Where is the heart located, and what its parts?
The heart is located under the rib cage -- 2/3 of it is to the left of your breastbone (sternum) -- and between your lungs and above the diaphragm. It is about the size of a closed fist, weighs about 10.5 ounces and is somewhat cone-shaped. It is covered by a sack termed the pericardium or pericardial sack. The normal heart anatomy consists of a four-chambered, hollow organ. It is divided into the left and right side by a muscular wall called the septum. The right and left sides of the heart are further divided into two top chambers called the atria (also termed the right and left atrium), which receive blood and then pump it into the two bottom chambers called ventricles, which pump blood to the lungs and to the body.
The coronary arteries are on the heart surface (left main, right coronary). The coronary arteries and veins comprise the heart’s own mini-circulatory system. Two major coronary arteries branch off from the aorta near the point where the aorta and the left ventricle meet:
- Right coronary artery supplies the right atrium and right ventricle with blood. It branches into the posterior descending artery, which supplies the bottom portion of the left ventricle and back of the septum with blood.
- Left main coronary artery branches into the circumflex artery and the left anterior descending artery. The circumflex artery supplies blood to the left atrium, side and back of the left ventricle, and the left anterior descending artery supplies the front and bottom of the left ventricle and the front of the septum with blood.
- These arteries and their branches supply all parts of the heart muscle with blood.
Normal heart anatomy and physiology need the atria and ventricles to work sequentially, contracting and relaxing to pump blood out of the heart and then to let the chambers refill. When blood leaves each chamber of the heart, it passes through a valve that is designed to prevent backflow of blood. There are four heart valves within the heart:
- Mitral valve between the left atrium and left ventricle
- Tricuspid valve between the right atrium and right ventricle
- Aortic valve between the left ventricle and aorta
- Pulmonic valve (also called pulmonary valve) between the right ventricle and pulmonary artery
The heart valves work the same way as one-way valves in the plumbing of your home. They prevent blood from flowing in the wrong direction. Each valve has a set of flaps, called leaflets or cusps. The mitral valve has two leaflets; the others have three. The leaflets are attached to and supported by a ring of tough, fibrous tissue called the annulus. The annulus helps to maintain the proper shape of the valve. The leaflets of the mitral and tricuspid valves are also supported by tough, fibrous strings called chordae tendineae. These are similar to the strings supporting a parachute. They extend from the valve leaflets to small muscles, called papillary muscles, which are part of the inside walls of the ventricles.
The endocardium is the membrane composed of epithelial cells that line the heart chambers and valves. It provides a slick surface so that red blood cells, platelets and other substances in blood will not stick to the heart’s inner surface. It also contains Purkinje fibers (specialized muscle cells that can transmit electrical impulses that can cause heart muscle contraction) and collagen fibers to make the endocardium elastic.
In addition, a cluster of cells that are located in the upper right atrium is termed the SA (sinoatrial node or pacemaker), which generates electrical impulses. These impulses move down cells toward the AV node (atrioventricular node), another cluster of cells located near the center of the heart between the bottom of the right atria and the top of the ventricles. The AV node pauses the electrical impulse long enough to have the atria fully contract (squeeze blood out into the ventricles); then it allows the impulse to go into cells termed the bundle of His to the ventricles that split into the right and left bundle branches in the ventricles. The electrical impulse finally reaches Perkinje fibers and then cause the ventricles to contract to push blood into the lungs and aorta. The heart rate (pulse) and blood pressure are generated by ventricular contractions; the SA node impulse rate is influenced by the body’s autonomic nervous system. At rest, a normal heart beats around 50 to 99 times a minute. Exercise, emotions, fever, and some medications can cause your heart to beat faster, sometimes to well over 100 beats per minute.