How Does Blood Flow Through the Heart?
The Heart's Contractions
Almost everyone has heard the real or recorded sound of a heartbeat. When your heart beats, it makes a "lub-DUB" sound. Between the time you hear "lub" and "DUB," blood is pumped through your heart and circulatory system.
A heartbeat may seem like a simple, repeated event, but it's a complex series of very precise and coordinated events that take place inside and around your heart.
Each side of your heart uses an inlet valve to help move blood between the atrium and ventricle. The tricuspid valve does this between the right atrium and ventricle. The mitral valve does this between the left atrium and ventricle. The "lub" is the sound of the tricuspid and mitral valves closing.
Each of your heart's ventricles has an outlet valve. The right ventricle uses the pulmonary valve to help move blood into the pulmonary arteries. The left ventricle uses the aortic valve to do the same for the aorta. The "DUB" is the sound of the aortic and pulmonary valves closing.
Each heartbeat has two basic parts: diastole (di-AS-toe-lee), or relaxation, and atrial and ventricular systole (SIS-toe-lee), or contraction.
During diastole, the atria and ventricles of your heart relax and begin to fill with blood. At the end of diastole, your heart's atria contract (atrial systole) and pump blood into the ventricles. The atria then begin to relax. Next, your heart's ventricles contract (ventricular systole) and pump blood out of your heart.
The Pumping Action
Your heart uses its four valves to ensure your blood flows only in one direction. Healthy valves open and close in coordination with the pumping action of your heart's atria and ventricles.
Each valve has a set of flaps called leaflets or cusps. These seal or open the valves. This allows pumped blood to pass through the chambers and into your blood vessels without backing up or flowing backward.
Blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium.
Oxygen-rich blood returns from the lungs to your heart's left atrium through the pulmonary veins.
Atrial Systole (contraction)
As blood fills the heart's atrium, it contracts. This event also is called atrial systole.
Blood flows from your right atrium into your right ventricle through the open tricuspid valve. When the ventricles are full, the tricuspid valve shuts. This prevents blood from flowing backward into the atria while the ventricles contract.
Blood flows from your left atrium into your left ventricle through the open mitral valve. When the ventricles are full, the mitral valve shuts. This prevents blood from flowing backward into the atria while the ventricles contract.
Ventricular Systole (contraction)
As blood fills the heart's ventricles, it contracts. This event also is called ventricular systole.
The pulmonary valve opens and closes quickly. Blood is allowed to enter into your pulmonary arteries without flowing back into the right ventricle. This is important because the right ventricle begins to refill with more blood through the tricuspid valve. Blood travels through the pulmonary arteries to your lungs to pick up oxygen.
The aortic valve closes quickly to prevent blood from flowing back into the left ventricle, which is already filling up with new blood. Blood leaves the heart through the aortic valve, into the aorta and to the body. This pattern is repeated, causing blood to flow continuously to the heart, lungs and body.
How does blood flow through the lungs?
Once blood travels through the pulmonic valve, it enters your lungs. This is called the pulmonary circulation. From your pulmonic valve, blood travels to the pulmonary artery to tiny capillary vessels in the lungs.
Here, oxygen travels from the tiny air sacs in the lungs, through the walls of the capillaries, into the blood. At the same time, carbon dioxide, a waste product of metabolism, passes from the blood into the air sacs. Carbon dioxide leaves the body when you exhale. Once the blood is purified and oxygenated, it travels back to the left atrium through the pulmonary veins.