primarily upon the tissues, rather than upon 

 the blood. Cyanide, for example, blocks 

 cytochrome oxidase (p. 147) and perhaps 

 others of the oxidizing enzymes of the cells, 

 thus preventing a proper usage of oxygen 

 in the tissues. 



CONTROL OF BREATHING 



Active work may double or even treble the 

 oxygen consumption of the muscles and other 

 tissues, and this extra demand evokes a large 

 increase in the depth and frequency of 

 breathing, as well as an acceleration of the 

 circulation. The breathing reflexes are in- 

 tegrated in the respiratory center, which is a 

 specialized group of nerve cells situated in 

 the medulla part of the brain (p. 459). The 

 respiratory center is particularly responsive 

 to chemical conditions in the blood that 

 flows to this part of the brain, and these con- 

 ditions may greatly modify both the rate and 

 the intensity of the motor discharges of the 

 respiratory center, which are sent to the 

 diaphragm via the phrenic nerves. 



Chemical Control of Breathing. The chief 

 chemical factor controlling the activity of 

 the respiratory center is the quantity of car- 

 bon dioxide present in the blood, and the 

 respiratory center is extremely sensitive to 

 changes in the COo content of the blood. If, 

 for example, the breathing is not fast and 

 deep enough to prevent an accumulation of 

 C0 2 in the alveolar air, C0 2 tends to accu- 

 mulate in the blood. This stimulates the re- 

 spiratory center, increasing the rate and 

 depth of the breathing until the excess of 

 C0 2 has been "blown off" from the lungs. 

 Such an automatic control is very effective. 

 An increase in the alveolar C0 2 from 5.5 

 to 5.7 percent is enough to double the breath- 

 ing rate; and a decrease to 5.3 percent re- 

 sults in a temporary cessation of breathing. 

 Thus after a series of very deep voluntary 

 inhalations and exhalations, the alveolar C0 2 

 concentration is significantly reduced, and 

 the subsequent breathing of the individual is 

 usually suspended for about half a minute. 



Respiration - 367 



Also the efficacy of 10 percent C0 2 , used to 

 initiate the breathing of newborn infants, 

 depends upon the stimulating effect of C0 2 

 upon the respiratory center. 



The respiratory center is also sensitive, at 

 least indirectly, to a diminution in the oxy- 

 gen concentration of the blood. Thus breath- 

 ing becomes faster and deeper whenever the 

 aeration of the. blood, becomes inadequate. 

 For the most part this oxygen control is in- 

 direct, however. Lack of oxygen leads to an 

 accumulation of incompletely oxidized prod- 

 ucts such as lactic acid, and a very slight in- 

 crease of hydrogen ion concentration in the 

 blood strongly stimulates the respiratory cen- 

 ter. 



A person cannot "hold the breath" beyond 

 a certain relatively safe limit, because in- 

 hibitory impulses from the cerebrum (p. 464) 

 are quenched at the respiratory center as soon 

 as the chemical influences become strong 

 enough to revive the rhythmic discharges of 

 this center. 



Nervous Control of Breathing. Sensory 

 impulses from various receptors also modify 

 the rhythmic motor discharges of the respira- 

 tory center. At the crest of an inspiration, 

 the stretching of the pulmonary wall excites 

 the pressure receptors in the wall of the lung, 

 and these receptors discharge a series of in- 

 hibitory impulses, which reach the medulla 

 by way of the sensory fibers of the vagus 

 nerves (p. 467). Conversely, at the depth of 

 an expiration, other receptors in the pul- 

 monary wall may send excitatory impulses, 

 also via the vagus, to the central nervous 

 system. Severe pain in any region of the body 

 may act reflexly through the respiratory cen- 

 ter, and the breath begins to "come faster." 

 In swallowing, impulses from the pharyngeal 

 receptors inhibit breathing for a moment — 

 while food passes the glottis — and this pre- 

 vents food from being sucked into the trachea 

 and lungs. The gasp evoked by a plunge into 

 icy water is also mediated through the res- 

 piratory center, as is likewise true of the 

 coughing and sneezing reflexes. And last but 

 not least, there is an additional mechanism 



