The following is a brief review from a pharmacologic standpoint:

The circulatory organs are for the purpose of carrying certain materials to and from the tissues by means of the blood; and since all exchanges between the blood and the tissues are made through the capillary walls, it may be said that the function of the circulatory organs is to maintain an adequate capillary blood-flow. Hence the circulatory organs need treatment when they fail to maintain an adequate capillary blood-flow. This capillary blood-flow is dependent somewhat upon the viscosity of the blood, but mainly upon the relation between the general arterial blood-pressure (the driving force) and the caliber of the arterioles which lead to the capillaries (the peripheral resistance). These arterioles, being actively contractile, serve as adjustable gates by means of which the amount of blood passing to any given set of capillaries may be regulated. And it is obvious that if the general arterial pressure remains the same an increase in the caliber of any given set of arterioles will result in a greater supply of blood to the capillaries of that set; and that if the caliber of these arterioles remains the same, an increase in the general arterial pressure will have a similar result. The adjustment of the caliber of individual sets of arterioles without producing the same changes in other sets is, for the most part, impossible therapeutically; but the caliber of the arterioles as a class may be readily changed by remedial measures.

Capillary flow may be altered by changes in - (1) The total amount of blood in the arterial system; (2) the heart's output in a given time; (3) the general peripheral or arteriole resistance, and (4) the viscosity of the blood.

The amount of blood in the arteries may be decreased by its accumulation in the veins, by its loss from the body (as in hemorrhage or blood-letting), or by the excessive removal of other fluid from the body, as in cholera or other severe diarrheal conditions. It may be increased, especially after a preliminary loss, as in hemorrhage or cholera, by increased receipt from the veins or tissues, by transfusion of blood, by intravenous administration of saline solutions, and by rapid absorption of liquid, e. g., saline solutions, from the alimentary tract or elsewhere. The heart's output may be affected by measures which influence either the filling, the capacity, the rate, or the strength of the ventricles. The peripheral resistance may be altered by measures which change the caliber of the arterioles.

It will be obvious that the rate of capillary flow is not to be judged by the degree of general arterial pressure. For example, suppose the heart increases its output, but the arterioles dilate just enough to let the additional blood through. Then, though the general pressure remains unchanged, yet more blood flows through the capillaries and the circulation is more active. As a matter of fact, it has been found in man that the mechanisms which control blood-pressure are so neatly adjusted that it is well-nigh impossible to cause a decided rise in arterial pressure by a therapeutic dose of any slowly acting drug, and yet some such drugs, e. g., digitalis, do have great power to improve the circulation. So the therapeutic value of a circulatory drug cannot be measured by its ability to raise arterial pressure in man. However, in dogs and other laboratory animals we can inject toxic doses intravenously, and thus bring about a concentration of the drug in the blood which will produce effects of sufficient degree and with sufficient rapidity to submerge the dissipating influences. And these give us valuable information as to the real sites and modes of action of a drug.