Alveolus and capillary network based on a scanning electron microscopy (SEM). De-oxygenated red blood cells are sent by the right side of the heart through the pulmonary artery into the vessels of the lungs to be refilled with oxygen for their next circuit through the body. The blood is carried through the lung tissues, where it exchanges its carbon dioxide for oxygen in the alveoli. It is then returned through the pulmonary veins to the left side of the heart and sent out to the rest of the body. The walls of the alveolar sacs are made up of a single layer of very thin cells, and are intimately surrounded by a bed of capillaries. These walls are so delicate that the inhaled oxygen can seep from the air sacs to bind to the hemoglobin in the blood, while the carbon dioxide and other waste gasses leave the blood and diffuse into the lungs where they can be exhaled.