486 RESPIRATION 



during the latter and falls during the former period. These changes, 

 which are commonly referred to as the respiratonj variations in blood 

 pressure, are reversed during inlBation. It is easily conceived that 

 the establishment of a positive pressure in the pulmonary pas- 

 sages, corresponding to the normal inspiratory motion, must tend to 

 compress the pulmonary capillaries, thereby producing a stagnation in 

 the venous channels and right side of the heart and a deficiency in 

 its left side and arterial outlets. Just the opposite effect is produced 

 during the period of deflation. Inasmuch as the pressure is now 

 removed from the alveolar walls, the pulmonary blood-bed must be 

 enlarged, allowing a greater quantity of blood to reach the arteries. 

 For this reason, we obtain an inspiratory fall in arterial pressure and 

 an expiratory rise, while, on the venous side, the pressure rises during 

 inflation and falls during deflation. 



The methods of artificial respiration previously enumerated are 

 intended to effect either a rhythmic expansion or a rhythmic infla- 

 tion of the lungs. But it should not be forgotten that these organs 

 may also be retained in a distended condition by the procedure of 

 constant insufflation.^ A long rectangular piece of tubing is inserted 

 through the larynx until its free end comes to lie at the bifurcation 

 of the bronchi. A steady stream of air is then permitted to flow 

 through this tube until the thorax assumes a position of moderate 

 distention. Care must be exercised, however, that the outflow of 

 air along the sides of this tube be not hindered in any way, because 

 an excessive positive pressure gives rise to an immediate fall in arterial 

 pressure dependent upon a compression of the pulmonary capillaries. 



CHAPTER XXXIX 

 THE CHEMISTRY OF RESPIRATION 



The Character of the Inspired and Expired Air. — The gaseous 

 metabolism of the tissues consists, on the one hand, in a constant 

 acquisition of oxygen and, on the other, in an evolution of carbon 

 dioxid. This change from one into the other is not accomplished 

 in a direct way, but only with the help of several intermediate reactions 

 which together constitute the process of oxidation. Obviously, 

 the purpose of these reactions is the reduction of the carbon and 

 hydrogen of the food and the liberation of energy in its different forms. 

 The blood and lymph serve as the medium in which this assimilation 

 and dissimilation is effected, while the lungs enable these body 



1 Meltzer, Jour. Am. Med. Assoc, Ivii, 1911, 521, also, Zentralbl. fiir Physiol., 

 xxvi, 1912, 161. 



