Regulation of blood vessels.

THE CARDIOVASCULAR system serves the biologic organism as a transportation and mixing system. The demalnd by the organism for this service varies in different tissues and at different times while the volume of blood in the organism tends to remain constant. The cardiovascular system fulfills its function, therefore, by varying the rate and the distribution of blood flow. Blood vessels play a major role in controlling the rate of flow of blood by affecting the resistance to flow, and they possess the entire responsibility for the distribution of the blood and its flow pathways. The primary aspect of blood vessels that affects the flow and distribution of blood is their caliber, i.e., their internal radius. This discussion will deal primarily with the factors that control the caliber of blood vessels and with the role of these factors in the regulation of the circulation of blood. The concept of control of the caliber of blood vessels by the nervous system is relatively new. Moreover, the mechanisms by which the caliber of blood vessels vary as a result of nervous influences are only beginning to be understood and evaluated. Indeed, the youthfulness of many of our concepts of physiology and medicine is so striking that I cannot avoid mentioning it. As late as 1831, Dr. E. Weber, a well-known Germnan physician, said, "I believe that flushing and pallor could be explained if it could be taken for granted that the caliber of arteries becomes larger and smaller under nervous influence. "1 Only slowly has evidence begun to accrue during the past century that supports the idea of nervous control of the heart and blood vessels. It was not until after the