412 THE RESPIRATION 



arteries. It is obvious that increased depth and frequency of the respira- 

 tory movements will accelerate the bloodtlow and tend to raise the arte- 

 rial blood pressure. 



The above factors will come into play during most kinds of muscular 

 exercise such as walking, running, or swinging dumbbells, etc. There 

 are certain types of muscular effort, however, in which, the mechanical 

 factors produce decidedly disturbing effects on the circulation. During 

 a sustained effort as, for example, in pulling against a resistance or in 

 attempting to lift a heavy load, the respirations are suspended, often after 

 a deep inspiration, and the contracted abdominal muscles press the dia- 

 phragm up into the thoracic cavity. After a preliminary squeezing out 

 of blood first of all from the veins of the abdomen into the thorax and 

 then from those of the latter into the systemic arteries, with a consequent 

 rise in arterial pressure, there comes to be a damming back of blood into 

 the peripheral veins, causing them to swell and, if continued, marked 

 cyanosis may develop. When such efforts are maintained for long, the 

 arterial pressure begins to fall, and this fall is very pronounced indeed 

 at the end of the effort, because, the compression being removed from the 

 abdominal and thoracic veins, these open up and form a large unfilled 

 blood reservoir. 



A similar mechanism comes into play during expulsive acts such as 

 defecation, parturition, etc. In these the glottis is closed, usually after 

 a preliminary inspiration, and a powerful expiratory movement is per- 

 formed, with the consequence that the intrathoracic and intraabdominal 

 pressures rise considerably, greatly augmenting the systolic discharge 

 and causing the blood pressure to rise. Because of the obstruction to 

 the bloodflow in the large veins of the abdomen and thorax, however, 

 the later effect of the effort is to diminish the systolic discharge, but the 

 fall in blood pressure which this would be expected to occasion is masked. 

 The pressure remains high because other factors increasing the peripheral 

 resistance come into play. The fall in blood pressure following these acts 

 may be very marked indeed. Similar mechanical effects are produced 

 in the acts of coughing, sneezing, etc. 



The capacity of the veins varies considerably with the position of the 

 body, and it is in order that we may cause alterations in this capacity 

 and therefore encourage a more rapid bloodflow that Ave stretch the body 

 after sitting for some time in a cramped position. 



The Nervous Factor. — The vagus, vasoconstrictor and respiratory cen- 

 ters are all excited during muscular effort. In the earlier stages the 

 excitation depends entirely on nervous impulses transmitted to the cen- 

 ters, but later il depends on changes in the composition and temperature 

 of the blood flowing through them — the hormone factor. The initial 



