Hemodynamic changes in the small vessels in man as analyzed by digital plethysmography.

ROBERT L.LJOHNSON*, M.D., EDWARD D.,FREIS, M.D. AND HAROLD W. SCHKAPER, MB. G Although fairly adequate methods are available for the study of the hemodynamics of the arterial circulation .and overall blood flow to various areas, little is known of the dynamic.changes which occur in the small vessels in man. Aside from the observations of “Thomas Lewis (1) on the terminal circulation in the human skin, and measurements of static capillary pressure by Landis (2), further explorations have been hampered by the lack of suitable methods of st.udy. On the other hand much has been learned about, the minut.e vessels from investigations in living animals and from anatomic specimens. Anatomically the capillary bed begins with the metarterioles jvhich arise as branches or prolongations of the terminal arterioles. The metarterioles form the initial portions of the so-called preferential or t,horoughfare channels described by Chambers and Zweifach (3) which, after a relatively long course, join similar channels and terminate in the venules. Numerous sphincteric muscular buds, the precapillary sphincters (4), which originate at, various interyals along the early portion of the thoroughfare rhannels, make up the proximal ends of the true capillaries. These small endothelial vessels in turn empty into the distal segments of the same or other thoroughfare channels. In addition to these components of the small vessel unit there occasionally are relatively large short, vessels, the arteriovenous anastomoses, &ieh arise from metarterioles or arterioles and empty directly into venules. Changes in the diameter of t,he proximal portions of the thoroughfare channels permit flow through these vessels to be independent of pressure and flow conditions in the systemic circulation. Further, the pulsatile quality of the blood flow in the larger arterial vessels is replaced by a relatively continuous flow through the thoroughfare channels. Alternate opening and closing of the precapillary sphincters produces an intermittent type of flow through the capillaries. These alternating periods of spontaneous const,riction and relasation of the metarterioles and of the precapillary sphincters, a process known as vasemotion (5), allows adjustment of the circulation through the capillary unit as necessary to provide for the metabolic requirements of the tissue at any moment and in the skin to serve as radiators of body heat. The arteriovenous anastomoses (6) are usually closed during normal conditions but open under special circumstances of blood flow.