Part 3. The Heart

The heart lies in the thoracic cavity, just behind the breastbone and between the lungs.

The heart is a pump consisting of four chambers: two upper chambers called atria, and two lower chambers called ventricles. It is a double pump synchronized very carefully.

Deoxygenated blood enters the heart through the venae cavae, superior and inferior. They bring blood to the right atrium. It contracts to force blood through the tricuspid valve into the right ventricle. The cusps of the tricuspid valve form a one-way passageway designed to keep the blood flowing only in one direction. As the right ventricle contracts to pump deoxygenated blood to the lungs through the pulmonary artery, the tricuspid valve stays shut, thus preventing blood from pushing back into the right atrium.

The newly oxygenated blood enters the left atrium of the heart from the pulmonary vein. The walls of the left atrium contract to force blood through the mitral valve into the left ventricle.

The left ventricle which has the thickest walls of all four heart chambers must pump blood with great force so that the blood travels through arteries to all parts of the body. The blood is pumped out of the left ventricle through the aortic valve and into the aorta which branches to carry blood all over the body. The aortic valve prevents the return of aortic blood to the left ventricle.

The four chambers of the heart are separated by muscular partitions called septa. They are interatrial septum and interventricular septum.

The heart wall is composed of three layers. The endocardium is a smooth layer of cells which lines the interior of the heart, and also is the material of which the valves of the heart are formed. The myocardium is the middle, muscular layer of the heart wall and is the thickest layer. The epicardium is a thin layer and forms the outermost layer of the heart wall. The pericardium is a delicate, double-folded membrane which surrounds the heart like a sac. It is attached to the breastbone in front and to the diaphragm below, while an inner portion of the membrane adheres to the heart.

There are two phases of the heartbeat, diastole and systole. Diastole is the relaxation phase of the heartbeat when the atria and ventricles fill with blood. Systole begins as diastole ends. Systole is the active contraction phase of the heartbeat when the ventricles pump blood out of the heart.

Primary responsibility for initiating the heartbeat (systole and diastole) rests with a small region of specialized muscle tissue in the posterior portion of the right atrium, where an electrical impulse originates. The region is called the sinoatrial node. It is also called the pace-maker of the heart.

The wave of electricity passes from the pace-maker to another region of the myocardium. This region is at the posterior portion of the interatrial septum and is called the atrioventricular node. This node sends the excitation wave along to a region deep in the ventricle wall, called the bundle of His. From there the electrical wave passes to all parts of the ventricles and stimulates them to contract, pumping blood from the heart.

The heartbeat can be regulated by nervous impulses from the autonomic nervous system (parasympathetic and sympathetic nerves).

Date: 2014-12-29; view: 1440