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		So, do not maximize concentration difference. Mammalian lungs are much less efficientat extracting O2than fish gills. 4. Mechanics: Negative pressure breathing42.23 - rib muscles and diaphragm contact and expand chest cavity and lungs Breathing is the alternate filling and emptying of the lungs. There are two possible mechanisms; air can either be forced in and then allowed out, or sucked in and then pushed out. Human lungs use the second strategy; the rib muscles and the diaphragm contract pulling the lung open. The muscles don't act on the lung itself, they act on a sealed chamber, the pleural cavity, that contains the lungs. Slight negative pressure is maintained even when muscles are relaxed to keep alveoli from collapsing. During shallow, resting breathing, most of the lung expansion is due to the diaphragm. During vigorous exercise, contraction of the rib muscles greatly increases the lung volume. Try to feel the difference while you're breathing under different conditions. Take a really deep breath, exhale, then contract your abdominal muscles to squeeze out all of the air that you can. The total air exchanged with deep breathing is known as the vital capacity. The amount of air exchanged with normal, shallow breathing is the tidal volume. The difference between them, the additional air exchanged with a deep breath, is known as the reserves. Vital capacity = tidal volume + reserves Even after you exhale all that you can, about 1 L of air remains in your lungs; this is known as the residual volume. Factors that decrease the elasticity of the lungs, like smoking and emphysema, increase the residual volume. Q. What is the effect of that? - fresh air coming in is mixed with more O2-depleted air The numbers for an average human female are: Vital capacity (total air exchanged with deep breathing) = 3.2 L Tidal volume (normal volume air exchanged ) = 0.5 L Reserves (additional volume exchanged with deep breath) = 2.7 L Residual volume (that remains in lungs) = 1.0 L Total volume = residual volume + vital capacity = 4.2 L