Chap. 13 THE RELEASE OF ENERGY — RESPIRATION 239 



contraction of the diaphragm creates a flat floor. The floor now presses on the 

 organs beneath, the muscles of the abdominal wall relax and the abdomen 

 bulges. 



The chest expands because the muscles contract. But why do the lungs 

 follow its expanding walls? In the first place lungs are free to move, since they 

 are attached only by the bronchial tubes and the partition between them. 

 Thus they can slide easily on the lining (pleura) of the lung cavity. In 

 addition, there are many elastic fibers in the lung, all of them stretched 

 and trying to shorten just as they do in the arteries. Thus their action keeps 

 the lungs in a state of trying to pull away from the walls about them. But 

 they meet the strong opposition of the low pressure in the space between 

 the lungs and chest wall. Within the lungs the pressure is near that of the 

 atmosphere, slightly below at the beginning of inspiration, the reason that 

 air enters them. On the other hand, in the space outside the lungs, there is 

 no air, only a little fluid and a suction or negative pressure. This exerts a 

 pull on the lungs that is stronger than they can resist. It is why they chng to 

 the thoracic wall as long as that is intact. When it is perforated by accident, 

 or in the treatment of tuberculosis to give one lung a rest that lung instantly 

 collapses. This is the rather well-known state of pneumothorax. 



Expiration. Often called breathing out, expiration is a purely passive relaxa- 

 tion of muscles; the ribs are dropped; the dome of the diaphragm once more 

 presses upward against the lung cavity. Ordinary quiet breathing is an inspi- 

 ration and expiration repeated about 1 6 times per minute, the number differing 

 slightly in different individuals. 



Chemical Control. As carbon dioxide increases in arterial blood, it acts 

 upon the respiratory center of the medulla and indirectly on the chemorecep- 

 tors of the carotid body, which are thus stimulated to discharge impulses that 

 quicken respiration. In contrast, a decrease in carbon dioxide affecting the 

 respiratory center diminishes or stops breathing. And the content of these 

 gases in the blood depends upon the proportions of oxygen or carbon dioxide 

 in the lungs. 



Nervous Control. You can hold your breath but not your heartbeat, and 

 you cannot even hold your breath for long. But the fact that you can hold 

 your breath at all shows that messages come from the higher centers of the 

 brain and act upon the respiratory center in the medulla (Fig. 13.13). The 

 failure to continue holding the breath means that the chemical control by 

 accumulated carbon dioxide has gotten the upper hand of any nervous control. 

 Branches of the truly named vagus (wandering) nerves help control ordinary 

 breathing. During inspiration their receptor endings in the pleura are stretched 

 and the messages from them to the medulla are more frequent. The expansion 

 of the chest finally causes the respiratory center to stop sending the impulses 

 which stimulate inspiration. As soon as this occurs, another group of receptors 



