[Vasomotor activity and vascular geometry].

The circulation may be divided into several anatomical and functional segments placed in series. The vessels of distribution (great arteries) are designed to decrease the cardiac periodicity. The tone of the resistance vessels (arterioles) control the blood pressure. In situ changes of their diameter must be assessed with respect to variations in pressure in order to evaluate the active or passive mechanism of a physiological or therapeutic intervention. The vessels of exchange (capillaries, veinules) which have no smooth muscle in their walls are passively affected by changes in flow and transmural pressure. The capacitance vessels (veins) are partially affected by the blood flow entering this segment and influence the cardiac output by the effects of changes of volume on venous return to the heart.