In the normal heart, oxygenated blood (red) enters the left atria, from the lungs, via the pulmonary veins. The oxygenated blood then drains from the left atria, through the mitral valve, into the left ventricle.
When the left ventricle contracts, the mitral valve is forced closed and blood is propelled through the aortic valve and onward to the rest of the body. Simultaneously, de-oxygenated blood (blue) is forced out of the right ventricle through the pulmonic valve and into the lungs.
The National Heart, Lung and Blood Institute provides excellent consumer learning resources for parents