The heart is a part of the circulatory system, and is considered one of the most vital organs of the human body. It is about the size of a fist, has a mass of approximately 300 grams, and its main function is to contract and produce enough pressure to circulate blood throughout the body. The top of the heart where the veins and arteries all connect is called the base. The pointed lower tip of the heart is called the apex. The coronary sulcus is a deep groove that runs horizontally across the surface of the heart and separates the atria from the ventricles.

The heart is further divided into four inner chambers. The two upper chambers are the left atrium, which receives oxygenated blood; and the right atrium, which receives deoxygenated blood. The two lower chambers are the left ventricle, which sends blood to the body cells; and the right ventricle, which sends blood to the lungs. The interatrioventricular septum is a thick muscle separating the left and right sides of the heart, making them functionally separate. There are valves known as atrioventricular valves that separate each atrium from its corresponding ventricle. They are made of cusps of fibrous tissue that prevent the backflow of blood, and they are attached to the connective tissue in the heart called the chordae tendinae and further to the papillary muscle. The tricuspid valve separates the right atrium from the right ventricle with three cusps. The mitral valve (or bicuspid valve) separates the left atrium from the left ventricle with two cusps. There are also two valves made up of fibrous tissue that cover the openings of two major arteries. The pulmonary semilunar valve separates the right ventricle from the pulmonary arteries and ensures that there is no backflow of blood back into the right ventricle. The aortic semilunar valve is located between the left ventricle and the aorta and ensures that the oxygenated blood does not leak back into the left ventricle. The veins of the cardiovascular system carry blood to the heart, while the arteries carry blood away from the heart. The lumen, or openings, in the vessels provide the resistance needed to allow the blood to flow. The precapillary sphincters are made up of smooth muscle cells and constrict or dilate to help control blood flow.

The entire heart is protected by a sac called the pericardium. It has a double membrane of pericardial fluid, which nourishes the heart and prevents it from shocks. The outer layer is called the fibrous pericardium, and its job is to protect the heart, anchor it to surrounding structures, and prevents the heart from over-filling itself with blood. The inner layer provides a smooth lubricated sliding surface for the heart to move against when it contracts and in response to the movement of other structures, such as the diaphragm and lungs. Directly within the pericardial sac is the heart wall, which is made up of three layers. The outer layer is called the epicardium and consists of connective tissue and epithelial cells. The middle layer of the heart, called the myocardium, is made up of cardiac muscle tissue, which allows the heart to contract, as well as blood vessels and nerves. The inner layer is called the endocardium and consists of simple squamous epithelial cells that are in contact with the blood. The left ventricular wall has a much thicker myocardium than the other parts of the heart, giving it enough power to push the blood through the aorta and into the rest of the body.

The circulation of blood begins with the pulmonary circuit. Blood flows into the heart through the superior and inferior vena cavae, which are two large veins located near the back of the heart. They both pour into the right atrium, and then the right ventricle. It is then pumped through the pulmonary valve, through the pulmonary trunk, and along the pulmonary arteries. It then moves into the capillaries of the lungs, and returns back to the heart through the pulmonary veins into the left atrium. This entire circuit sends deoxygenated blood to the lungs, where it drops off carbon dioxide and picks up oxygen. The second part of circulation is called the systemic circuit. The blood moves to the left ventricle and is then pumped up through the ascending aorta, through the aorta, and out to the capillaries of the body. It then comes back through the vena cavae. This circuit delivers oxygen to the cells of the body and picks up carbon dioxide produced as waste products by the cells.

The heartbeat is the atria contracting, following with the ventricles contracting. There are two types of cells that work together to cause the heart to contract. The contractile cells are the normal cardiac muscle cells that pump the blood. The specialized non-contractile muscles cells are further divided into two types: nodal cells and conducting cells. The nodal cells establish the rate of contraction. The sinoatrial node is located on the back upper wall of the right atrium and is known as the heart’s pacemaker. A normal heartbeat is approximately 60-90 beats per minute. This rate can be adjusted by the autonomic nervous system. The atrioventricular node is located on the bottom of the right atrium, and it acts as the backup pacemaker if the SA node is damaged. It also delays the signal for 0.1 seconds to allow the atria time to finish contracting before the ventricles begin to contract. The conducting cells send signals throughout the heart. The AV bundle and bundle branches send signals through the interventricular septum and into the ventricular walls. The Purkinje fibers send the signal to all the other ventricular cardiac muscle cells.

The cardiac cycle is defined as the time between the start of one heartbeat and the start of the next heartbeat. During one cycle, both the atria and ventricles contract and relax. Contraction is referred to systole when a chamber forces blood into another chamber or vessel. Relaxation is referred to as diastole when a chamber fills with blood. A cardiac cycle begins with atrial systole. At this point, it should be noted that the ventricles are filled about 70 percent with blood. This stage fills the ventricles completely as the atria contract and force the blood down. When atrial systole ends, both atrial diastole and ventricular systole begin. During this time the AV valves close and the semilunar valves open so blood can flow through to the arteries. When ventricular systole ends, ventricular diastole begins. The semilunar valves close and the AV valves reopen so blood can flow into the ventricles. When both the atria and the ventricles are in diastole, blood can flow from veins into atria, and then into the ventricles.

The heart sounds are associated with the cardiac cycle. The first sound a person can hear through a stethoscope is the “lubb,” which is produced at the beginning of ventricular systole when the AV valves close and the semilunar valves open. The second sound is the “dupp,” which is produced at the beginning of ventricular diastole when the semilunar valves close. There are two other heart sounds produced, but they are not generally heard through a stethoscope. When a sphygmomanometer is used to take a person’s blood pressure, the clinical professional is measuring the peak force of the vessels during systole (which produces the top number of a blood pressure reading) and diastole (which produces the bottom number of a blood pressure reading).

An arrhythmia is any cardiac electrical rhythm that is abnormal. Normal rhythm is called sinus rhythm. Tachycardia is defined as a rapid heartbeat, usually over 100 beats per minute. Bradycardia is defined as a slow heartbeat, usually less than 50 beats per minute. A common cardiac problem is commonly known as a heart attack. Clinically, a heart attack is called a myocardial infarction, which means death of the muscle cells in the heart. This is caused by prolonged cardiac ischemia, which is when oxygen is not able to reach the tissue of the heart, usually due to a blocked artery. Angina is chest pressure that results from myocardial ischemia, and is usually indicative of a future MI.

Image Caption: Anatomy of the human heart, in English, by Ties van Brussel. Credit: Tvanbr/Wikipedia