308 RESPIRATION 



relative effect would be produced as by inspiration. The immediate effect 

 during inspiration of the diminished intrathoracic pressure upon the pul- 

 monary vessels is to produce an initial dilatation of both artery and veins, 

 and this delays for a moment the passage of blood toward the left side of 

 the heart, resulting in an initial fall in the arterial pressure, but the fall of 

 blood pressure is immediately followed by a steady rise, since the flow is 

 increased by the initial dilatation of the vessels. The converse is the case 

 with expiration. As, however, the pulmonary veins are more easily di- 

 latable than the pulmonary artery, their greater distensibility increases the 

 flow of blood as inspiration proceeds, while during expiration, except at its 

 beginning, this property of theirs acts in the opposite direction, and diminishes 

 the flow. Thus, at the beginning of inspiration the diminution of blood 

 pressure, which commenced during expiration, is continued, but after a time 

 the diminution is succeeded by a steady rise. The reverse is the case with 

 expiration, i.e., there is at first a rise and then a fall of blood pressure. 



As regards the effect of expiration, the capacity of the chest is diminished 

 and the intrathoracic pressure returns to the normal, which is still slightly 

 below the atmospheric pressure. The effect of this on the veins is to in- 

 crease their extravascular and so their intravascular pressure, and to di- 

 minish the flow of blood into the left side of the heart. This will, of course, 

 react to decrease the general blood pressure. Ordinary expiration does not 

 produce a distinct obstruction to the circulation, as even when the expiration 

 is at an end the intrathoracic pressure is less than the atmospheric pressure. 

 The effect of violent expiratory efforts, however, does have a distinct action 

 in obstructing the current of blood through the lungs, as seen in the con- 

 gestion in the exaggerated condition of straining, this condition being pro- 

 duced by pressure on the entire group of pulmonary vessels. 



There are other mechanical factors, such, for example, as the effect of 

 the abdominal movements, both in inspiration and in expiration, upon the 

 arteries and veins within the abdomen and of the lower extremities. Also 

 the influence of the varying intrathoracic pressure upon the pulmonary 

 vessels, which ought to be taken into consideration. The effect of the 

 abdominal movements during inspiration is twofold. On the one hand, 

 blood is sent upward into the chest by compression of the vena cava inferior; 

 on the other hand, the passage of blood downward from the chest through 

 the abdominal aorta, and upward in the veins of the lower extremity, is to 

 a certain extent obstructed. 



LABORATORY EXPERIMENTS IN RESPIRATION. 



i. Respiratory Rate in Man. Count your respirations for from 

 2 to 4 minutes while sitting quietly, and determine the average number per 

 minute. Repeat the counting after standing for 5 minutes, and after brisk 



