Alveoli of Lung
When you breathe, oxygenated air flows through your lungs and ends up in thousands of small air sacs in the lungs called alveoli. The right side of your heart sends deoxygenated blood to the capillaries surrounding these alveoli. The walls between the alveoli and the capillaries are extremely thin, so that the inhaled oxygen can seep from the air sacs to bind to the hemoglobin molecules in the erythrocytes. Carbon dioxide and other waste gases leave the blood and diffuse into the air sacs, where they are exhaled through the lungs. This gas exchange is passive: oxygen goes from the higher concentration in the lungs to the lower concentration in the blood. Similarly, carbon dioxide goes from the blood to the lungs.