Tissue Deficiencies

Among the internal excitants which arise from tissue deficiencies, those associated with thirst are very prominent. An abnormal decrease in the water content of the body is accompanied by the appearance of more crystalloid substances (salts) in the serum of the blood. This increases the density of the blood and makes it more nearly equal to that of the cells which form the salivary glands.

Lack of oxygen is another important tissue deficiency which arouses the neuromuscular apparatus to vigorous activity. Smothering and suffocation are produced by withholding adequate ventilation by way of the lungs. The manner in which afferent currents are set up under these conditions is not well understood. Two explanations are possible--pressure changes in the respiratory passages may excite interoceptors lying therein, or the effect may be due to deficiencies or excesses in the blood. From what we know of the mechanism of respiration the latter seems more probable. The respiratory rate is controlled by a chemically sensitive center in the medulla oblongata. Changes in the acidity or CO 2 tension of the arterial blood determine the rate at which this center discharges impulses to the respiratory muscles. The effect of CO 2 tension in the center is really the effect of the hydrogen ion concentration of the blood, and this tends to be adjusted to the mean barometric pressure at which a man is living. In high altitudes the alkalinity or O 2 tension of the blood is greatly reduced; and in order that the hydrogen ion concentration may remain constant, breathing becomes faster and deeper. It may be that the general activity induced by a sudden cutting off of oxygen is also to be explained by this mechanism, the change in the composition of the blood acting first in the medullary centers to produce violent movements of the diaphragm, and then spreading to include other centers.

The decrease in O 2 tension of the blood in high altitudes and in suffocation is not directly effective upon the respiratory and other nerve centers; the effect is produced by the ratio of O 2 tension to CO 2 tension. A simple experiment proves this. If a person breathes into an air-tight collapsible bag, which is arranged so that the air exchange between it and the lungs passes through a flask containing an alkali, the outgoing carbon dioxide will be absorbed but no renewal of oxygen is possible. Under these conditions neither suffocation nor discomfiture is experienced, and the blood may attain a very low O 2 : tension while retaining its normal CO 2 -O 2 ratio. No effect on breathing movements is produced, even though the person may become blue in the face as the result of oxygen deprivation and consequent reduction of hemoglobin.

Other tissue deficiencies besides lack of food, water, and air may act as excitants to general activity; but they have not yet been identified. Possibly the acquired craving for alcohol and other narcotics can be traced to a similar mechanism; that is, constant usage of these drugs and stimulants creates in the blood antitoxic substances which, if not neutralized by the substance for which they have been specifically created, will excite the neuromuscular apparatus and occasion restless behavior.