sac is continuous with the tunica adventitia of the great blood vessels above and is adherent to the diaphragm below. Its inelastic, fibrous nature prevents overdistension of the heart.

The outer layer of the serous membrane, the parietal pericardium, lines the fibrous sac. The inner layer, the visceral pericardium, or epicardium, which is continuous with the parietal pericardium, is adherent to the heart muscle. A similar arrangement of a double membrane forming a closed space is seen also with the pleura, the membrane enclosing the lungs (see Fig. 10.15, p. 243).

The serous membrane consists of flattened epithelial cells. It secretes serous fluid into the space between the visceral and parietal layers, which allows smooth movement between them when the heart beats. The space between the parietal and visceral pericardium is only a potential space. In health the two layers lie closely together, with only the thin film of serous fluid between them.

Myocardium

The myocardium is composed of specialised cardiac muscle found only in the heart (Fig. 5.12). It is not under voluntary control but is striated, like skeletal muscle. Each fibre (cell) has a nucleus and one or more branches. The ends of the cells and their branches are in very close contact with the ends and branches of adjacent cells. Microscopically these ‘joints’, or intercalated discs, are thicker, darker lines than the striations. This arrangement gives cardiac muscle the appearance of being a

sheet of muscle rather than a very large number of individual cells. Because of the end-to-end continuity of the fibres, each one does not need to have a separate nerve supply. When an impulse is initiated it spreads from cell to cell via the branches and intercalated discs over the whole ‘sheet’ of muscle, causing contraction. The ‘sheet’ arrangement of the myocardium enables the atria and ventricles to contract in a coordinated and efficient manner.

Figure 5.12

Cardiac muscle fibres.

Running through the myocardium is also the network of specialised conducting fibres responsible for transmitting the heart’s electrical signals. The myocardium is thickest at the apex and thins out towards the base (Fig. 5.13). This reflects the amount of work each chamber contributes to the pumping of blood. It is thickest in the left ventricle, which has the greatest workload.

Figure 5.13

Interior of the heart.

Fibrous tissue in the heart

The myocardium is supported by a network of fine fibres that run through all the heart muscle. This is called the fibrous skeleton of the heart. In addition, the atria and the ventricles are separated by a ring of fibrous tissue, which does not conduct electrical impulses. Consequently, when a wave of electrical activity passes over the atrial muscle, it can only spread to the ventricles through the conducting system that bridges the fibrous ring from atria to ventricles (p. 83).

Endocardium

This lines the chambers and valves of the heart. It is a thin, smooth, glistening membrane that permits smooth flow of blood inside the heart. It consists of flattened epithelial cells, and it is continuous with the endothelium lining the blood vessels.

Interior of the heart

The heart is divided into a right and left side by the septum (Fig. 5.13), a partition consisting of myocardium covered by endocardium. After birth, blood cannot cross the septum from one side to the other. Each side is divided by an atrioventricular valve into the upper atrium and the ventricle below. The atrioventricular valves are formed by double folds of endocardium strengthened by a little fibrous tissue. The right atrioventricular valve (tricuspid valve) has three flaps or cusps and the left atrioventricular valve (mitral valve) has two cusps. Flow of blood in the heart is one way;

blood enters the heart via the atria and passes into the ventricles below.

The valves between the atria and ventricles open and close passively according to changes in pressure in the chambers. They open when the pressure in the atria is greater than that in the ventricles. During ventricular systole (contraction) the pressure in the ventricles rises above that in the atria and the valves snap shut, preventing backward flow of blood. The valves are prevented from opening upwards into the atria by tendinous cords, called chordae tendineae, which extend from the inferior surface of the cusps to little projections of myocardium into the ventricles, covered with endothelium, called papillary muscles (Fig. 5.14).

Figure 5.14

The left atrioventricular valve: A. Valve open. B. Valve closed.

Flow of blood through the heart (Fig. 5.15)

The two largest veins of the body, the superior and inferior venae cavae, empty their contents into the right atrium. This blood passes via the right atrioventricular valve into the right ventricle, and from there is pumped into the pulmonary artery or trunk (the only artery in the body which carries deoxygenated blood). The opening of the pulmonary artery is guarded by the pulmonary valve, formed by three semilunar cusps. This valve prevents the backflow of blood into the right ventricle when the ventricular muscle relaxes. After leaving the heart the pulmonary artery divides into left and right pulmonary arteries, which carry the venous blood to the lungs where exchange of gases takes place: carbon dioxide is excreted and oxygen is absorbed.

Figure 5.15

Direction of blood flow through the heart.

Two pulmonary veins from each lung carry oxygenated blood back to the left atrium. Blood then passes through the left atrioventricular valve into the left ventricle, and from there it is pumped into the aorta, the first artery of the general circulation. The opening of the aorta is guarded by the aortic valve, formed by three semilunar cusps (Fig. 5.16).

Figure 5.16

The aorta cut open to show the semilunar cusps of the aortic valve.

From this sequence of events it can be seen that the blood passes from the right to the left side of the heart via the lungs, or pulmonary circulation (Fig. 5.17). However, it should be noted that both atria contract at the same time and this is followed by the simultaneous contraction of both ventricles.

Figure 5.17

Circulation of blood through the heart and the pulmonary and systemic circulations.

The muscle layer of the walls of the atria is thinner than that of the ventricles (Fig. 5.13). This is consistent with the amount of work they do. The atria, usually assisted by gravity, propel the blood only through the atrioventricular valves into the ventricles, whereas the ventricles actively pump the blood to the lungs and round the whole body.

The pulmonary trunk leaves the heart from the upper part of the right ventricle, and the aorta leaves from the upper part of the left ventricle.

Blood supply to the heart (the coronary circulation) Arterial supply (Fig. 5.18)

The heart is supplied with arterial blood by the right and left coronary arteries, which branch from the aorta immediately distal to the aortic valve (Figs 5.16 and 5.18). The coronary arteries receive about 5% of the blood pumped from the heart, although the heart comprises a small proportion of body weight. This large blood supply, especially to the left ventricle, highlights the importance of the heart to body function. The coronary arteries traverse the heart, eventually forming a vast network of capillaries.

Figure 5.18

The coronary arteries.

Venous drainage

Most of the venous blood is collected into a number of cardiac veins that join to form the coronary sinus, which opens into the right atrium. The remainder passes directly into the heart chambers through little venous channels.

Conducting system of the heart

The heart possesses the property of autorhythmicity, which means it generates its own electrical impulses and beats independently of nervous or hormonal control, i.e. it is not reliant on external mechanisms to initiate each heartbeat. However, it is supplied with both sympathetic and parasympathetic autonomic nerve fibres, which increase and decrease respectively the intrinsic heart rate. In addition, the heart responds to a number of circulating hormones, including adrenaline (epinephrine) and thyroxine.

Small groups of specialised neuromuscular cells in the myocardium initiate and conduct impulses, causing coordinated and synchronised contraction of the heart muscle (Fig. 5.19).

Figure 5.19

The conducting system of the heart.