Explain the path a red blood takes in the body

Let's follow the flow of blood beginning with a Red Blood Cell (RBC) erythrocyte in the Right Atrium.

The RIGHT ATRIUM holds the deoxygenated blood that has returned to the heart through the VENA CAVA. This blood is held until it can flow through the TRICUSPID VALVE into the RIGHT VENTRICLE. The RIGHT VENTRICLE contracts sending the blood up through the PULMONARY VALVE into the PULMONARY ARTERY. The PULMONARY ARTERY carries this deoxygenated blood away from the HEART through a series of PULMONARY ARTERIOLES which branch into the CAPILLARIES which surround the ALVEOLI in the LUNGS. The ALEOLI walls and CAPILLARY walls are very thin and allow for the rapid diffusion of carbon dioxide  ( C O2 )  into the ALVEOLI from the blood and Oxygen  ( O 2 )  into the blood from the ALVEOLI.

The now oxygenated now returns to the HEART by way of PULMONARY VENULES and the PULMONARY VEINS. The PULMONARY VEINS bring the blood to the LEFT ATRIUM where it is held until it can drop through the BICUSPID VALVE into the LEFT VENTRICLE. The LEFT VENTRICLE contracts sending the blood up through the AORTIC VALVE into the AORTA. The blood then branches off of the AORTA into ARTERIES. Arteries branch into smaller ARTERIOLES and then to CAPILLARIES that diffuse the oxygen into the tissues of the body and pick up the waste product carbon dioxide.

The deoxygenated blood begins its return to the heart in VENULES and then VEINS returning to the VENA CAVA which dumps the blood back in the RIGHT ATRIUM.