Gas exchange

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.

Humans

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.

Eukaryotes

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.

Gas Exchange Media

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  Gas exchange is driven by a concentration gradient(High to Low). Gases move from areas of high concentration/partial pressure to areas of low concentration/partial pressure.

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  Fig. 1. Fick's law for gas-exchange surface

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  Fig. 3. An alveolus (plural: alveoli, from Latin alveus, "little cavity"), is an anatomical structure that has the form of a hollow cavity. They occur in the mammalian lung. They are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood.

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  Fig. 5. The changes in the composition of the alveolar air during a normal breathing cycle at rest. The scale on the left, and the blue line, indicate the partial pressures of carbon dioxide in kPa, while that on the right and the red line, indicate the partial pressures of oxygen, also in kPa (to convert kPa into mm Hg, multiply by 7.5).

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  Fig. 8. Gills of tuna showing filaments and lamellae

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  Fig. 10. Inhalation-exhalation cycle in birds.