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The function of any cell group depends largely upon the pressure of the blood in the vessels applying it. The cell is bathed in lymph, and this is derived from the capillaries. The flow of the nutrient lymph, while probably not altogether independent of a certain secretory activity on the part of the capillary endothelium, is yet almost absolutely subject to the laws of osmosis and diffusion. The variations in lymph flow due to changes in osmotic tension depend upon changes in the quality of the blood, in the quality of the waste material thrown off by the cells, and in the rapidity of the flow of the lymph by which the products of cell metabolism and the unabsorbed foods derived from the blood are carried away. The lymph varies normally according to the pressure in the veins and arteries, and the presence of substances in the blood which are the result of the metabolism of the various organs of the body, or are taken with the food. Under abnormal conditions, the lymph flow is affected by very many factors.
The lymph flow depends in part upon the capillary pressure. This follows the pressure within the arterioles, and this in turn depends upon several other factors. Arterial pressure is increased by an increased rate and force of the heart’s action, by an increased quantity of the blood in the vessels, by the contraction of the arterioles in the tissues of any marked area, or by any interference with the circulation through any organ or group of vessels. An increased secretion of certain of the ductless glands, notably the supra-renal capsules, increases the blood pressure both by initiating an increased force of the heart’s action and by decreasing the caliber of the vessels. The manner in which this increase in the activity of the non-striated muscles is produced by these internal secretions is not yet understood. The blood pressure is decreased by a diminished rate and force of the heart’s action, by anything which decreases the amount of blood in circulation, or by the dilatation of any considerable area of blood vessels. The blood pressure is practically the same in all vessels of equal rank and caliber all over the body. The differences due to gravity, to the pressure of other organs and to other factors are not great under normal conditions. Under abnormal conditions, however, these factors become matters of serious import. When the vascular walls lose their tone, the influence of gravity dilates them most painfully, and the elevation of the part affected affords great temporary relief.
When the arterioles in a given area are dilated, and the systemic pressure is low, the blood flows slowly through the dilated arteries and the capillaries. The interchange of gases and of foods is very slow. The diffusion of the proteids of the blood serum is always a matter of difficulty, only secured under normal circumstances by the maintenance of a high arterial pressure. During a period of low pressure, these may be scarcely diffused at all, and as a result the cells of the body may be insufficiently nourished, even though the blood itself be fairly normal. The blood itself does not remain even fairly normal if the pressure remain abnormally low for any great length of time. The hematopoietic organs are just as dependent upon the maintenance of a normal pressure within the blood vessels as are any other organs of the body. Digestion and assimilation fail in the presence of persistently low blood pressure, and the blood itself soon becomes very poor, in its serum constituents as well as in its anatomical elements.
The absorption of the waste products of cell metabolism is also hindered during periods of low pressure. The return flow of the lymph is often retarded in these cases, and the cells are forced to maintain their functions as well as they may in the presence of their own excretions, as well as with a poor food supply. Normally, the carbon dioxid is carried from the tissues chiefly by the veins. The amount of any gas which can be absorbed by any liquid depends upon the temperature and pressure. Now since the temperature of the body remains fairly constant, it is evident that the absorption of gas by the blood varies directly with the pressure within the capillaries. In the presence of a low pressure the carbon dioxid normally formed by the living cells is not properly eliminated. This failure of the elimination of the carbon dioxid, together with the oxygen deficiency usually associated with it, is a source of several abnormal conditions of more or less discomfort and danger.
During digestive activity, a low pressure almost invariably causes the accumulation of gas within the digestive tract. The habitual occurrence of this symptom is itself a cause of an abnormal distention and later a dilation of the stomach or some part of the intestinal tract. Other evils follow in the train of such dilatation in the due course of events. The accumulation of gas, in itself, is a matter of grave discomfort and annoyance, and under some conditions may be a source of danger. The gases formed by the action of the digestive juices and bacteria upon the food stuffs in the alimentary canal are, under normal conditions, absorbed by the blood and eliminated from the lungs. In the presence of an abnormally low blood pressure, there are retained within the stomach and intestines, causing pain and considerable annoyance from borborygmi and eructations.
