=Ventilation= [image:] During ''inspiration'' (breathing in) the the '''intercostal muscles''' between the '''ribs''' contract making the ribs swing outward and upward. The '''diaphragm''', a sheet of muscle underneath the lungs, contracts and is pulled downward. Both of these actions increases the internal volume of the lungs, reducing internal air pressure. As a result air is forced into the lungs. During ''expiration'' (breathing out) the intercostal muscles relax and ribs swing downward again. The diaphragm also relaxes and its elastic properties cause it to rise to its original position. As a result the volume of the lungs decreases and internal pressure increases forcing air out of the lungs. [image:] This process ensures the lungs are ventilated and that a concentration gradient is maintained in order for adequate gas exchange. Mammals are often large and warm-blooded and therefore have higher metabolic demands. The conducting pathways leading to the lungs (bronchial tree) are too long that without active ventilation diffusion would be too slow to maintain this concentration gradient. The rate of ventilation (breathing rate & volume) can be adjusted to ensure an adequate concentration gradient is maintained during strenuous activities (that result in increased oxygen demands / carbon dioxide production).
Credit: Ben Himme