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 weight about 225g in women and is about 310g in men.
Position
The heart lies in the thoracic cavity in the mediastinum. 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.
Organs associated with the heart
Inferiorly - the apex rest on the central tendon of the diaphragm
Superiorly - the great blood vessels I.e. aorta, inferior vena cava, pulmonary artery and pulmonary vein.
Posteriorly - the oesophagus , trachea, left and right bronchus, descending aorta, inferior vena cava and thoracic vertebrae
Laterally - the lungs- the left lung overlaps the left side of the heart
Anteriorly - the sternum , ribs and intercostal muscles.
The heart wall
The heart wall is composed of three layers of tissue:
1- pericardium
2- myocardium
3- endocardium
Pericardium
The pericardium is the outermost layer and made up of two sacs. The outer sac consists of fibrous tissue and the inner of a continuous double layer of serous membrane.
The fibrous pericardium 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 great.
The outer layer of the serous pericardium, the parietal pericardium, lines the fibrous pericardium. The inner layer, the visceral pericardium, which is continuous with the parental 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, called pericardial fluid, into the space between the visceral and parietal layers, which allows smooth movement between them when the heat 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 pericardial fluid between them.
Myocardium
The myocardium is composed of specialised cardiac muscle found only in the heart. It is striated, like skeletal muscle, but is not under voluntary control. Each fibre has a nucleus and one or more branches. The end of the cells and their branches are in very close contact with the ends and branches are in very close contact with the ends and branches of adjacent cells.
Endocardium
This lines the chambers and valves of the of the heart. It is a thin, smooth membrane to ensure smooth flow of blood through 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, 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 valve are formed by double folds of endocardium strengthened by a little fibrous tissue. The right atrioventricular valve has three flaps and the left atrioventricular valve has two flaps. 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. They open when the pressure in the atria is greater than that in the ventricles. During ventricular systole 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 extends from the inferior surface of the cusps to little projections of myocardium into the ventricles, covered with endothelium, called papillary muscles.
Flow of blood through the heart
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 opening of the pulmonary artery is guarded by the pulmonary valve, formed by three semilunar cups. This valve prevents the back flow 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 exchanges of gasses takes place: carbon dioxide is excreted and oxygen is absorbed.
The muscle layer of the walls of the atria is thinner than that of the ventricles. This is consistent with the amount of work they do. The atria, usually assisted by gravity, pump the blood only through the atrioventricular valve into the ventricles, whereas the more powerful ventricles pump the blood to the lungs and round the whole body. The pulmonary trunk leaves the heart from the upper part of the right ventricles, and the aorta leaves from the upper part of the left ventricle.
Arterial supply
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. The coronary arteries receive about 5% of the blood pumped from the heart, although the heart comprises a small proportion of the body weight.
This large blood supply, of which a large proportion goes 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.
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 heart beat. However , it is supplied with both sympathetic and parasympathetic nerve fibres, which increase and decrease respectively the intrinsic heart rate. In addition , the heart responds to a number of circulating hormones, including adrenaline and thyroxine.
Small groups of specialised neuromuscular cells in the myocardium initiate and conduct impulses , causing coordinated and synchronised contraction of the heart muscle.
Sinoatrial node (SA node)
This small mass of specialised cells lies in the wall of the right atrium near the opening of the superior vena cava. The sinoatrial cells generate these regular impulses because they are electrically unstable. This instability leads them to discharge regularly, usually between 60 and 80 times a minute. Depolarisation is followed by recovery, but almost immediately their instability leads them to discharge again, setting the heart rate. Because the SA node discharges faster than any other part of the heart, it normally sets the heart rate and is called the pacemaker of the heart. Firing of the SA node triggers atrial contraction.
Atrioventricular node (AV node)
This small mass of neuromuscular tissue is situated in the wall of the atrial septum near the atrioventricular valves. Normally , the AV node merely transmits the electrical signals from the atria into a ventricles. There is a delay here; the electrical signal takes 0.1 of a second to pass through into the ventricles. This allows the atria to finish contracting before the ventricles start.
The AV node also has a secondary pacemaker function and takes over this role if there is a problem with the SA node itself, or with the transmission of impulses from the atria. Its intrinsic firing rate, however, is slower than that set by the SA node.
Nerve supply to the heart
The heart is influenced by autonomic nerves originating in the cardiovascular centre in the medulla obligata .
The vagus nerve supplied mainly the Stand AV node and atrial muscles. Vagal stimulation reduces the rate at which impulses are produced, decreasing the rate and force of the heartbeat.
Sympathetic nerves supply the SA and AV nodes and the myocardium of atria and ventricles, and stimulation increases the rate and force of the heartbeat.
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