New blood entering the right atrium is pumped by the tricuspid valve, to the right ventricle.  Then, the pulmonary valve opens to the pulmonary artery.  This artery has to carry the blood to the lungs to get oxygen.  This is the only artery in the body that carries deoxygenated blood.  Once the blood reaches the lungs, carbon dioxide is diffused into the lungs. Carbon dioxide is a cell's waste product after using oxygen. Then the blood must get its oxygen.  This is where the circulatory system and the respiratory system intertwine.

     The oxygen in the lungs is diffused through the alveoli sacs and then through the wall of the lungs into the bloodstream.  The blood carries the oxygen to the various cells in the body.  To get the oxygen to the actual cells, the arteries branch off into smaller arterioles.  These even branch off to capillaries, the smallest of blood vessels.  Their walls are extremely thin and elastic.  In these vessels, the red blood cells must travel single file to pass through.  The oxygen is released from the hemoglobin and diffuses across the capillary wall.  It then travels to a nearby cell and enters through the cell membrane.  Once inside the cell, the oxygen molecule travels into the mitochondrial matrix where it plays a role in ATP synthesis.

     The carbon dioxide that returns to the blood is taken back to the lungs through the capillaries.  These then fork into venules, which then fork into veins.  The veins carry the carbon dioxide along with the blood back to the heart.  This completes the cycle of the circulatory system.