The urgent need in struggle or flight is a generous supply of oxygen to oxidize the metabolites of muscular contraction, and a quick riddance of the resultant carbon-dioxide from the body. The moment vigorous exercise is begun the breathing at once changes so as to bring about a more thorough ventilation of the lungs. And one of the most characteristic reactions of animals in pain and emotional excitement is deep and rapid respiration. Again the reflex response is precisely what would be most serviceable to the organism in the strenuous efforts of fighting or escape that might accompany or follow distress or fear or rage. It is known that by such forced respirations the carbon-dioxide content of the blood can be so much reduced that the need for any breathing whatever may be deferred for as much as a minute or even longer.[31] And Douglas and Haldane[32] have found that moderately forced breathing for three minutes previous to severe muscular exertion results in greatly diminishing the subsequent respiratory distress, as well as lessening the amount of air breathed and the amount of carbon-dioxide given off. Furthermore, the heart beats less rapidly after the performance and returns more quickly from its increased rate to normal. The forced respirations in deeply emotional experiences can be interpreted, therefore, as an anticipatory reduction of the carbon-dioxide in the blood, a preparation for the augmented discharge of carbon-dioxide into the blood as soon as great muscular exertion begins.[*]
[*] The excessive production of heat in muscular work gives rise to sweating. The evaporation of sweat helps to keep the body temperature from rising unduly from the heat of exertion. Again in strong emotion and in pain the “cold sweat” that appears on the skin may be regarded as a reaction anticipatory of the strenuous muscular movements that are likely to ensue.
As the air moves to and fro in the lungs with each respiration, it must pass through the fine divisions of the air tubes or bronchioles. The bronchioles are provided with smooth muscle, which, in all probability, like smooth muscle elsewhere in the body, is normally held in a state of tonic contraction. When this tonic contraction is much increased, as in asthma, breathing becomes difficult, and even with the body at rest unusual effort is then required to maintain the minimal necessary ventilation of the lungs. During strenuous exertion, with each breath the air must rush through the bronchioles in greatly increased volume and speed. Thus in a well person “winded” with running, for example, the bronchioles might become relatively too small for the stream of air, just as they are too small in a person ill with asthma. And then some extra energy would have to be expended to force the air back and forth with sufficient rapidity to satisfy the bodily needs. It is probable that even under the most favorable conditions, the labored breathing in hard exercise involves to some degree the work of accelerating the tidal flow of the respiratory gases. This extra labor would obviously be reduced, if the tonic contraction of the ring-muscles in the wall of the bronchioles was reduced, so that the tubules were enlarged. It has been shown by a number of investigators, who have used various methods, that adrenin injected into the blood stream has as one of its precise actions the dilating of the bronchioles.[33] The adrenin discharged in emotional excitement goes to the lungs before entering into relation with any other organ except the right heart chamber; it may, therefore, have as its first effect the relaxation of the smooth muscles of the lungs. This would be another very direct means of rendering the organism more efficient when fierce struggle calls for a bounteous supply of fresh air and a speedy discharge of the carbonaceous waste.
Effects Produced in Asphyxia Similar to Those Produced in Pain and Excitement
All the bodily responses occurring in pain and emotional excitement have thus far been considered as anticipatory of the instinctive acts which naturally follow. And as we have seen, these responses can reasonably be interpreted as preparatory to the great exertions which may be demanded of the organism. This interpretation of the facts is supported by the discovery that a mechanism exists whereby the changes initiated in an anticipatory manner by emotional excitement are continued or perhaps augmented by the exertion itself.
Great exertion, such as might attend flight or conflict, would result in an excessive production of carbon-dioxide. Then, although respiratory and circulatory changes of emotional origin may have prepared the body for struggle, the emotional provisions for keeping the working parts at a high level of efficiency may not continue to operate, or they may not be adequate. If there is painful gasping for breath in the course of prolonged and vigorous exertion, or for a considerable period after the work has ceased, a condition of partial asphyxia has evidently been induced. This condition, as everyone knows, is distinctly unfavorable to further effort. But the asphyxia itself may act as a stimulus.[34]
In our examination of the influence of various conditions on the secretion of the adrenal glands, Hoskins and I[35] tested the effects of asphyxia. By use of the intestinal segment as an indicator we compared the action of blood, taken as nearly simultaneously as possible from the vena cava above the adrenal vessels and from the femoral vein before asphyxia, with blood taken from the same sources after asphyxia had been produced. The femoral venous blood after passing the capillaries of the leg thus acted as a standard for the same blood after receiving the contribution of the adrenal veins. Asphyxia was caused by covering the tracheal cannula until respiration became labored and slow, but capable of recovery when air was admitted. It may be regarded, therefore, as not extreme.
The results of the degree of asphyxia above described are shown by graphic record in [Fig. 36]. Blood taken from the vena cava and from the femoral vein before asphyxia (“normal”) failed to cause inhibition of the contractions. Blood taken from the femoral vein after asphyxia produced almost the same effect as blood from the same vein before; asphyxia, therefore, had wrought no change demonstrable in the general venous flow. Blood taken from the vena cava after asphyxia had, on the contrary, an effect markedly unlike blood from the same region before (compare the record after 1 and after 7, [Fig. 36])—it caused the typical inhibition which indicates the presence of adrenal secretion.[*]
Figure 36.—At 1 normal vena-cava blood applied, at 2 removed. At 3 normal blood from femoral vein applied, at 4 removed. At 5 blood from femoral vein after asphyxia applied, at 6 removed. At 7 blood from the vena cava after asphyxia applied. Time, half-minutes.