The metabolism of the cells in the absence of a proper exchange of oxygen and carbon dioxid varies greatly from the normal. The waste products of katabolism are not thoroughly oxidized. In normal metabolism the waste materials are almost neutral in reaction. The blood remains alkaline during any amount of metabolism when the normal oxygen supply and carbon dioxid removal is maintained. But when the proper balance between these gases is disturbed, the metabolism also is disturbed, and the katabolic products include complex, poorly oxidized compounds, acid in reaction, very variable in their chemical relationships and almost invariably toxic to the cells of the body tissues, and the alkalinity of the blood is decreased. The liver and the lymphatic glands, and perhaps other adenoid elements of the body render these substances inert. The kidneys, liver, and other depurating organs eliminate them from the body as rapidly as possible, but the powers of these organs are limited, and the symptoms of auto-intoxication are almost sure to occur sooner or later. The alkalinity of the blood is lessened by the retention of the normal wastes, by the formation of the abnormal wastes, and by other factors concerned in the oxygen and carbon dioxid relation. The bacteriolytic power of the blood is decreased with the decrease in its alkalinity.
The activity of all the glands of the body is subject to variations in answer to variations in the blood pressure. Any gland, whether it possesses secretory nerves or not, is more active in the presence of high pressure than during a decrease in pressure. The effects of the action of the secretory nerves may mask the variations due to changing blood pressure. In the case of the sweat glands, for example, the secretion may be almost or quite suspended in the presence of high pressure, or be increased during periods of low pressure. In all such cases, however, the action of the secretory nerves is effective only as a stimulation to the katabolic processes, the nerve influences are not effective in the absence of the zymogen granules from which the secretion is derived. Secretory nerves initiate the discharge of the substances already potentially formed within the substance of the cell protoplasm. The effects of the stimulation of the secretory nerves fail utterly after a comparatively short period of activity with a low blood pressure, or when the blood supply is deficient. When the blood supply is good, and the pressure is high, the serum proteids are more easily diffusible and the gland is properly nourished. The oxygen supply and the carbon dioxid removal are also facilitated by the higher pressure. Variations in the action of the kidneys, especially, are known to depend almost, if not quite, upon variations in the rapidity of the blood flow. Secretory nerves have not been demonstrated in them. Given an abnormally low arterial pressure, the action of the kidneys is insufficient. Given an abnormally high venous pressure, the action of the kidneys is also deficient. Given a somewhat increased pressure, the action of the kidneys is also somewhat increased, but the persistent increase in the blood pressure so injures the renal cells that malfunction is produced, with the associated structural lesions. Local changes in the vascular musculature, as well as changes in the systemic pressure, affect the activity of all glands. The changes in the metabolism of the ductless glands in these conditions offer a field for some very profitable work in investigation. The little study that has been made of clinical cases in which malfunction of these glands has exercised very deleterious effects upon the general body metabolism, seems to indicate that the action of these glands may be very dependent upon vaso-motor influences.
The brain itself is not exempt from changes in its activity due to changing pressure in its vessels. Sleep is accompanied, and, in part at least, produced, by a lowered pressure due to a general dilatation of the systemic arterioles. Any considerable lowering of the blood pressure due to any cause is marked by a sleepiness, or by a stupid, dull feeling. In a series of experiments performed in the investigation of physical phenomena associated with mental conditions, it was noticed that the usual effect of mental effort is to increase the systemic blood pressure. Conversely, the lowering of the systemic pressure by the experimental dilatation of the splanchnic arterioles was followed by decided sleepiness and an inability to concentrate the attention in the degree to which the subject was accustomed. The contraction of the splanchnic arterioles by stimulating manipulations raises the systemic pressure and renders the mental processes more speedy and the mental pictures more vivid. If the increase in the blood pressure does not exceed the degree normal to the individual, the experimental increase of blood pressure was followed by a consciousness of well being, and by a very rapid, pleasant, and vivid flow of mental processes.
