The Respiratory System 157 



muscles. This apparently results in increased efficiency in the removal 

 of waste products from the muscles, and in an increase in the supply 

 of available oxygen. It occurs more rapidly in the trained than in the 

 untrained individual. 



Effects of Altitude. — It is a well-known fact that upon first going 

 to a higher altitude, a feeling of distress is experienced. If the individual 

 remains at this altitude for a period of time, this sensation of breathless- 

 ness gradually disappears. 



This response is due to the fact that the air pressure is greatly 

 reduced at high altitudes, and the amount of oxygen available to the cells 

 is reduced. Acclimatization is due to certain physiological responses 

 on the part of the individual. One very important factor is the increase 

 in the number of the red blood cells for transporting the available oxygen ; 

 another, in the actual increase in lung ventilation. 



This problem of the efifect of higher altitudes has become of im- 

 portance in our modern air age. At altitudes above 12,000 feet, symp- 

 toms of distress are noted in the unadapted individual. At 25,000 feet 

 consciousness is lost. As planes fly at higher altitudes, these problems 

 become increasingly important in aviation engineering. The use of 

 pressurized cabins and oxygen masks have made possible flying at these 

 high altitudes. 



Basal Metabolism. — The sum total of life process involving the 

 use of food for energy and repair is known as metabolism. The 

 rate of metabolism is dependent upon such things as physical exercise, 

 amount of food eaten, and exposure to cold. This rate can be measured 

 by measuring the amount of oxygen consumed in a given length of time. 

 If this measurement is made when the individual is at rest and has not 

 eaten for several hours, it is known as the basal metabolic rate. This 

 rate is important, for it is indicative of the state of health of the individual. 



Some Respiratory Modifications. — Sneezing, yawning, coughing, 

 hiccuping, and snoring are recognized as unusual aspects of respiration. 

 Coughing and sneezing result from local irritations of the linings of 

 the respiratory passages. They are useful for expelling unwanted ma- 

 terials. They result when the pressure within the lungs is elevated due to 

 the closing of the glottis, and the air thus enclosed in the lungs is 

 suddenly forced out. If the air is expelled through the mouth, it is a 

 cough ; if through the nose, a sneeze. 



A hiccup results when the diaphragm contracts spasmodically. The 

 resulting inspiration is cut short by a rapid closing of the glottis. Snor- 



