CHANGES ACCOMPANYING MUSCULAR EXERCISE 429 



be called upon to maintain the circulation. In isolated heart-lung prep- 

 arations Starling and his coworkers have very clearly demonstrated the 

 close dependence of cardiac output upon rate of venous filling and the 

 enormous range through which the systolic discharge can be made to 

 vary by altering this factor. As explained elsewhere (page 441), when the 

 reserve power of the heart is lessened, the rise in blood pressure following 

 exercise is longer in attaining its maximum, which is set at a higher level 

 and persists for a longer time. Observation of the extent of these changes 

 furnishes a most useful functional test of cardiac efficiency. 



Other mechanical factors that augment the cardiac output depend on 

 the increased respiratory movements. During each respiration the in- 

 crease in capacity in the thorax causes both an opening up of the thin- 

 walled veins, so that blood is aspirated towards them from the extra- 

 thoracic venous system, and a dilatation of the blood vessels of the lungs, 

 so that the blood finds its way from right to left heart more readily. 

 Although this dilatation will at first tend to cause more blood to collect 

 in the intrathoracic vessels and less to be pumped out of them, the expira- 

 tory act when it supervenes will, by compressing the veins, cause the 

 extra blood to be expelled into the left ventricle and thence into the 

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

 tory movements will accelerate the bloodflow 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- 



