In humans, the respiratory system facilitates the intake of oxygen and removal of carbon dioxide in the body. The lungs, heart, blood vessels, and blood cells are all important figures in respiration. When we inhale, oxygen-rich air fills our lungs and the hundreds of thousands of tiny pockets called alveoli. Each alveolus is surrounded by a membrane full of capillaries. Oxygen moves across this membrane and into our blood, where it attaches to red blood cells. These red blood cells are circulated out of the lungs and around the body, where it nourishes all of our bodily tissues  by "dropping off" some of that oxygen. In place of the oxygen, gaseous bodily waste like carbon dioxide can attach and return to the lungs. When these red blood cells return to the lungs, they drop off the carbon dioxide and pick up some fresh oxygen to carry throughout the body! All of this occurs in a continuous cycle.