Pressor reflexes can be evoked by stimulation of almost any of the sensory nerves of the skin, the digestive tract or other abdominal viscera. The chief depressor nerve originates in the heart and in the first part of the aorta. In the rabbit this nerve runs in a separate and easily accessible trunk. Stimulation of the central end of the depressor produces such a large dilation of most of the blood vessels that the arterial blood pressure falls to a very low level. In the intact animal the physiological stimulus which brings this depressor mechanism into operation is the high blood pressure in the aorta. Distension of the aorta by pressure stimulates the depressor nerve and leads to a com pensatory dilation or arterioles and to a fall in pressure. The nerve can be regarded as a safety valve which prevents excessive rises of blood pressure.
It is highly probable that in the case of an excessive fall of pressure another reflex mechanism comes into action, a pressor reflex which also originates in the heart and aorta and which tends to antagonize the fall. Recently it has been shown that the carotid artery at the point of its division into the internal and external branch is also sensitive to pressure. Increase of pressure in this part leads to a reflex vasodilation, and fall of pressure leads to a vasoconstriction. Besides these two influences, it seems that the vasomotor centre itself can be excited or in hibited by variation in the blood pressure. Fig. 2 2 gives an example of the effect on the general blood pressure of changes in the pres sure of the brain (which was perfused through the carotid arteries).
Thus we see that the organism possesses several mechanisms by means of which it can keep the blood pressure constant and counteract any influences which would tend to raise or lower it. In the case of haemorrhage, a considerable amount of blood may be removed from the body, but the blood pressure soon returns to the normal. Blood may be withdrawn from the system slowly to the extent of about 4%, or rapidly to the extent of about 2% without lowering the arterial pressure, owing to the compensatory constriction of the arterioles. Such withdrawal of blood, though it does not affect the blood pressure, will greatly diminish the intensity of the circulation and will reduce the output of the heart. Conversely, injection of salt solution (0.9%) or transfusion of blood will not, within certain limits, affect the arterial blood pressure owing to compensatory vasodilation. Transfusion will,
however, increase the circulation of blood and thus lessen the harmful consequences of haemorrhage. Considerable haemorrhages will not be effectively compensated and will therefore lead to fall of blood pressure, and transfusion will be absolutely necessary to save the animal from death.
Since most blood vessels are innervated by vasoconstrictor and vasodilator fibres, it is obvious that a pressor or depressor reflex could be carried out by several means. Vasodilation could for instance be produced by means of a reflex inhibition of the vaso constrictor tone, or by a stimulation of the vasodilator fibres which would depress the tonic state of the blood vessels. What actually happens is that the two sets of fibres in all known cases act in a reciprocal fashion. Thus a depressor reflex involves a simultaneous diminution of the vasoconstrictor influence and increase of the vasodilator influence, and vice versa in the case of a pressor reflex.
Chemical Factors.—In recent years there has been a con siderable accumulation of facts which show that various chemical factors affecting the blood vessels, apart from any nervous influ ence, are of great importance for the regulation of the circulation, and that these factors can exercise their action either on the arterioles or on the capillaries.
It is known for instance that in many conditions, e.g., pain, asphyxia, the physiological state that accompanies emotion, there is a considerable rise in the production of adrenaline by the suprarenal glands. Adrenaline, as has been mentioned before, stimulates the nerve endings of the sympathetic nervous system, and it theref ore also stimulates the nerve endings of the vaso constrictor nerves. Thus, in all cases associated with increased production of adrenaline, there is widespread vasoconstriction and a rise in the blood pressure; this vasoconstriction is independent of the integrity of the vasomotor nerves. In the case of activity of an organ, vasodilation occurs in most cases not on account of concurrent depressor reflexes but on account of the production of chemical substances within the organ. These substances mainly affect the tone of the capillaries, and in their presence a great number of hitherto closed capillaries open or dilate widely. It has been shown that in the case of a contracting muscle the capil laries may be increased from 2% to 15% of the total volume of the muscle.