Under abnormal conditions the effects of changing blood pressure in modifying mentality are much more pronounced. Melancholia and the apathetic psychoses are marked by very low pressure. Anything that raises the blood pressure in these cases in slight degree exercises a beneficial effect upon the neurosis. The excitable manias are characterized by abnormally high pressure. The insane manifestations of these are somewhat relieved if the systemic pressure can be decreased. The pressure changes in the psychoses are probably in part a cause of the symptoms observed, but it is also true that emotional reactions produce vaso-motor effects. Under fairly normal conditions, a persistently gloomy attitude toward one’s surroundings lowers blood pressure, and, on the other hand, a low blood pressure renders the gloomy view the easy one, and the cheerful aspect a matter of considerable effort. Reasoning from the normal conditions, it appears that the so-called functional psychoses are in part referable to the metabolic changes due to deficient or excessive blood pressure, together with the faulty elimination of waste products usually associated with such conditions.
It is evident that the normal activity of any cell group requires the maintenance of a sufficiently high pressure in the arterioles. The effects of an abnormally high pressure are not less disastrous than are the effects of an abnormally low pressure. Too high pressure, if long continued, leads to an abnormal activity of certain organs of the body, and to their too speedy fatigue. The kidneys especially are very easily injured by persistently high blood pressure. The phenomena of arterio-sclerosis ensue, if the walls of the blood vessels are long subject to too high pressure, especially if the blood contain toxic irritants which render the cells more unstable than is their wont. The walls of the blood vessels are subject to various pathological conditions when they are kept under too great tension for a sufficient length of time. The heart, also, is injured by the maintenance of too high a pressure.
The action of the arterioles in maintaining a normal degree of pressure in the blood vessels is kept regulated by the vaso-motor nerves. These are axons of the sympathetic neurons, which receive their stimulation from coordinating centers in the viscero-motor nuclei in the spinal cord, medulla, pons and mid-brain. Any interference with the pathway by means of which the nerve impulses are carried to or from the coordinating centers, must lessen the normal relation between the vascular dilatation and the functional activity of the different tissues. Any condition which abnormally increases the activity of the vaso-motor centers causes an abnormal vaso-constriction, and any condition which abnormally decreases the activity of the vaso-motor centers causes an abnormal dilatation of the vessels. If the vessels are permitted to remain for a long time dilated, the area of their distribution is injured, and the other organs of the body are subjected to a decreased blood pressure. If the vessels of any tissue are forced to remain contracted for any length of time the tissue undergoes a degenerative process, and usually, if the condition be not relieved, gangrene ensues. The gangrene of ergotism is of interest in this connection. Under normal conditions, the vessels of any organ become dilated during its activity. At the same time, and in answer to the same nerve impulses which cause the local dilatation, the vessels in other parts of the body become contracted, so that the pressure of the blood in the dilated arterioles remains high,--perhaps even becomes higher than before the local dilatation occurred. Sometimes, under abnormal conditions, the vaso-motor impulses are not properly coordinated, and the dilatation of the vessels in the active structure is not accompanied by the general vaso-constriction. In such a case, the general blood pressure is lowered, the functional activity of the whole body is lessened, and the elimination of the waste products of such metabolism as does occur is decreased. The active structure whose vessels are dilated lacks the normal pressure, and its function is thereby lessened. This malfunction and the abnormal conditions of pressure and nutrition initiate sensory impulses, which, reaching the vaso-motor centers, effect still further dilatation of the vessels in the organ or cell group whose activity has caused the whole series of reactions. The coordination of all, or nearly all, of the vaso-motor nerves of the body is essential to the normal activity of any important organ or cell group. This coordination fails under the following conditions:-- Local abnormalities of structure may interfere with the normal passage of sensory impulses from any part of the body or of vaso-motor impulses to it. Nerve trunks may be subjected to the steady pressure which decreases their power to transmit impulses, or to pressure which varies, as the pressure of a pulsating artery, etc., and so exercises a continual stimulating effect upon its fibers. Malpositions of the vertebrae may either increase or decrease the activity of the subsidiary centers, by initiating abnormal sensory impulses. The neuron threshold of any center may be either abnormally raised or abnormally lowered by abnormal conditions of nutrition or function. Abnormal structural conditions of the heart or of the vessel walls may render them inefficient, in the presence of the normal nerve impulses. Abnormal impulses from the brain, especially from the basal ganglia, may interfere with the action of the lower centers.
Ergotism, Osler’s Practice of Medicine. Raynaud’s Disease, McConnell and Teall. Raynaud’s Disease, J. L. Adams in A. O. A. Case Reports. |