Exchange of gases or gas exchange is carried out by organisms, where different gases move in different directions. Animals, microorganisms and plants breathe out carbon dioxide and breathe in oxygen. To balance the composition of air, plants use carbon dioxide to carry out photosynthesis, so oxygen is made. Plants only carry this out during the day, when there is sunlight. Plants respire all day and night.
In the human respiratory system, gas exchange happens in the alveolus (alveoli for plural). These are in the lungs and help with diffusion. Oxygen diffuses into the bloodstream, from the air. Oxygen is used in aerobic respiration. Carbon dioxide diffuses out of the bloodstream, into the air. Carbon dioxide is a waste product made by respiration.