In this section...
Structure of Heart
From the right atrium, blood collects into the right ventricle. The AV node, located in the lower wall of the right atrium sends a signal via the bundle of His to the Purkinje fibers that make the ventricles contract and make blood flow from the right ventricle through the pulmonary semilunar valve to the pulmonary artery. Like all other valves 1, the pulmonary semilunar valve prevents blood from moving backward.
Note that this is the only artery that caries deoxygenated blood. This artery takes the blood to the lungs. In the lungs, membrane surface area is increased with the presence of cellular sacs called the alveoli. These tiny sacs contain capillaries that serve to oxygenate blood with inspiration and release carbon dioxide with expiration.
After gas exchange occurs (CO 2 exchanges with O 2), the blood comes back to the heart via the pulmonary vein.
The blood, now oxygenated, collects in the left atrium. When the right atrium is contracted, the left atrium is contracted simultaneously. The blood passes through the mitral or bicuspid valve to the left ventricle.