Living cells need oxygen and glucose to chemically react with each other in order to produce aerobic respiration. The respiration reaction releases energy that the body's cells need to function and creates carbon dioxide and water as by-products. Both glucose and oxygen are distributed to individual cells by the bloodstream.
Glucose is a carbohydrate created by food digestion that can be absorbed through the small intestine walls into the bloodstream. Glucose blends with blood plasma to travel through the body and diffuses into cells through the capillaries.
Oxygen enters the body through the lungs and diffuses into the bloodstream through the alveoli. In the bloodstream, it bonds with haemoglobin in the red blood cells. It diffuses through the capillaries in a similar manner as glucose.
When oxygen and glucose meet in the cells, the respiration reaction occurs. The carbon dioxide created diffuses back into the bloodstream and travels to the alveoli where it is expelled through the lungs.
The process by which oxygen is ingested and carbon dioxide is expelled is called ventilation. The alveoli handle the exchange of gases. During this exchange, some of the water created by respiration is also expelled, which is why a person can see their breath on a cold day.