Figure 17.—The arrows indicate the points at which the thorax began to be compressed in order to lessen the output of blood from the heart.
The region of 90 to 100 millimeters of mercury may therefore be regarded as the critical region at which a falling blood pressure begins to be accompanied by a concurrent lessening of the efficiency of muscular contraction, when the muscle is kept in continued activity. It is at that region that the blood flow is dangerously near to being inadequate.
An Explanation of the Effects of Varying the Arterial Pressure
How are these effects of increasing and decreasing the arterial blood pressure most reasonably explained? There is abundant evidence that fatigue products accumulate in a muscle which is doing work, and also that these metabolites interfere with efficient contraction. As Ranke[3] long ago demonstrated, if a muscle, deprived of circulating blood, is fatigued to a standstill, and then the circulation is restored, the muscle again responds for a short time to stimulation, because the waste has been neutralized or swept away by the fresh blood. When the blood pressure is at its normal height for warm-blooded animals (about 120 millimeters of mercury, see [Fig. 13]), the flow appears to be adequate to wash out the depressive metabolites, at least in the single muscle used in these experiments, because a large rise of pressure produces but little change in the fatigue level. On the other hand, when the pressure is abnormally low, the flow is inadequate, and the waste products are permitted to accumulate and clog the action of the muscle. Under such circumstances a rise of pressure has a very striking beneficial effect.
It is noteworthy that the best results of adrenin on fatigued muscle reported by previous observers were obtained from studies on cold-blooded animals. In these animals the circulation is maintained normally by an arterial pressure about one-third that of warm-blooded animals. Injection of adrenin in an amount which would not shut off the blood supply would, by greatly raising the arterial pressure, markedly increase the circulation of blood in the active muscle. In short, the conditions in cold-blooded animals are quite like those in the pithed mammal with an arterial pressure of about 50 millimeters of mercury (see [Fig. 16]). Under these conditions the improved circulation causes a remarkable recovery from fatigue. That notable results of adrenin on fatigue are observed in warm-blooded animals only when they are deeply anesthetized or are deprived of the medulla was claimed by Panella.[4] He apparently believed that in normal mammalian conditions adrenin has little effect because quickly destroyed, whereas in the cold-blooded animals, and in mammals whose respiratory, circulatory, and thermogenic states are made similar to the cold-blooded by anesthesia or pithing, the contrary is true. In accordance with our observations of the effects of blood pressure on fatigued muscle, we would explain Panella’s results not as he has done but as due to two factors. First, the efficiency of the muscle, when blood pressure is low, follows the ups and downs of pressure much more directly than when the pressure is high. And second, a given dose of adrenin always raises a low blood pressure in atonic vessels. The improvement of circulation is capable of explaining, therefore, the main results obtained in cold-blooded animals and in pithed mammals.
Oliver and Schäfer reported unusually effective contractions in muscles removed from the body after adrenal extract had been injected. As shown in [Fig. 16], however, the fact that the circulation had been improved results in continued greater efficiency of the contracting muscle. Oliver and Schäfer’s observation may reasonably be accounted for on this basis.
The Value of Increased Arterial Pressure in Pain and Strong Emotion
As stated in a previous paragraph, there is evidence that the vessels supplying a muscle dilate when the muscle becomes active. And although the normal blood pressure (about 120 millimeters of mercury) may be able to keep adequately supplied with blood the single muscle used in our investigation, a higher pressure might be required when more muscles are involved in activity, for a more widely spread dilation might then reduce the pressure to the point at which there would be insufficient circulation in active organs. Furthermore, with many muscles active, the amount of waste would be greatly augmented, and the need for abundant blood supply would thereby to a like degree be increased. For both reasons a rise of general arterial pressure would prove advantageous. The high pressure developed in excitement and pain, therefore, might be specially serviceable in the muscular activities which are likely to accompany excitement and pain.
In connection with the foregoing considerations, the action of adrenin on the distribution of blood in the body is highly interesting. By measuring alterations in the volume of various viscera and the limbs, Oliver and Schäfer[5] proved that the viscera of the splanchnic area—e. g., the spleen, the kidneys, and the intestines—suffer a considerable decrease of volume when adrenin is administered, whereas the limbs into which the blood is forced from the splanchnic region actually increase in size. The action of adrenin indicates the relative degrees of sympathetic innervations. In other words, at times of pain and excitement sympathetic discharges, probably aided by the adrenal secretion simultaneously liberated, will drive the blood out of the vegetative organs of the interior, which serve the routine needs of the body, into the skeletal muscles which have to meet by extra action the urgent demands of struggle or escape.
But there are exceptions to the general statement that by adrenin the viscera are emptied of their blood. It is well known that adrenin has a vasodilator, not a vasoconstrictor, action on the arteries of the heart; it is well known also that adrenin affects the vessels of the brain and the lungs only slightly if at all. From this evidence we may infer that sympathetic impulses, though causing constriction of the arteries of the abdominal viscera, have no effective influence on those of the pulmonary and intracranial areas and actually increase the blood supply to the heart. Thus the absolutely and immediately essential organs—those the ancients called the “tripod of life”—the heart, the lungs, the brain (as well as its instruments, the skeletal muscles)—are in times of excitement abundantly supplied with blood taken from organs of less importance in critical moments. This shifting of the blood so that there is an assured adequate supply to structures essential for the preservation of the individual may reasonably be interpreted as a fact of prime biological significance. It will be placed in its proper setting when the other evidence of bodily changes in pain and excitement have been presented.