Need-to-know anatomy: blood flow through the healthy heart

Although the heart is a muscle, the primary pumping action of the heart is more like a spring that contracts and relaxes. When the spring is relaxed, the ventricles simultaneously fill with blood. When the spring contracts, the ventricles eject a portion of the blood to either the lungs or the rest of the body. That exact portion is called the ejection fraction and in a healthy heart is greater than 60 percent. Like a spring, if it is continually overextended, the ventricle will ultimately lose the ability to contract.

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The right side of the heart is responsible for pumping blood to the lungs. In a healthy heart, oxygen-poor blood enters the heart at the right atrium on the top floor. When the right atrium fills with blood, the atrium contracts slightly and the blood is gently pumped through the open tricuspid valve down into the right ventricle. When the right ventricle is filled with blood, the tricuspid valve closes to prevent blood from leaking back into the right atrium. The heart then contracts strongly and pumps the blood out through the open pulmonary valve into the pulmonary artery. The pulmonary valve then closes to prevent blood leaking back into the right ventricle. The blood briefly leaves the heart at this point to pick up oxygen in the lungs.

The oxygenated blood leaves the lungs and returns to the heart via the pulmonary veins, re-entering the heart at the left atrium, one of the upper chambers in the heart. The blood exits the left atrium through the mitral valve to flow into the left ventricle. The heart then contracts strongly, pumping the blood out through the aortic valve, the main exit door from the heart. The aortic valve keeps the oxygen-rich blood from leaking back into the left ventricle while maintaining flow into the aorta to deliver oxygen and nutrients to the rest of the body. The coronary arteries branch directly off the aorta and distribute the oxygenated blood to the heart itself. The left side of the heart is responsible for circulating blood to the entire body, powered by the left ventricle--normally the strongest pumping chamber in the heart where the pressures are greatest.