The human heart is a four-chambered muscular organ , shaped and sized roughly like a man's closed fist with two-thirds of the mass to the left of midline. The heart is enclosed in a pericardial sac that is lined with the parietal layers of a serous membrane. The visceral layer of the serous membrane forms the epicardium.
Three layers of tissue form the heart wall. The outer layer of the heart wall is the epicardium, the middle layer is the myocardium , and the inner layer is the endocardium. The internal cavity of the heart is divided into four chambers:. The two atria are thin-walled chambers that receive blood from the veins. This is because oxygen is removed from your blood as it circulates through your body's organs and tissues.
Once it has received oxygen, your blood returns directly to the left side of your heart, which then pumps it out again to all parts of your body. The left ventricle of your heart is larger and thicker than the right ventricle. This is because it has to pump the blood further around the body, and against higher pressure, compared with the right ventricle.
To make sure your blood flows in the correct direction, valves guard the entrance and exits of your hearts chambers. This publication is provided for education and information purposes only. It is not a substitute for professional medical care. The top chambers of the heart are called the left atrium and right atrium. The bottom chambers of the heart are the left ventricle and right ventricle, which have thicker walls. The heart itself needs oxygen-rich blood to function, which is supplied by the coronary arteries running along its surface.
A double-layered sac called the pericardium surrounds the heart, with the inner layer attached to the heart muscle and the outer layer attached by ligaments to your spinal column, diaphragm, and other parts of the body.
Fluid between the two layers of the pericardium allows it to move as the heart beats. Heart rhythm issues or other heart disease can occur when any part of this complex system stops functioning properly. Forgot Password? Enter an Access Code. Sign in via: Open Athens Shibboleth. Become a subscriber to watch this video. If you have received this message in error for example, you have purchased a new subscription or activated a trial , please log out and log back in to start a new session and receive access.
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