Heart
describe the main factors determining heart rate and cardiac output.
The heart is a roughly cone-shaped hollow muscular organ. It is about 10 cm long and is about the size of the owner's fist. It weighs about 225 g in women and is heavier in men (about 310 g).
The heart lies in the thoracic cavity in the mediastinum (the space between the lungs). It lies obliquely, a little more to the left than the right, and presents a base above, and an apex below. The apex is about 9 cm to the left of the midline at the level of the 5th intercostal space, i.e. a little below the nipple and slightly nearer the midline. The base extends to the level of the 2nd rib.
Position of the heart in the thorax.
Inferiorly. the apex rests on the central tendon of the diaphragm
Superiorly. the great blood vessels, i.e. the aorta, superior vena cava, pulmonary artery and pulmonary veins
Posteriorly(Behind the heart). the oesophagus, trachea, left and right bronchus, descending aorta, inferior vena cava and thoracic vertebrae
Laterally(Beside the heart).the lungs the left lung overlaps the left side of the heart
Anteriorly(Infront of the heart).the sternum, ribs and intercostal muscles.
Structure
The heart wall
The heart wall is composed of three layers of tissue pericardium, myocardium and endocardium.
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
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. 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.
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 . 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.
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.
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.
The two largest veins of the body, the superior and inferior venae cavae, empty right atrium. This blood passes via the right atrioventricular valve into the right mere is pumped into the pulmonary artery or trunk (the only artery in the body genated blood). The opening of the pulmonary artery is guarded by the pulm by three semilunar cusps. This valve prevents the backflow of blood into the The ventricular muscle relaxes. After leaving the heart the pulmonary artery divide Pulmonary arteries, which carry the venous blood to the lungs where exchange carbon dioxide is excreted and oxygen is absorbed.