Although the human heart is only about 10 oz in an adult (about the size of a closed fist), it pumps 30x its weight/ minute and beats 100,000 times a day. It pumps 7000 liters/day; pumps 5 million times a year and in 70 years it will move 220 million quarts. If you turned on a typical faucet, you'd have to let it run for 45 years before approximating the amount of blood pumped in a lifetime. A normal human body has 60,000 miles of blood vessels; this is equivalent to a trip 2.5 times around the world. Your heart is a pump which matches its output to your body's needs: the amount of blood pumped per minute can be varied from 5 to 30 liters.

The heart is not located entirely on the midline of the body: 2/3 of its mass lies on the left half of the midline

The heart is composed of 4 chambers: a left and right atrium and a left and right ventricle. The cardiac muscle of the heart is depicted below.

heart muscle

The muscular wall of the heart (myocardium) is divided by septa. The 2 atria are separated from each other and from the ventricles. Atria are thin-walled because their contractions only have to deliver blood to the ventricles. The contractions of the right force blood to the lungs while the contractions of the left ventricle force blood throughout the body. Because the left ventricle must create greater pressure to accomplish this, the wall of the left ventricle is 2-4x as thick as that of the right. In section, the right ventricle looks like it's attached to the muscle wall of the left. A sheep heart is depicted below.

The valves of the heart prevent backflow and permit the flow of blood in one direction only. They open and close in response to pressure changes in heart chambers. Atrioventricular valves (AV valves) separate an atria from the ventricle beneath it and tendonlike chordae tendineae connect the ends of the cusps to papillary muscles in the ventricles. When ventricular pressure is low, AV valves open and blood flows into the ventricle but when the ventricle contracts, pressure increases and the cusps close; tendons stop from moving into the atria.

heart valvesheart valves
Semilunar valves separate each ventricle from the arteries leading form it; each are composed of 3 cusps. The pulmonary valve separates the right ventricle and the pulmonary trunk and the aortic valve separates the left ventricle and the aorta.


Deoxygenated blood from the tissues of the body arrive at the right atrium from the superior and inferior vena cava. This blood brings the carbon dioxide which was produced as a waste in the cells of the body, nutrients from the digestive tract from digested food, and hormones from glands.


The blood passes through the right atrium, past the tricuspid, into the right ventricle.


When the right ventricle contracts, blood is ejected from the heart through pulmonary arteries to the lungs. There, carbon dioxide leaves the blood and oxygene enters the blood.



Oxygenated blood returns to the heart through four pulmonary veins into the left atrium.


From the left atrium, blood passes past the bicuspid (mitral) valve into the left ventricle. When the left ventricle contracts, oxygenated blood is sent to the tissues of the body through the aorta.