Heart. An anatomic structure of heart

Heart of the person - hollow muscular body. Heart is divided by continuous vertical partition on two half: left and right. The second partition going in a horizontal direction forms four cavities in heart: the top cavities are auricles, bottom - ventricles. The weight of heart of newborns is on the average equal 20 g. Weight of heart of the adult person makes 0,425-0,570 kg. The length of heart at the adult person reaches 12-15 cm, the cross-section size is 8-10 cm, anteroposterior - 5-8 cm. Weight and the sizes of heart increase at some diseases (heart diseases), and also at people, long time engaged the intense physical work or sports. The wall of heart consists of three layers: internal, average and external. The internal layer is presented by endothelium environment (endocardium), which covers an internal surface of heart. The average layer (myocardium) consists of a cross-section-striped muscle. The muscles of auricles are separated from muscles of ventricles by connective tissue partition, which consists of dense fibrous fibers - a fibrous ring. The muscular layer of auricles is developed more poorly, than a muscular layer of ventricles that is connected with features of functions, which each department of heart is carried out. The external surface of heart is covered by a serous membrane (epicardium) which is internal leaf of pericardial pleura - pericardium. Under a serous environment the largest coronary arteries and veins which provide blood supply of tissues of heart, and also the big congestion of nervous cells and nervous fibers, innervated heart are located.

The pericardium (cardiac shirt) surrounds heart as a bag and provides its free movement. The pericardium limits a stretching of heart to blood filling it and is a support for coronary vessels. It is possible to distinguish two kinds of valves in heart - atrioventricular and semilunar. Atrioventricular valves settle down between auricles and corresponding ventricles. Semilunar valves separate an aorta from left ventricle and a pulmonary trunk from right ventricle.

In activity of heart it is possible to allocate two phases: a systole (reduction) and diastole (relaxation). Duration of various phases of cardiac cycle depends upon frequency of cardiac reductions. At more frequent cardiac reductions activity of each phase decreases, especially diastole.

During diastole of auricles atrioventricular valves are opened also the blood acting from corresponding vessels, fills not only their cavities, but also ventricles. During a systole of auricles ventricles are completely filled with blood. Thus return movement of blood in hollow and pulmonary veins is excluded. By the end of a systole of ventricles pressure in them becomes more then pressure in an aorta and a pulmonary trunk. It promotes opening semilunar valves, and blood from ventricles acts in corresponding vessels. During diastole of ventricles pressure in them sharply falls, that creates conditions for return movement of blood aside ventricles. Thus, opening and closing of valves of heart is connected with change of value of pressure in cavities of heart.

The cardiac muscle possesses the next properties: excitability, ability to spend excitation and contractility.

Cardiac muscle is less excited, than skeletal. For excitation of cardiac muscle it is necessary to apply stronger irritation, than for skeletal. It is established, that the size of reaction of a cardiac muscle does not depend upon force of irritations (electric, mechanical, chemical). The cardiac muscle is as much as possible reduced both on threshold, and to stronger on size irritation.

Conductivity. Waves of excitation are spent on fibers of a cardiac muscle and a so-called special tissue of heart with unequal speed. Excitation on fibers of muscles of auricles extends with a speed of 0,8-1,0 m/sec, on fibers of muscles of ventricles - 0,8-0,9 m/sec, on a special fabric of heart - 2,0-4,2 m/sec.

C ontractility. Contractility a cardiac muscle has the features. The first reduce muscles of auricles, then - papillary muscles and a layer of muscles of ventricles. In the further reduction covers also an internal layer of ventricles, providing movement of blood from cavities of ventricles in an aorta and a pulmonary trunk. Physiological features of a cardiac muscle is extended refractory period and automatism.

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