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		B. Mammalian (human) lungs (see table)42.22 1. Exchange in air 2. O2content of air Unlike water, air is a gas, and can expand (and become less dense) or contract depending on environmental conditions. So, the concentration of O2also can vary. The content of each of the gases that make up air is referred to as their partial pressure. At sea level, the pressure exerted by the atmosphere is equivalent to a column of mercury 760 mm high. Dry air is 20.9% oxygen, so the partial pressure of oxygen (PO2) at sea level is: PO2= 760 mm Hg x 0.209 = 159 mm Hg Q. How is this affected by altitude? -at high altitude, the barometric (total) pressure is lower, PO2is only 50 mm Hg on top of Mt. Everest (8,848 m) 3. Structure tracheaÆbronchusÆbronchioleÆalveoli Air is brought through the trachea into the lung, a complex branching structure of bronchi and bronchioles. Each bronchiole ends in air sacs known as an alveolus. The 300 million alveoli have a combined surface area of over 70 m2 and are made up of flat thin-walled epithelial cells. Each alveolus is covered with a mesh of capillaries. The internal location and the narrow opening to the outside lessen water loss. In mammals the regions above the lungs moisten and warm the incoming air, to reduce the drying of the lung surface. Q. What are the advantages of this structure, considering Fick's law again? - large surface areaand short diffusion pathboth favor exchange Q. What are the limitations of this structure? - respiration has to be tidal(in and out), so can't set up countercurrent exchange - not possible to fully empty lungs; fresh air coming in is mixed with O2- depleted air