13 



Tlie Release oi Energy- 

 Respiration 



The respiration of living organisms depends upon gases that originate 

 mainly in the atmosphere. Whether they are in the atmosphere or dissolved 

 in water, the conditions that govern these gases deeply affect the lives of plants 

 and animals. 



Air. The earth is completely surrounded by the atmosphere, a covering of 

 mixed gases and water vapor. It is about 100 miles deep and is held to the 

 earth by gravity. In dry air the mixture of gases is mainly nitrogen, approxi- 

 mately 78 per cent, and oxygen, 21 per cent (Fig. 13.1). The other one per 

 cent is carbon dioxide with minute amounts of hydrogen, helium, argon, and 

 some other rare gases. When water vapor is abundant, the air is humid. In 

 dry air the proportions of gases do not change at different atmospheric levels, 

 but the total amount of gases does. At low levels molecules bombard one 

 another at close quarters. With lessened pressure the gases expand and the 

 molecules are not even near neighbors. At greater and greater heights there 

 is less and less gas in the air. 



Atmosphere is piled up on the surface of the earth like hay in a stack 

 (Fig. 5.14). The hay at the bottom bears the pressure of all that is above it. 

 This pressure is evenly distributed within and without in all directions. The 

 distribution prevents the existence of weight in its ordinary sense although 

 atmospheric pressure is usually expressed in terms of weight. It is calculated 

 as the weight of a column of air one inch square and reaching from sea level 

 to the upper limit of the atmosphere. At sea level it is 14.7 pounds per square 

 inch. The pressure upon the air drives it into the lungs and acts as the first 

 step in inhaling. At 18,000 feet it is not strong enough to force the oxygen 

 from the lungs into the blood, the next necessary step. The effects of oxygen- 

 lack in the body have long been known as mountain sickness, the weakness, 



224 



