The heart is a hollow pear-shaped muscular organ placed between the lungs in the middle of the chest that pumps blood through the body, supplying cells with oxygen and nutrients.
The heart in humans, quadrupeds, birds, and some reptiles is composed of four cavities, two auricles and two ventricles. It is enveloped in a membrane called the pericardium, and is situated toward the left of the cavity of the chest, between the lungs. With each beat the apex of the heart strikes against the wall of the chest in the space between the 5th and 6th ribs, a little below and to the right of the left nipple. The right auricle communicates with the right ventricle, besides which there are in it three openings, that of the vena cava inferior, that of the vena cava superior, and that of the coronary vein. The communication between this auricle and ventricle is closed by a valve when the ventricle contracts. The right ventricle communicates with the pulmonary artery, the opening into the artery being guarded by a valve formed of three flaps. When these are brought together they interrupt the communication between the ventricle and the artery. The left auricle communicates through a valved opening with the left ventricle, and contains the orifices of the four pulmonary veins. The left ventricle, besides the communication with the left auricle, contains the orifice of the aorta, also provided with a valve similar to that of the pulmonary artery.
The auricles and ventricle of one side are separated from those of the other by a complete muscular partition, the septum cordis. The valves at the openings of the arteries are called semilunar, that at the orifice of the right auricle tricuspid, that at the orifice of the left auricle mitral, and that at the orifice of the vena cava inferior the Eustachian valve.
The heart is formed of a firm thick muscular tissue, composed of fibres interlacing so as to form a figure of eight. It also contains nerves and vessels.
The arteries carry the blood from the heart to all parts of the body. They terminate in the capillary vessels, a series of extremely minute tubes which pass over into the veins.
The veins are the channels by which the blood passes back from the body to the right auricle of the heart. The blood which is returned from the veins is purplish red, from the presence of carbon dioxide and deficiency in oxygen, and is called venous; that which leaves the heart is bright red, being oxygenated, and is called arterial.
The venous blood parts with its carbon dioxide and receives new supplies of oxygen in the capillary system of the lungs, flows into the pulmonary veins, thence into the left cavities of the heart, thence it passes into the aorta, and is transmitted to all parts of the body, returning to the veins by the capillary system. It has now become venous, passes through the veins from the extremities towards the heart, receiving the chyle and the lymph, and is emptied into the right cavities of that organ, which returns it through the pulmonary artery to the capillary vessels of the lungs, where it is subjected to the influence of the air, resumes the qualities of red or arterial blood, and is ready for a new course.
The mechanism of the circulation is as follows: The blood contained in the two venae cavae is poured into the right auricle, which contracts, and thus forces the fluid to escape; but the venae cavae oppose to its backward passage the column of blood which they contain, and it must therefore pass into the right ventricle. The ventricle then contracts, and the tricuspid valve closing the passage through which the liquid entered, it is forced into the pulmonary artery, along which it must flow (return to the ventricle being prevented by the semi-lunar valve) into the capillary system of the lungs, whence it passes into the pulmonary veins, which pour it into the left auricle by four orifices.
The contraction of the auricle impels it into the left ventricle, by which it is driven forward into the aorta (the mitral valve preventing its return into the auricle), and thence into the general circulation. The two auricles contract and relax simultaneously with each other, as do also the two ventricles. The relaxation is called diastole; the contraction systole.
The quantity of blood projected at each systole is generally estimated at six ounces. The causes of the alternate contraction and relaxation are entirely involuntary and dependent on the nervous system to a large extent. The systole of the ventricles is the cause of the motion of the blood in the arteries, which dilate with each wave driven into them.
The heart is the seat of various and generally dangerous diseases. One of these is pericarditis or inflammation of the pericardium, the double lining membrane or bag enveloping the heart. The cause of this disease may be exposure to cold, or an injury, or it may be complicated with other diseases. Inflammation of the inner lining is termed endocarditis.
Valvular disease is a common affection of the heart, the valves becoming thickened, contracted, rigid, or otherwise affected, so that they cannot properly perform their duty. The mitral valve, for instance, may become too narrow and contracted, and the result is that all the blood does not pass into the aorta. In other cases of valvular disease, the same result follows, that is imperfect depletion of the ventricles and auricles, the return of blood being termed regurgitation. The heart consequently becomes weakened, while the entire system suffers.
Overgrowth or hypertrophy and dilatation are frequent results of valvular disease. The use of digitalis is often successful in strengthening and soothing the heart. Certain diseases produce atrophy, in which the heart becomes feeble in action, while fatty degeneration occurs, when the muscular fibres are replaced by oleaginous particles. Research Heart
The vena cava is a large vein that runs along the right side of the body. It is divided into two portions, the superior vena cava and the inferior vena cava. The superior vena cava is the great vein that returns the blood from the head, arms and upper body to the right atrium of the heart. The inferior vena cava, which is larger than the superior vena cava, returns blood from the lower body and legs. Research Vena Cava
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Heart
