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Exchange of gases or gas exchange is carried out by organisms. Animals breathe out carbon dioxide and breathe in oxygen. Plants use carbon dioxide to carry out photosynthesis, making oxygen. Plants only do this when there is light. Plants breathe 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.
In eukaryotes, respiration is carried out by the mitochondria. However, in some cells it is carried out by the cytoplasm. For more information visit cellular respiration.