A Normal Heart
The heart has four chambers. The two lower pumping chambers of the heart are called the right and left ventricle, while the two upper filling chambers are the right and left atrium.
In the normal circulation, the blood that returns from the body through the superior and inferior vena cava and to the right atrium is low in oxygen. This blood passes through the tricuspid valve into the right ventricle, and then travels to the lungs, through the pulmonary valve and pulmonary arteries, to receive oxygen. The oxygen–rich blood returns through the upper and lower pulmonary veins into the left atrium, then moves to the left ventricle through the mitral valve.
The blood is then pumped out to the body from the left ventricle through the aortic valve and the aorta, which is a large blood vessel that carries oxygenated blood to the smaller blood vessels in the body.
The right and left atrium are separated by a middle wall, called the interatrial septum. The right and left ventricle are separated by a middle wall, called the interventricular septum.
The main function of the right heart (right atrium and right ventricle) is receiving the oxygen-poor blood coming from the upper and lower body into the heart and pumping it into the lungs so that it can get oxygenated.
The main function of the left heart (left atrium and left ventricle) is to receive oxygen-rich blood from the lungs and pump it into the body through the aorta.
There are four heart valves:
Tricuspid valve. This is the valve between the right atrium and right ventricle. When it opens it permits blood to move forward from the right atrium to the right ventricle. When it closes, it does not allow the blood to return back to the right atrium.
Pulmonary valve. This is one of the two semi-lunar valves. When the pulmonary valve opens, it permits blood to pass from the right ventricle into the pulmonary arteries (and then into the lungs).
Mitral valve. This atrial-ventricular valve is in between the left atrium and left ventricle. When the mitral valve opens it moves blood forward from the left atrium into the left ventricle. When it closes, it seals off and does not permit any leakage of blood back into the left atrium.
Aortic valve. This is the most important heart valve. It connects the left ventricle with the aorta.
The two great arteries are:
Pulmonary artery. This is the main artery coming off of the right ventricle. The main pulmonary artery bifurcates (forks) into the left and right pulmonary arteries so that the blood can be carried into the right and left lung.
Aorta. This is the main artery originating from the left ventricle. It carries oxygen-rich blood from the left heart to the body.
The main veins bringing oxygen-poor blood from the body into the right heart are called the superior vena cava and inferior vena cava.
The veins that carry oxygen-rich blood from the right and left lung into the left atrium are called pulmonary veins. There are four pulmonary veins. Two drain blood from the left lung while the other two drain the oxygen-rich blood from the right lung into the left atrium.