<unk>  is the physical process by which<unk> es move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a<unk>  bubble in a liquid, a<unk> -permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment.<unk> es are constantly consumed and produced by cellular and metabolic reactions in most living things, so an efficient system for<unk>  between, ultimately, the interior of the cell(s) and the external environment is required. Small, particularly unicellular organisms, such as bacteria and protozoa, have a high surface-area to volume ratio. In these creatures the<unk>  membrane is typically the cell membrane. Some small multicellular organisms, such as flatworms, are also able to perform sufficient<unk>  across the skin or cuticle that surrounds their bodies. However, in most larger organisms, which have a small surface-area to volume ratios, specialised structures with convoluted surfaces such as gills, pulmonary alveoli and spongy mesophyll provide the large area needed for effective<unk> . These convoluted surfaces may sometimes be internalised into the body of the organism. This is the case with the alveoli, which form the inner surface of the mammalian lung, the spongy mesophyll, which is found inside the leaves of some kinds of plant, or the gills of those molluscs that have them, which are found in the mantle cavity. In aerobic organisms,<unk>  is particularly important for respiration, which involves the uptake of oxygen () and release of carbon dioxide (). Conversely, in oxygenic photosynthetic organisms such as most land plants, uptake of carbon dioxide and release of both oxygen and water vapour are the main<unk>  processes occurring during the day. Other<unk>  processes are important in less familiar organisms: e.g. carbon dioxide, methane and hydrogen are<unk> d across the cell membrane of methanogenic archaea. In nitrogen fixation by diazotrophic bacteria, and denitrification by heterotrophic bacteria (such as Paracoccus denitrificans and various pseudomonads), nitrogen<unk>  is<unk> d with the environment, being taken up by the former and released into it by the latter, while giant tube worms rely on bacteria to oxidize hydrogen sulfide extracted from their deep sea environment, using dissolved oxygen in the water as an electron acceptor. Diffusion only takes place with a concentration gradient.<unk> es will flow from a high concentration to a low concentration. A high oxygen concentration in the alveoli and low oxygen concentration in the capillaries causes oxygen to move into the capillaries. A high carbon dioxide concentration in the capillaries and low carbon dioxide concentration in the alveoli causes carbon dioxide to move into the alveoli.
Gas exchange
Breathing circuit
Fresh gas flow
Thermoregulation
Respiratory burst
Cardiovascular physiology
Diffusing capacity
Ventilatory threshold