The second factor, namely the increase in the output of the heart, is far more important. Amongst all the factors involved in this increase in the blood flow, we will first consider the heart. This organ plays only a subordinate role. The output of the heart depends on the inflow and, under normal conditions, changes in the activity of the heart have little or no effect on blood flow. The main factor in the augmentation of the inflow concerns the peripheral blood vessels. Considerable changes in the heart beat must, however, take place in order to make it possible for the heart to cope with the larger inflow. The first effect of the larger inflow will be to increase the filling of the heart and therefore the output on account of the stronger beat. This is a purely auto matic reaction of the cardiac muscle, and is independent of the central nervous system. The adaptation of a denervated heart stops at this point. The limit to this adaptation of the heart muscle is set by the pericardium. The adaptation of the innervated heart does not, however, stop here. The increase in the strength of contraction in this case is out of proportion to the inflow, so that the heart becomes able to give progressively stronger beats. It is not only more filled in diastole but it also empties better during systole. This additional strengthening of contraction de pends on the diminution of the influence of the vagus nerve, and on the excitation of the augmentor fibres of the sympathetic nerve.
But usually in addition to this increase in strength of the beat there is a quickening of the heart rate. This is also based in the first instance on a diminution of the vagus tone, and probably in most cases on an augmentation of the sympathetic tone. Further increase of the inflow is therefore met by a corresponding quicken ing of the heart beat. The acceleration of the heart in muscular exercise is a purely reflex phenomenon, based (especially at the be ginning of the exercise) partly on impulses descending to the cardiac medullary centres from the cortex of the hemispheres, which are probably conditioned reflexes. In part the acceleration is due to concurrent changes in the respiratory activity and to reflexes originating within the vascular system of the animal itself. We have seen that each inspiratory phase is accompanied by a diminution of the vagus tone and hence by an acceleration of the heart. Thus if there is a greater number of respirations per minute, the periods of acceleration become more frequent and finally fuse, so that the periods of retardation typical of the expiratory state have no time to develop.
Amongst the reflexes which affect the cardiac rhythm and which originate within the organs of the body itself, the reflexes from the heart should be considered first. It has been shown by several ob
servers that an increased inflow into the heart brings about a reflex acceleration of the heart beat. Changes in the activity of the heart should not be regarded as the cause of increased intensity of circulation, but as an adaptation which enables the increase to be performed.
The chief causes of increased total blood flow during muscular exercise can be grouped under the following headings: I. The increase in the aspiration by the thorax ; 2. The pumping action of the contracting muscles ; 3. The increase in the amount of circulating blood ; 4. The diminution of the total vascular resistance. To these should be added the factors which are not likely to affect the total blood flow, but which alter the distribution of the blood between the various organs. These are : I. The constriction of the splanchnic area; 2. The dilation of the blood vessels of the muscles.
These factors have all been discussed in the preceding pages and need only be mentioned shortly. The increased and more frequent expansion of the chest, which follows the deepening and acceler ation of respiration, accentuates the negative pressure in the tho rax, and tends to diminish the pressure in the large veins. These are therefore more quickly and more completely filled with blood from the periphery. The working muscles expel, by each contraction, blood from their capillaries to the veins, and thus actively help the propulsion of blood into the heart. The spleen contracts and empties its blood into the vascular system, and an extra amount of blood thus becomes available. The dilation of the blood vessels in the muscles is so intense that the general resistance diminishes and the blood passes from the aorta into the venous system at a considerably faster rate. All these factors are helped by the con traction of the splanchnic vascular area, which diverts the blood to the muscles. The blood vessels in the muscle dilate, and the capillaries open so that the capillary bed in the muscles may in crease 4o to too times during maximal activity. This dilation is chiefly if not exclusively due to the local production of vasodilator substances. It is not at present known how far the vasomotor innervation participates in this dilation.
Thus we see that during muscular exercise the organism meets the demand for an increased oxygen supply to the muscle by an intricate series of correlated mechanisms. The necessary adjust ments involve the respiratory system and the whole cardio-vascular system, and probably afford one of the finest examples of co ordination that has yet been discovered in the realm of physiology.