Heart (сердце) – the central organ of the circulatory system in animals and humans, forcing blood into the arterial system and ensuring its movement through the vessels.
Circulatory system by cells and tissues of the body supplied with nutrients and oxygen, and removed from the products of metabolism. Therefore, the circulatory system is sometimes called the transport or distribution, system.
In humans, the heart is located in the thoracic cavity asymmetrically: 1/3 it is the right of the median plane of the body, 2/3 – on the left. The base of the heart facing up, back and to the right; tip – down, forward and to the left.
The rear surface of the heart adjacent to the diaphragm. Since it is surrounded on all sides with light, except for the front surface immediately adjacent to the chest wall (Fig. 1):
Fig. 1. Projection of the heart, the valves and large vessels on the anterior chest wall (semi-schematic): 1 – the trachea; 2 – right common carotid artery; 3 – brachiocephalic trunk; 4 – subclavian artery; 5 – subclavian Vienna; 6 – opening of the aorta (aortic semilunar valves); 7 – right atrioventricular orifice (tricuspid valve); 8 – external carotid artery; 9 – internal jugular Vienna; 10 – the thyroid gland; 11 – left brachiocephalic Vienna; 12 – aortic arch; 13 – pulmonary trunk; 14 – bronchi; 15 – hole pulmonary trunk (pulmonary artery valve); 16 – left atrioventricular orifice (mitral valve); 17 – apex of the heart.
In adults, the heart is the length of 12-15 cm, the transverse dimension of 8-11 cm anteroposterior size of 5-8 cm. The weight of the heart 220 – 300 g, is 1/215 of the body weight in men and 1/150 part – in women. Atrium – cavities, perceiving the blood from the veins. In the right atrium fall lower and upper vena cava carrying venous blood from the systemic circulation and the veins of the heart (including coronary sinus), the left – 4 pulmonary veins, which flows arterial blood from the lungs, oxygenated. Both are connected to the ventricles atrium atrioventricular openings which are closed in reducing ventricular flap valve. The inner surface of the crossbar and the ventricles are conical projections, called papillary muscles. From the tops of these muscles to the free edges of the flaps atrioventricular valves stretch tendon strings, preventing vyvёrtyvaniyu valve leaflets toward the atria.
At the base of the pulmonary trunk and aortic valve located pulmonary trunk and aortic valve. These valves consist of three semilunar cusps’s opening towards the respective vessels (Fig. 2), resulting in reductions of blood from the right ventricle of the heart enters the pulmonary trunk and the left of – the aorta (Fig. 3).
Fig. 2. Right (tricuspid) and left (mitral) atrioventricular valves. The valves of the aorta and pulmonary trunk (cross-sectional removed pulmonary trunk and aorta atrium; top view): 1 – pulmonary valve stem (crescent-shaped pockets); 2 – left annulus; 3 – left ventricle; 4 – left atrioventricular valve (mitral); 5 – aortic valve; 6 – Right annulus; 7 – right ventricle; 8 – right atrioventricular valve (tricuspid).
Fig. 3. Aortic valve (part of the wall of the left ventricle and the aorta bulbs cut and deployed): 1 – right coronary artery; 2 – aortic valve; 3 – the left ventricle (opened and deployed); 4 – papillary muscle; 5 – aorta (opened and deployed); 6 – left coronary artery; 7 – left atrioventricular valve; 8 – tendon strings.
The wall of the heart consists of three layers: the inner – endocardium, medium – infarction and outdoor – epicardium. Endocardium lines the cavity of the heart, is built of connective tissue containing collagen, elastic fibers and smooth muscle, blood vessels and nerves. On the free surface is covered with endocardium endothelium. Heart valves are the folds of endocardium. Myocardium – the thickest shell, divided into 2-3 layers. In atrial thickness reaches 2.3 mm in the right ventricle – 5.8 mm, left – 10-15 mm. The difference in thickness is associated with different functional load.
The myocardium is composed of striated muscle cells – myocytes. Their length varies from 50 to 120 microns, the width is equal to 15-20 microns. In the central portion 1-2 located myocyte nucleus. Contractile elements – myofibrils occupy the peripheral part of the sarcoplasmic. The ability of the heart to the continuous operation associated with mitochondria contained in myocytes – native enzymes involved in redox processes that provide cells with energy. Between adjacent myocytes are intercalated discs in which are combined into myocytes muscle fibers (Fig. 4):
Fig. 4. Schematic representation of the intercalated disks of cardiac muscle of mammals: A – a place close contact cell membranes of myocytes (nexus); B – the lateral edge portion of the muscle fiber, which have intimate contact between the membranes myocytes: gap separating membrane, according to the extracellular space.
Through the intercalated disks held excitation from one cell to another. Muscle fibers as atrial and ventricular start fibrotic heart rings surrounding the atrio-ventricular openings. Muscles of the atria, ventricles detached from the muscles, consists of 2 layers: the outer circular and deep longitudinal fibers which in folded cover the mouth of the vena cava draining into the atrium.
Muscles of the ventricles has three layers: the outer and inner – longitudinal, transverse between them – circular (Fig. 5).
Fig. 5. Muscle layers of the heart; left (deleted portions of the myocardium to show the location of its layers, pulmonary trunk and the aorta removed at their base): 1 – the external oblique layer; 2 – middle circular layer; 3 – deep longitudinal layer.
Partition between the ventricles constructed mainly of muscular tissue and it leaves the endocardial lining, except for the upper portion, where the ventricles are separated by only two sheets with a layer of fibrous endocardial tissue therebetween. At the heart of education contained atypical muscle tissue, the cells of which are poor and rich sarcoplasm myofibrils. The tissue forms the so-called cardiac conduction system made up of the sinoatrial node, located in the wall between the right atrium and the superior vena cava right eye; atrioventricular node, located in the wall between the atria of the right atrioventricular valve; atrioventricular bundle branch coming from the atrioventricular node in the interventricular septum. Bundle of His divides into right and left legs branching in the ventricular myocardium as the Purkinje fibers. The cells of the conduction system generate rhythmic excitation pulses and transmit them to the first atrial myocardium, and then on the myocardium of the ventricles, causing a reduction in series of chambers of the heart. Tightly to the epicardium and myocardium consists of connective tissue. Free its surface is covered mesothelium. At the base of the heart epicardium wrapped and goes into the pericardial sac – pericardium. Between the epicardium and pericardium is slit-like cavity containing a small amount of serous fluid friction reducing heart wall while it is running.
Blood supply to the heart is done right and left coronary arteries extending from the ascending aorta. Large veins of the heart are collected in coronary sinus, which flows into the right atrium, which coincide, moreover, and small veins. In the heart there is a dense capillary network, each muscle fiber is accompanied by capillaries. Lymph from the heart and mediastinal drains into the left tracheobronchial nodes.
Innervate the heart of wandering and sympathetic nerves. Located within the heart intracardiac ganglia containing the so-called efferent nerve cells that transmit impulses from them suitable fibers of the vagus nerve on the myocardium and coronary vessels. Furthermore, in the ganglia and heart are sensitive (afferent) nerve cells shoots which form closure sensitive devices (receptors) on the coronary vessels and the myocardium. These cells are in contact with intracardiac efferent neurons, forming intracardiac reflex mechanisms. (O.N. Aksenova)