Facts about the heart
The heart is a hollow organ located in the thorax and between the lungs; it is above the diaphragm. Its shape is similar to a blunt cone. It is suspended by the great vessels, on the broader end, also called the base, which enables this portion of the heart to be directed upward, backward and to the right. The pointed end is called the apex and points downward, forward and to the left. In other words, the human heart is placed in the body in an oblique position, with the right side almost in front of the left.
The heart is an involuntary organ, which means that its action is not under our conscious control. Some individuals can voluntarily relax, causing the heart rhythm to slow. It is also possible to increase the heart rate when exercising, but the level of control is limited. We cannot stop it and cannot re-initiate it.
The heart wall is composed of three layers: the epicardium, the myocardium and the endocardium. The prefix "myo" means muscle. The prefix "endo" means "inside." The prefix "epi –" means "on, upon, at, by, near, over, on top of, toward, against, among." In this particular case, the closest meaning is "on top of."
The heart is composed of cardiac muscles, which are interconnected by junctional complexes called gap junctions. Each set of neighboring cardiac muscle cells may be connected by a large number of gap junctions, which physically lock the cells together and allow the transfer of electrical signals from cell to cell. The gap junctions among the whole heart ensure that all the cells in the heart will participate of the beating action.
A cardiac muscle excites itself and can generate an electrical impulse and contraction independent of the nervous system. Nerves that come to the heart are directly related to rate and strength of contraction, but the heart will beat in their absence. These nerves do not initiate the heartbeat. In contrast, skeletal muscles contract only when they receive a nerve impulse.
The Cavities of the Heart
The heart is divided into two halves by a muscular partition called the ventricular septum. This partition allows the heart to be divided into left and right sides, which after birth, do not communicate with each other. Both sides of the heart contract and relax almost simultaneously. The right side contains venous blood. The left side contains arterial blood. Each side is subdivided into two cavities of unequal size; the smaller, called the atrium, is on top, and the larger, called ventricle, is on the bottom.
The ventricle located on the left side propels blood at a higher pressure than the right ventricle, which does not really requires as much pressure, due to the fact that its action will be only applied on the shorter pulmonary circulation. The amount of blood pumped into the circulatory system will be analyzed in the section called "Math of the Heart."
Valves of the Heart
There are four valves in the heart, but the most famous are the tricuspid and the bicuspid valves, which are located between the atria and ventricles. The right atrio-ventricular valve is the tricuspid valve, which is formed by three flaps. The left atrio-ventricular valve is the bicuspid valve, which is formed by two flaps. The bicuspid valve is also called the mitral valve. In order to remember the position of these valves, we can imagine the number 32 written on our chest, meaning that the tricuspid valve is located on the right and the bicuspid valve is on the left.
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