Central and reflex regulation of the circulation

The variety of means which the organism uses in adjusting its general pressure and the blood flow through different organs according to their needs has been made the subject of a great many researches, the results of which comprise a voluminous literature. In the great majority of cases the results of the experiments are based on observations of the changes in the mean blood pressure which occur under different conditions or on measurements of changes in the blood flow and of the volumes of different organs. In all these experiments the organs under observation retained their normal blood supply, a fact which makes the interpretation of the results sometimes very uncertain. In plethysmograph, calorimeter, blood pressure and blood flow experiments, as well as in direct microscopic observations of the calibre of small blood vessels, the changes observed are nothing but an expression of an algebraical sum of a number of different factors. The circulatory changes in peripheral organs are in any given instance determined by ‘passive’ and 'active’ influences, and there is often a large degree of uncertainty as to which of the two is the predominant factor. This is rendered still more complicated if the mean blood pressure is not steady, especially in plethysmographic measurements, since most of these are usually made with highly sensitive instruments with a dangerous degree of magnification. A few attempts have been made to eliminate the passive circulatory changes by means of stabilizing the hæmodynamic conditions in the organ under observation. The use of Bayliss’s mercury valve (1), which was devised for controlling the blood pressure of the animal, is valuable in preventing slow changes in the blood pressure, but is inadequate in all cases of sudden changes. Stefani (2) has introduced for the same purpose a method of perfusing at a known and constant pressure organs which were kept in connection with all their nerves. Sollman and Pilcher (3), Foa (4) and others have applied this method for perfusion of the spleen and limbs. If properly performed so as to avoid all possibility of a collateral circulation, this method does eliminate all passive variations in the blood flow, and the changes observed will be determined by active changes in the calibre of the blood vessels of the perfused organ. The few crossed circulation experiments which are on record are mostly of the type introduced by Leon Frédéricq (5), with a double intercarotid anastomosis between two animals. In some cases it serves as a good method for deciding whether certain changes are of central or of reflex origin, or whether the stimulus is of a nervous or of a hormone nature. But, as stated in another place, the blood pressure changes must be always distorted by the different rate of bleeding of one animal into the other, especially when the blood pressures in them are different.