where does the actual gas exchange take place most often
lungs
title: Gas exchange text: 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, gas exchange 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 gas-exchange processes occurring during the day. Other gas-exchange processes are important in less title: Gas exchange text: than 80% of the oxygen in the water flowing over the gills is generally transferred to the blood. Alternative arrangements are cross current systems found in birds. and dead-end air-filled sac systems found in the lungs of mammals. In a cocurrent flow system, the blood and gas (or the fluid containing the gas) move in the same direction through the gas exchanger. This means the magnitude of the gradient is variable along the length of the gas-exchange surface, and the exchange will eventually stop when an equilibrium has been reached (see upper diagram in Fig. 2). Cocurrent flow gas exchange title: Gas exchange text: their basement membranes and the endothelial cells of the pulmonary capillaries (Fig. 4). The large surface area of the membrane comes from the folding of the membrane into about 300 million alveoli, with diameters of approximately 75-300 µm each. This provides an extremely large surface area (approximately 145 m) across which gas exchange can occur. Air is brought to the alveoli in small doses (called the tidal volume), by breathing in (inhalation) and out (exhalation) through the respiratory airways, a set of relatively narrow and moderately long tubes which start at the nose or mouth and end in the alveoli title: Gas exchange text: systems are not known to be used in nature. The gas exchanger in mammals is internalized to form lungs, as it is in most of the larger land animals. Gas exchange occurs in microscopic dead-end air-filled sacs called alveoli, where a very thin membrane (called the blood-air barrier) separates the blood in the alveolar capillaries (in the walls of the alveoli) from the alveolar air in the sacs. The membrane across which gas exchange takes place in the alveoli (i.e. the blood-air barrier) is extremely thin (in humans, on average, 2.2 μm thick). It consists of the alveolar epithelial cells, title: Gas exchange text: main group of terrestrial arthropod, the arachnids (spiders, scorpion, mites, and their relatives) typically perform gas exchange with a book lung. Gas exchange Gas exchange is the physical process by which gases 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 gas bubble in a liquid, a gas-permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment. Gases are constantly consumed and produced by cellular and metabolic reactions in most living things, so an efficient system for gas