One of the most essential and functional parts of the human body is the heart. Its size could be identical to that of a closed fist and it lies beneath the sternum and between the second and the sixth ribs. However, the exact position of it varies to some extent in each person.
The heart acts as the body’s pumping station, by which it pumps blood to the lungs and to the systemic arteries. It is surrounded by pericardium, a sac which is composed of outer (fibrous) and inner (serous) layers. Pericardial space, on the other hand, delicately protects and cushions the heart by having pericardial fluid to lubricate its inner layers. In addition, the wall of the heart has three layers, namely:
- Epicardium – It is a thin layer involving the outer portion of the heart as well as the visceral layer of the inner part of the pericardium.
- Myocardium – It is the muscular, the largest, and the middle portion of the heart’s wall. Its function is to contract properly to every heart beat.
- Endocardium – It is the innermost portion of the heart which contains endothelial tissue composed of small blood vessels and bundles of smooth muscle.
It is composed of four pumping chambers; the right atrium, left atrium, right ventricle, and the left ventricle. Both the right and left atria are low pressured chambers which serve mainly as reservoirs for blood. On the contrary, the ventricles are high pressured chambers, which produce sufficient load of force to maintain a normal arterial pressure. Furthermore, the four chambers have their specific functions which are listed below:
- Right Atrium – It receives deoxygenated blood going back to the body.
- Left Atrium – It receives oxygenated blood from the lungs.
- Right Ventricle – It receives blood from the right atrium and pumps the blood through the pulmonary arteries going to the lungs where the blood picks up oxygen and drops off carbon dioxide.
- Left Ventricle – It receives the oxygenated blood from the left atrium and pushes it towards the aorta and to the rest of the body.
The atrial septum separates the right and the left atria; to make sure that the blood going in and out of the atria does not mix up. A thick muscular structure called ventricular septum, on the other hand separates the two ventricles and contains the conduction tissue. This conduction tissue is important in the transmission of impulses.
In order to keep the flow of the blood in one direction, there are heart valves that open and close passively as the result of pressure changes in the four chambers. These include two types which are the atrioventricular and the semilunar valves. Atrioventricular (AV) valves pertain to bicuspid and tricuspid valves. Mitral or bicuspid valve can be seen between the left atrium and the left ventricle and it is composed of two leaflets of fibroelastic tissue overlap each other when the valve is closed. The tricuspid valve, in addition, is composed of three leaflets and separates the right atrium and the right ventricle. Its leaflets are fibrous in origin but thinner than those of that of the mitral valve. Semilunar valves comprised of the pulmonary and the aortic valves. The pulmonary valve lies between the right ventricle and the pulmonary artery while the aortic valve is positioned between the left ventricle and the aorta. The leaflets which keep the valves tightly closed anchor the heart wall through the help of chordate tendinae; which is controlled by papillary muscles.
The great vessels where blood goes and out of the heart are the following:
- Aorta – The main trunk of the systemic artery system and carries the blood away from the left ventricle.
- Superior and inferior vena cavae – They carry the deoxygenated blood from the body towards the right atrium.
- Pulmonary artery – It carries the blood away from the right ventricle and also carries the blood to the right and left lungs.
- Pulmonary veins – They are four of them, two on the left and two on the right which all carry oxygenated blood from the left and right lungs to the left atrium.
Coronary arteries supply the needed amount of blood to the heart. It can be seen on the heart’s surface. The coronary ostium is the opening in the aorta that leads blood to the coronary arteries. The right coronary artery is responsible for supplying blood to the right atrium and ventricle as well as some portion of the left ventricle. It also delivers blood to the bundle of His and the sinoatrial (SA) node. The SA node to be specific is essential during the establishment of rhythm of cardiac contractility. The left coronary artery lies along the left atrium which divides into two branches. These are the left anterior descending artery and the left circumflex artery.
Physiology of the Heart involves the electrical and the mechanical activities of the myocardial muscle. The electrical events are initiated by the sinus node which is affected by the autonomic nervous system; involving depolarization and repolarization. On the other hand, the mechanical events are regulated by the following: the preload (degree of stretch before contraction, afterload (resistance), contractility (force of contraction), and the heart rate.
In connection, the cardiac activity has two major phases: the systole referring to contraction and the journey of blood out of the heart and the diastole which is termed for the relaxation and filling of the ventricles. It begins when the deoxygenated blood from the superior and inferior vena cavae enters the right atrium which passes through the bicuspid valve down to the right ventricle. When the pressure in the right ventricle is greater than that of the pulmonary artery, the right ventricle contracts and the blood is delivered through the pulmonary artery going to the left and right lungs. There in the lungs, the blood picks up oxygen and drops carbon dioxide and goes back to the heart through the pulmonary veins, entering the left atrium. The oxygenated blood then passes the tricuspid valve towards the left ventricle. And lastly, it goes out through the aorta towards the rest of the body.