THE BREATHING SYSTEM 



479 



from the same or other organisms in the 

 lungs, since the air has not been filtered in 

 the nasal passages as it normally would be. 



Once past the nasal passages, the air is 

 carried down into the pharynx and into the 

 larynx on its way to the lungs. The larynx, 

 or voice box, is a cartilaginous enlargement 

 of the upper end of the trachea. It ofiFers 

 a rigid support for the vocal cords, which 

 are stretched bands of tough tissue that 

 vibrate to produce a sound when they are 

 tightened and air is forced over them. 

 While the initial sound comes from the 

 vocal cords, the ultimate sound produced is 

 highly modified by the nasal chambers and 

 mouth cavity. These resonating chambers, 

 together with their movable parts (soft 

 pallate, tongue, and cheeks), are responsi- 

 ble for the quality of the voice, either 

 speaking or singing. Any interference with 

 these passages has a marked effect on the 

 quality of the voice, a familiar phenomenon 

 in anyone suffering from a cold. 



Immediately beyond the larynx is the 

 trachea, a single tube whose walls are kept 

 from collapsing by means of cartilaginous 

 rings. The "windpipe," as it is sometimes 

 called, lies at the front of the throat where 

 the rings can be felt through the skin. The 

 trachea passes into the chest cavity where 

 it divides into two bronchi, one going to 

 each lung. These subdivide into bronchioles 

 which after many divisions terminate in 

 tiny blind sacs, the alveoli. 



In the alveoli the actual exchange of 

 gases takes place. All of the mechanism up 

 to this point functions in getting the air to 

 and from these alveoli. The combined sur- 

 face of all the alveoli of the human lungs is 

 over 1000 square feet, and all of this is in 

 intimate contact with capillaries. The lungs 

 are covered by a thin layer of epithelium 

 called the pleura and the spaces between 

 the alveoli and bronchioles are filled with 

 elastic connective tissue. The lungs remain 

 partially inflated at all times. When re- 

 moved from the thoracic cavity they col- 

 lapse to only a fraction of their inflated size, 



because of the constant tension on the walls 

 of the bronchioles. But, even in this con- 

 tracted condition they resemble a sponge in 

 consistency and will readily float in water. 



THE BREATHING MECHANISM 



The lungs completely fill the thoracic 

 cavity, which is lined with pleura identical 

 to that covering the lungs. These two layers 

 normally lie close together with no actual 

 space between them, but when deflated, 

 the space that results is known as the 

 pleural space. The two surfaces are held 

 together because there is no air in the 

 pleural space. In other words, there is a 

 partial vacuum in which the pressure is 

 below that of the outside atmosphere. 

 Therefore, if the walls of this closed cavity 

 move out so as to enlarge the cavity, the 

 walls of the lungs must follow. This 

 creates a decreased pressure in the lungs 

 themselves which, in turn, causes air to 

 rush into the low pressure region deep in 

 the alveoli. The opposite effect results when 

 the cavity is reduced in size. The cavity 

 can be increased and decreased in size by 

 simply raising the ribs ( Fig. 18-3 ) and then 

 allowing them to return. Simultaneous with 

 the raising of the ribs, the dome-shaped 

 diaphragm is pulled downward and flat- 

 tened. The ribs are raised by the contrac- 

 tion of the muscles lying between them ( in- 

 tercostal muscles) and when these muscles 

 are relaxed the ribs fall back into position. 

 At the same time, the diaphragm is re- 

 turned to its dome-shape by a ligament that 

 keeps it in this position. Therefore, inspira- 

 tion is brought about by the contraction of 

 muscles, expiration by relaxation. As a re- 

 sult of these two actions the air in the 

 alveoli is changed. 



Under extreme exertion the abdominal 

 muscles also are involved in the breathing 

 movements. On inspiration they relax in 

 order to allow the diaphragm to descend 

 farther, thereby compressing the abdominal 

 organs. In forced expiration the abdominal 



