1054 



PHYSIOLOGY 



^70il5 420 



5?5 630 735 

 Work per Minute 



described by Bainbridge. The increasing tension on the venous side of the 

 heart evokes reflexly a quickening of the heart rhythm, chiefly by inhibition 

 of the vagus tone, possibly also by reflex stimulation of the sympathetic 

 accelerator nerves. Further increase in the inflow into the heart is met by 

 corresponding quickening of the heart rhythm. Distension of the big veins 

 is thus prevented, and the output of each ventricle per minute is increased 

 seven, ten or ev<an twelve times. The part played by increase of output per 



beat and by increase of pulse 

 rate respectively in augmenting 

 the total output of the heart is 

 shown in Fig. 483. In this 

 figure the first rise in pulse 

 rate from sixty-eight to ninety- 

 eight and the corresponding 



j~ increase in output per beat 



can be regarded as associated 

 with the initial changes origi- 

 nated by the act of attention 

 and volition. It will be seen 

 that between 270 to 600 kilo- 

 grammetres' work per minute 

 the pulse rate remains prac- 

 tically unchanged, while the 

 output per beat increases 

 steadily with the work. After 

 this point there is very little 

 further increase in the output 

 per beat, which towards the 



^ begins to diminish, while there is a steady increase in the pulse rate. 

 By this means the blood is driven through the lungs at a rate correspond- 

 ing to the increased needs of the muscles for oxygen. The passage of this 

 blood through the muscles is provided for by two mechanisms. In the first 

 place we have the contraction of the splanchnic vessels, so that the blood 

 pressure is raised and all the available blood can be driven through the 

 working tissues. In the second place the muscles in their activity produce 

 lactic acid, C0 2 , and possibly other metabolites, which cause dilatation of 

 the arterioles and capillaries in the muscles themselves. During rest it is 

 probable that the majority of the capillaries are closed; during activity 

 these dilate and are filled with blood, so that the capillary bed in the muscles 

 may be increased many times in area, and each element of the muscle is 

 brought into close relation with a dilated capillary through which is flowing 

 a rapid stream of oxygenated blood. Krogh has shown that the number of 

 blood-containing capillaries in each square millimetre cross-section of the 

 muscle may be increased 40 to 100 times during maximal activity of the 

 niuscle. As a result the oxygen tension in the muscle fibres becomes almost 

 equal to that in the capillaries themselves. 



Flu. 483 Chart showing the effect of increasing 

 amounts of .muscular work on the pulse rate P, 

 shown by dots ; on the heart output per beat, 

 VpB, and on the co-efficient of oxygen utilisa- 

 tion in the blood, C. (From MEANS and NEW- 

 BUEQH.) 



