When naturally occurring, inspiration and expiration occur with contraction and relaxation of the diaphragm and intercostal muscles. On inspiration, the diaphragm contracts and lowers and the ribs contract and rise. This enlarges the rib cage, lowers internal pressure, and forces air into the lungs.
The basic function of the respiratory system is to supply the organism with oxygen (O2) and to remove from it the gaseous product of cellular metabolism, that is, carbon dioxide (CO2). In unicellular organisms, gas exchange occurs directly between the cell and the surrounding medium through simple diffusion. In multicellular organisms, diffusion between the external environment and the interior of the cell mass occurs slowly, due to the distance to be covered by the gases.
With this information, we can conclude that When naturally occurring, inspiration and expiration occur with contraction and relaxation of the diaphragm and intercostal muscles. On inspiration, the diaphragm contracts and lowers and the ribs contract and rise. This enlarges the rib cage, lowers internal pressure, and forces air into the lungs.
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