When we breathe, the oxygen reaches our lungs via the mouth, nose, and trachea. The lungs have a number of very tiny cavities known as air sacs which are covered by a network of very fine blood vessels. An exchange of gases takes place here. Impure blood reaches these capillaries and releases the waste gas carbon dioxide in the air sacs. This is exhaled out of the body, and freshly inhaled oxygen is absorbed into the air sacs. This oxygen is released into the blood via the capillaries.
