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Anatomy 101: The Heart

Anatomy 101: The Heart

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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.

External Features of the Heart

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 front surface of the heart.

External view of the back 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.

Coronary Arteries

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:

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, front surface of the heart.

Coronary arteries and cardiac veins, back surface of the heart.

Coronary arteries and cardiac veins, back surface of the heart.

Cardiac Veins

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:

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.

Internal Features of the Heart

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.

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:


The walls of each of the four heart chambers consist of three layers: Epicardium, Myocardium, and Endocardium.

Layers of the heart.

The walls of each of the four heart chambers consist of three layers:

Heart Valves

Fibrous skeleton and valves of the heart.

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:

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.