Evidence That Vascular Conductance Is Regulated At the Capillary

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Abstract
The present study suggests that the local, nonemergency regulation of blood flow may take place at the capillary level, in conjunction with the fundamental exchange functions of the circulation. Such a relationship results from the hydrodynamic behavior of a permeable soft-walled capillary enclosed in a capsule. This thesis is illustrated in experiments on the vascular conductance of intact and perfused lung lobes, heart and kidney. Similar results were obtained on models. The pulmonary model consisted of an arteriovenous perfusion head, which produced flow through a soft-walled capillary in an enclosure (alveolus). The model of the systemic circulation consisted of a soft-walled permeable capillary in a capsule. These models exhibit critical closing and opening, and basal tone; the hyperemias of the postocclusion, compression, exercise and metabolic varieties; venous and arterial autoregulations; shunt and nutritional flows; and other phenomena of vascular beds. The roles of arteriolar and venular constriction are indicated to be limited to emergency functions. Neurohumoral mechanisms and the effector apparatus, such as smooth muscle, thus are viewed as moderating the transient effects of changes in position, sudden loss of blood and other emergencies. Nerves, hormones and pharmacologic materials may also produce hemodynamic effects by acting through capillary permeability. The present study suggests that some aspects of the regulation of the circulation may operate independently of the vascular smooth muscle. Reevaluation of the vascular pharmacodynamics from the point of view of the hydrodynamics of the capillaron may assist in the clarification of the complexities and numerous paradoxes of the responses of the vascular system to hormones and drugs.