CIRCULATORY CHANGES DURING MUSCULAR EXERCISE 1053 



imply a corresponding increase in the blood supplied to these organs. This 

 increased blood flow to the muscles involves in its turn an increase in the 

 blood flow through the lungs and in the ventilation of the lungs. In the next 

 chapter we shall have occasion to study the method by which the respiratory 

 centre is enabled to adjust its activity, and therewith the rate at which the 

 air in the pulmonary alveoli is renewed, in exact proportion to the needs of the 

 body for oxygen. We are concerned here chiefly with the mechanism by 

 which the circulation through the lungs and muscles can be and is increased 

 in like proportion. The measurement of the circulation through the lungs is 

 identical with the measurement of the output of the right ventricle. This 

 has been investigated by Krogh and Lindhard and by Means and Newburgh 

 in the healthy man during rest and during exercise. In Fig. 482 are shown 

 results obtained by the two last-named observers. It will be seen that the 

 blood flow through the lungs, or the output of the right ventricle per minute, 

 increases in a manner almost absolutely proportional to the consumption of 

 oxygen, and that both increase pari passu with the work done per minute. 

 How is this admirable adjustment of the activity of the heart and circulation 

 to the oxygen needs of the muscles effected ? The output of the heart depends 

 on the inflow into this organ, so that our problem is to determine the 

 factors which increase the inflow into the heart in proportion to the needs of 

 the muscles. At or before the onset of muscular exercise, unless this is quite 

 moderate, there is contraction of the splanchnic vessels, so that the blood is 

 diverted from the viscera to the muscles and later on also to the skin. Every 

 muscle as we have seen acts as an accessory heart, the muscular contractions 

 emptying the capillaries into the veins, and in the latter driving on the fluid 

 towards the heart in virtue of the valves present in these vessels. The more 

 active the muscles therefore, the more rapidly the blood which enters them 

 is passed on with force towards the big veins and the heart. The circulation 

 through the big veins of the abdomen and chest is aided by the respiratory 

 movements, which are also augmented in proportion to muscular activity, 

 each inspiration driving the blood out of the big veins in the abdomen and 

 aspiring it into the veins and heart cavities within the thorax. The blood 

 flow into the heart is thus increased in proportion to the activity of the 

 muscles. Under resting conditions it seems probable that the filling of the 

 heart is what Krogh has described &s ' inadequate ' i. e. the amount of blood 

 entering the heart during each diastole is not sufficient to fill this organ up to 

 the limits set by the fibrous and inextensible pericardium. The first effect of 

 muscular exercise will be to increase the filling of the heart and therefore 

 the output at each beat, and this will go on until the filling during each 

 diastole has become * adequate.' The heart therefore, at the beginning of 

 muscular exercise, automatically reacts by increasing the output per beat. 

 Whether diastolic filling of the heart be adequate or inadequate, the pressure 

 in its cavities just before systole will be approximately zero. If now the 

 inflow be still further increased, the diastolic pressure within the heart and 

 in the big veins will begin to rise, since the heart cannot further increase 

 appreciably its output per beat. Now comes in the reflex mechanism 



