Capillary Blood Vessel

Abstract

EVERY cell of the body is brought into material relationship with all other cells in virtue of the existence o£ a common medium, the blood, which is maintained in constant circulation throughout the body. Substances absorbed into the blood from the exterior, either through the external or internal surfaces of the body, are thus brought round and presented to every cell, to be taken up or rejected according to the needs of the latter. In the same way the products of the chemical changes occurring in any cell are distributed to all other cells, so that the blood represents the internal environment integrating the metabolic activities of all parts of the body. The interchange between blood and tissues takes place only in the capillaries and smaller veins, so that we may say that the whole vascular system-heart, arteries, and veins-exists to ensure an adequate passage of blood through the capillaries. It is therefore rather surprising that the physiology of the capillaries has been comparatively neglected until the last few years. There have been isolated observations with regard to their structure and contractility and the properties of their walls. Some twenty-five years ago, when the question of lymph production and absorption was brought into prominence by the researches of Heidenhain, the functions of the cells forming the capillary walls were hotly debated, but after a few years, interest in the matter died down, and physiologists failed to appreciate or to follow up the many other problems concerning the capillaries which were implicit in the problems of lymph production.