The heart is a four-chambered organ with four main vessels, which either bring blood to or carry blood away from the heart. The four chambers of the heart are the right atrium, the right ventricle, the left atrium, and the left ventricle.
The great vessels of the heart include the superior and inferior vena cava, which bring blood from the body to the right atrium, the pulmonary artery, which transports blood from the right ventricle to the lungs. The last of the great vessels is the aorta, the body's largest artery, which transports oxygen-rich blood from the left ventricle to the rest of the body.
If we remove some of the tough fibrous coating of the heart and great vessels, you can get a better look at the heart beating. If you look carefully, you can see a series of one-way valves that keep the blood flowing in one direction. If we inject dye into the superior vena cava, you can watch it pass through the heart as it goes through the cardiac cycle.
The blood first enters the heart into the right atrium. Blood passes from the right atrium through the tricuspid valve and into the right ventricle. When the right ventricle contracts, the muscular force pushes blood through the pulmonary semilunar valve into the pulmonary artery. The blood then travels to the lungs, where it receives oxygen. Next, it drains out of the lungs via the pulmonary veins, and travels to the left atrium. From the left atrium, the blood is forced through the mitral valve into the critically important left ventricle. The left ventricle is the major muscular pump that sends the blood out to the body systems. When the left ventricle contracts, it forces the blood through the aortic semilunar valves and into the aorta. From here, the aorta and its branches carry blood to all the tissues of the body.
The papillary muscles keep the valve leaflets from flopping back into the atrium. The chords are designed to control the movement of the valve leaflets similar to ropes attached to the sail of a boat. When the RA is full, it contracts. This builds up pressure and pushes the tricuspid valve open. Blood now rushes from the RA into the right ventricle (RV). When the RV is filled, the walls of the ventricle begin to contract and the pressure within the RV rises. The increased pressure shuts the tricuspid valve and blood is pumped into the pulmonary artery (pronounced PULL-mun-narey) through the pulmonic valve (pronounced pull-MON-nick). The diagram below once again shows the four heart valves as viewed from the top of the heart, i.e., we are looking down at the two ventricles with the right atrium and left atrium removed.
The pulmonic valve is made up of three cusps or flexible cup-like structures, capable of holding blood. When the pressure in the right ventricle is low (as is the case when the RV is filling with blood) blood starts to move backward from the lungs toward the RV. The three cusps of the pulmonic valve fill with that blood and their sides touch each other, effectively shutting the valve. This prevents blood from leaking from the pulmonary artery into the right ventricle while the RV is filling. When the RV contracts to empty, the pressure within the RV rises above that of the pulmonary artery. This forces open the three cusps of the pulmonic valve and blood rushes through the pulmonary artery towards the lungs, where the red blood cells pick up oxygen.
The oxygenated blood from the lungs now returns to the left atrium (LA) via four tubes that are known as pulmonary veins (each draining a separate portion of the lungs). The pulmonary veins empty into the back portion of the LA. When the LA is completely filled it contracts. The mitral valve then opens, and blood is forced into the left ventricle (LV). When the LV is completely filled, it starts to empty its contents by contacting its walls. This increases pressure within the chamber, shuts the mitral valve and opens the aortic valve (AV, pronounced ey-OR-tick). The sequence is similar to that described for the RA, RV and pulmonic valve. The aortic valve also has three cusps.
The mitral and tricuspid valves open and the aortic and pulmonic valves shut while the ventricles fill with blood. In contrast, the mitral and tricuspid valves shut while the aortic and pulmonic valves open during ventricular contraction. This sequence ensures that the ventricles are filled to capacity before the ventricles start to pump blood and that the blood flows in only one direction.
After leaving the LV, blood now rushes through the aorta (pronounced a-OR-tah). The aorta is the main "highway" blood vessel that supplies blood to the head, neck, arms, legs, kidneys, etc. Blood is brought to these organs and limbs via branches that originate from the aorta. The cells within each part of the body pick up oxygen and nutrients from the blood. The oxygen-poor blood then returns to the RA, via the superior and inferior vena cava, and the beat goes on!!