Blood Flow through Heart

The right atrium receives oxygen-poor blood from two major veins: the superior and inferior vena cava, which enter the atrium through separate openings. From the right atrium the blood passes through the tricuspid valve, which consists of three flaps, or cusps, of tissue. This valve directs blood flow from the right atrium to the right ventricle. It remains open during diastole, or ventricular filling; however, when the ventricle contrast, the valve closes, sealing the opening and preventing backflow into the right atrium. Fine cords attached to small muscles (Papillary muscles) on the ventricle’s inner surface prevent the valve’s flaps from being pushed backward (a similar arrangement can be seen on the left ventricle’s mitral valve). From the right ventricle blood is pumped through the pulmonary, or semi lunar, valve, which has three half-moon-shaped flaps, into the pulmonary artery. This valve prevents backflows from the artery into the right ventricle. From the pulmonary artery, blood is pumped to the lungs where it gives up carbon dioxide and receives oxygen, and then is returned to the heart’s left side through four pulmonary veins (two from each lung) to the left atrium and then through the mitral valve, a two flapped valve also called a bicuspid valve, to the left ventricle. As the ventricle contrasts, the mitral valve prevents backflow of blood into the left atrium, and blood is driven through the aortic valve into the AORTA, the major artery, which supplies blood to the entire body. The aortic valve, like the pulmonary valve, has a semi lunar shape and a unidirectional function.

Before birth and additional opening exists in the septum between the left and right atria of the fetal heart. This allows the blood to flow directly from the right to the left atrium without passing through the right ventricle and thus to the lungs, which do not yet function. After birth this opening closes. Additionally, in the fetal heart the ductus arteriosus, a bridge between the pulmonary artery and the aorta, allows most of the blood to bypass the collapsed fetal lungs. The ductus arteriosus atrophies shortly after birth.