The muscle of your heart is called the myocardium. Most of the myocardium is located in the ventricles which are heart, has four chambers and four valves. Blood flows through the pig heart in the same manner as through a human's. This picture shows the pig heart from the front  with the portion on the right of the picture being the left side of the heart and vice versa.The aorta is clearly visible at the top, with an atrium on either side, while the ventricles are in the bottom left. A top view shows the aortic and pulmonary arteries as well as the pulmonary veins and superior vena cava.

The first incision is along the right ventricle, allowing us to see inside the right side of the heart. The right ventricle can be identified by squeezing the heart, since the myocardium on the right side is much less rigid than that of the left ventricle. This incision allows us to see the tricuspid valve and the right ventricular outflow tract which includes the pulmonary valve.

The right ventricle has been cut open from the apex of the heart (at the bottom) towards the top. In this picture, the myocardium is being held back. My finger is stuck underneath one leaflet of the tricuspid valve, which leads to the pulmonary valve.

The tricuspid valve allows blood to flow from the right atrium (above) into the right ventricle when the heart is relaxed during diastole (di-a-stol-ee). When the heart begins to contract the heart enters a phase called systole (sis-toll-ee), and the atrium pushes blood into the ventricle. Then, the ventricle begins to contract and blood pressure inside the heart rises. When the ventricular pressure exceeds the pressure in the atrium, the tricuspid valve snaps shut. The valve itself consists of three leaflets that are attached to the myocardium directly at the top. At the bottom, long thin fibers of collagen (a connective tissue protein) called chordae tendinae connect the leaflets to specialized heart muscles called papillary muscles. The chordae tendinae keep the valve leaflets in the right position so that they can close properly during systole.

Behind the posterior leaflet of the tricuspid valve is the right ventricular outflow tract. This leads up to the pulmonary valve and pulmonary artery. When the ventricles contract, blood is forced along the outflow tract and through the pulmonary valve. Then the blood flows to the lungs where gas exchange takes place.

When the heart is contracting during systole, the pulmonary valve is open because the blood pushes the cusps out of the way. However, at the end of systole, the ventricles begin to relax and the intra-ventricular pressure drops. When the ventricular pressure drops to below the pulmonary artery pressure, the pulmonary valve closes and preventsback-flow (called regurgitation) of blood into the ventricle.
In the two top views, the valve has been cut away from the top of the right ventricle by an incision through
the myocardium below the valve. The valve consists of three cusps, which are thin flaps of connective tissue. Because of the shape of the cusps, the pulmonary valve is described as being semi-lunar. The cusps look like little sacs that are attached to the wall of the pulmonary artery.


Basic Heart Physiology

The human heart is a four chambered pump, approximately the size of your two fists put together. It's located in the
middle of the chest, with the apex inclined slightly towards the left side. The heart is truly an incredible organ:

Total volume of blood pumped will be almost 200 million liters!
Normal pressures in the heart vary from 0 to 120 mm of mercury
During excercise, blood pressure commonly rises to 180 mm of mercury, this is equivalent to the pressure at the
bottom of a column of water 2.4 metres high.
All this and your heart weighs about a pound!

Blood Circulation in the Heart

Deoxygenated blood returns from the superior and inferior branches of the vena cava (see Figure), and drains into
the right atrium which is normally at a very low pressure. During diastole, the relaxation phase of the cardiac cycle,
the pressure in the right ventricle drops to near zero. The pressure gradient formed between the right atrium and
ventricle, plus slight contraction by the atrium, causes blood to flow into the ventricle. As the ventricle fills, the
blood passes through the tricuspid valve, pushing its leaflets aside. As systole, the pumping phase, starts, the
ventricle starts to contract, increasing intraventricular pressure. This causes the leaflets of the tricuspid valve to
snap shut, and the cusps of the pulmonary valve to open. Blood then flows out of the ventricle through the
pulmonary artery and on to the lungs. As the ventricle relaxes, intraventricular pressure drops below the pressure
in the pulmonary artery, causing the pulmonary valve to close. In this way, blood returns from the body to the right
side of the heart and is pumped to the lungs for gas exchange.

The story on the left side of the heart is practically identical to the right, but the names change. Oxygenated blood
from the lungs arrives at the left atrium from the pulmonary vein. During diastole, blood flows through the mitral
valve into the left ventricle. As systole begins, intraventricular pressure rises, shutting the mitral valve and opening
the aortic valve. Blood flows out the aorta to feed the systemic circulation. As the ventricle again begins to relax,
the aortic valve snaps shut and the ventricle begins to fill.

While the basic geometry and function of the two sides of the heart are very similar, they have different jobs. The
right supplies blood only to the lungs, while the left must pump blood to the rest of the body. It is not surprising that
the left side of the heart must develop pressures ten times higher than those on the right side. This must certainly
contribute to the fact that heart valves on the left side of the heart are most often affected by disease.