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CHE4284_5292_Respiratory_System

The table shows what happens to the composition of

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The table shows what happens to the composition of air when it reaches the alveoli. Some of the oxygen dissolves in the film of moisture covering the epithelium of the alveoli. From here it diffuses into the blood in a nearby capillary. It enters a red blood cell and combines with the hemoglobin therein. At the same time, some of the carbon dioxide in the blood diffuses into the alveoli from which it can be exhaled. Composition of atmospheric air and expired air in a typical subject. Note that only a fraction of the oxygen inhaled is taken up by the lungs. Component Atmospheric Air (%) Expired Air (%) N 2 (plus inert gases) 78.62 74.9 O 2 20.85 15.3 CO 2 0.03 3.6 H 2 O 0.5 6.2 100.0% 100.0%
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CHE-4284/CHE-5292 Industrial Safety _ _______________________________________________________________________ 3 Alveoli The ease with which oxygen and carbon dioxide can pass between air and blood is clear from this electron micrograph of two alveoli ( Air ) and an adjacent capillary from the lung of a laboratory mouse. Note the thinness of the epithelial cells ( EP ) that line the alveoli and capillary (except where the nucleus is located). At the closest point, the surface of the red blood cell is only 0.7 μm away from the air in the alveolus. (Reproduced with permission from Keith R. Porter and Mary A. Bonneville, An Introduction to the Fine Structure of Cells and Tissues , 4th. ed., Lea & Febiger, 1973.) This photo (courtesy of the Anatomical Institute, Bern) shows a rubber cast of human lungs.
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CHE-4284/CHE-5292 Industrial Safety _ _______________________________________________________________________ 4 Central Control of Breathing The rate of cellular respiration (and hence oxygen consumption and carbon dioxide production) varies with level of activity. Vigorous exercise can increase by 20-25 times the demand of the tissues for oxygen. This is met by increasing the rate and depth of breathing. It is a rising concentration of carbon dioxide — not a declining concentration of oxygen — that plays the major role in regulating the ventilation of the lungs. The concentration of CO 2 is monitored by cells in the medulla oblongata. If the level rises, the medulla responds by increasing the activity of the motor nerves that control the intercostal muscles and diaphragm. However, the carotid body in the carotid arteries does have receptors that respond to a drop in oxygen. Their activation is important in situations (e.g., at high altitude in the unpressurized cabin of an aircraft) where oxygen supply is inadequate but there has been no increase in the production of CO 2 .
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