The body cannot survive more than a few minutes without a heartbeat. About 86,400 times each day, it beats and delivers vital oxygen- and nutrient-rich blood throughout the body. In order to understand how the heart works, you need to know about several important external and internal features.
The heart is slightly larger than a typical fist. When viewed from the front, the heart is triangular, although a cone shape or toy top is a better three-dimensional description. The heart consists of a base and an apex. The base is its broader top end, from which the great vessels enter and leave the heart. From the base, the heart narrows to a blunt, rounded point called the apex, which projects downward and toward the left. Within the chest, the bulk of the heart is positioned behind the sternum, and the apex extends a short distance to the left of the sternum (not coincidentally, right where you put your hand when you say the Pledge of Allegiance).
External view of the front surface of the heart.
External view of the back surface of the heart.
The heart has four chambers: two atria and two ventricles. Two atria are at the base of the heart. Beneath them are the two ventricles that extend toward the apex. On the surface of the heart separating these chambers are visible grooves, called sulci (or sulcus, when it’s singular).
The groove that separates the atria from the ventricles and completely encircles the heart is the atrioventricular groove, also known as the coronary groove because it “crowns” the base of the heart. On the front (anterior) and back (posterior) surfaces between the right and left ventricles are additional shallow grooves called the anterior and posterior interventricular grooves, respectively. These grooves are on top of the muscle wall (the interventricular septum) that separates these chambers. Within these grooves are the coronary arteries and their branches, as well as the cardiac veins.
The heart has its own mini-circulatory system. It has a pair of coronary arteries, a right one and a left one, that supply it with arterial blood. It also has a system of veins, called cardiac veins, that return venous blood to the right atrium.
Each coronary artery arises from the beginning of the aorta. The right coronary artery courses within the atrioventricular groove between the right atrium and the right ventricle. It gives off branches to portions of these two chambers. The right coronary artery continues within this groove onto the backside of the heart. It typically ends by turning downward and coursing into the posterior interventricular groove. When it does this, its name changes to the posterior interventricular artery. On this surface of the heart, it provides branches to the posterior portions of the right atrium, the right and left ventricles, and the posterior third of the interventricular septum.
After leaving the aorta, the left coronary artery courses to the left within the atrioventricular groove (which separates the left atrium from the left ventricle) for a short distance before dividing into two branches:
- the anterior interventricular artery (also known clinically as the left anterior descending artery) which courses downward within the anterior interventricular groove. It gives rise to branches that supply the major part (the anterior two thirds) of the interventricular septum and adjacent walls of the right and left ventricles.
- the circumflex artery which continues around to the backside of the heart within the atrioventricular groove. It typically gives off branches to the posterior wall of the left ventricle.
There are a variety of different branching patterns of the coronary arteries, depending on the person, but this is the most common pattern.
Coronary arteries and cardiac veins, front surface of the heart.
Coronary arteries and cardiac veins, back surface of the heart.
The main path for the return of venous blood from the heart is via the cardiac veins into the coronary sinus and the right atrium. There are several cardiac veins:
- Great cardiac vein. This vein ascends in the anterior interventricular groove, alongside the anterior interventricular artery, to the coronary groove. It follows in this groove to the back side of the heart, where it enters the coronary sinus.
- Coronary sinus. This is a wide venous channel in the coronary groove on the back of the heart. It receives the great, middle, and small cardiac veins. The coronary sinus ends as an opening into the right atrium.
- Middle cardiac vein. This vein begins in the lower part of the posterior interventricular groove (along with the posterior interventricular artery). It courses upward in this groove before entering the coronary sinus near its entrance into the right atrium.
- Small cardiac vein. This vein begins on the front surface of the heart along the lower margin of the right ventricle. It then enters the atrioventricular groove and courses within it (along with the right coronary artery) to the back side of the heart. There it enters the coronary sinus.
- Anterior cardiac veins. Usually two or three in number, these veins return blood from the right ventricle. They enter the right atrium directly.
Ultimately, venous return from the heart itself is into the right atrium. There the blood mixes with venous blood delivered from the rest of the body via the superior and inferior venae cavae.
Beneath the external grooves are internal partitions, or walls, that divide the interior of the heart into four chambers. Each of these chambers contains unique structures and features.
Chambers of the heart.
The heart is hollow and consists of four chambers: two upper chambers called atria (a right one and a left one) and two lower chambers called ventricles (right and left). The heart functions as a two-sided pump. The right atrium and ventricle work together to pump oxygen-poor blood to the lungs for oxygenation. The left atrium and ventricle pump oxygen-rich blood to the rest of the body. Here’s more detail on the function of these four chambers:
- Right atrium. Its interior wall is partly smooth and partly rough. The smooth portion receives the openings of the superior vena cava, the inferior vena cava, and the coronary sinus. The rough portion of this chamber is the right auricle (an “ear-like” projection). Its wall contains pectinate muscles. These are delicate, comb-like ridges of cardiac muscle. The interatrial septum is a thin wall of tissue between the right and left atria. The right atrium communicates with the right ventricle through the right atrioventricular opening and its right atrioventricular (AV) valve.
- Right ventricle. The wall of this chamber is thick and marked by irregular muscle ridges called trabeculae carneae (“thick, fleshy beams”). Extending from the walls are short, conical projections of cardiac muscle called papillary muscles. There are three of these papillary muscles (anterior, posterior, and septal) in the right ventricle. They correspond in name and number to the cusps of the right AV (or tricuspid) valve. At the tips of the papillary muscles are thin, tough tendons called chordae tendineae that connect a papillary muscle to the free edges of valve cusps. The papillary muscles keep the cusps from being pushed into the atria when the ventricles contract. They also maintain a constant tension on the cusps. This chamber opens to the beginning portion of the pulmonary artery (also known as the pulmonary trunk) at the pulmonary opening. This is where the pulmonary semilunar valves are located. The right ventricle is a low-pressure pump because it has to push blood only a short distance to the lungs.
- Left atrium. The wall of this chamber is similar to that in the right atrium. There is a larger smooth portion and a smaller rough portion containing the comb-like pectinate muscles in the left auricle. This chamber receives two pairs (right and left) of pulmonary veins that return oxygenated blood from the lungs. It opens to the left ventricle by the left atrioventricular opening and its left valve.
- Left ventricle. The wall of this chamber is about three times thicker than the wall of the right ventricle. This chamber is a high-pressure pump that pushes oxygenated blood to the rest of the body. Its internal surface is roughened by trabeculae carneae. It contains two papillary muscles (anterior and posterior) whose chordae tendineae attach to valves (more on these later). Leading out of the left ventricle is the aortic semilunar opening, which contains the aortic semilunar valves. The interventricular septum is mostly muscular, but has a thin, tough membrane portion near its upper end.
Layers of the heart.
The walls of each of the four heart chambers consist of three layers:
- Epicardium. This is the outer layer of the heart. It is covered by a special tissue called mesothelium. Mesothelium is a simple squamous epithelial tissue.
- Myocardium. “Myo” means “muscle.” This middle layer consists of cardiac muscle cells. It’s the part of the heart that contracts during heartbeats. The myocardium is thin in each atrium, but very thick (several millimeters) in the ventricles. The myocardium is three times thicker in the left ventricle than in the right ventricle because it must pump blood to the far reaches of the body.
- Endocardium. The internal layer of the walls. It lines the chambers and valves of the heart and is exposed to blood. The inner surface of the endocardium is lined by a special simple squamous epithelium called endothelium, which also lines the inner surfaces of all blood vessels.
Fibrous skeleton and valves of the heart.
Heart valves are located at the openings between the four chambers. There are also valves between the chambers and the arteries leaving the heart. They control the direction the blood flows within and out of the heart.
The valves are normally smooth, delicate, supple flaps of tissue. Their outer margins are attached to the fibrous skeleton of the heart. The fibrous skeleton consists of dense connective tissue. It is located between the atria and the ventricles. This tissue encircles and firmly connects each of the four valve openings and serves as attachment for part of the heart muscle.
There are two varieties of valves:
- Atrioventricular valves (AVs). These are active valves that require muscular activity (contraction of papillary muscles) to close them. These valves are open during ventricular diastole (when the ventricles are relaxed) and closed during ventricular systole (when the ventricles are contracting). The right AV valve has three cusps and is also called the tricuspid valve. The left AV valve has two cusps and is also known as the bicuspid or mitral valve.
- Semilunar valves. These valves are passive valves and do not require energy to close them. The aortic semilunar valves are located between the left ventricle and the aorta, whereas the pulmonary semilunar valves are located between the right ventricle and the pulmonary trunk. Each semilunar valve has three thin, floppy cusps that look like back pockets on a pair of jeans. At the center of each cusp is a small nodule of fibrous tissue. These cusps become back-filled with blood at the end of ventricular systole when the column of blood within the arteries loses energy and begins to fall back toward the ventricles. The nodules of the valves act like weights to ensure that.
Now that you've learned how the heart works to pump blood through your body, check out our Quick Guide Anatomy 101: The Cardiovascular System. Have fun learning!
From The Complete Idiot’s Guide to Anatomy Illustrated by Mark F. Seifert, Ph.D.