tailieunhanh - Introduction to the Cardiovascular System - Part 5

Hình đường cong tuân thủ 5-10 tĩnh mạch. Tĩnh mạch phù hợp (độ dốc của đường tiếp xúc với một điểm trên đường cong) là rất cao ở áp suất thấp bởi vì tĩnh mạch sụp đổ. Khi tăng áp lực tĩnh mạch giả định một mặt cắt tròn hơn và những bức tường của nó trở nên kéo dài | VASCULAR FUNCTION 105 FIGURE 5-10 Compliance curves for a vein. Venous compliance the slope of line tangent to a point on the curve is very high at low pressures because veins collapse. As pressure increases the vein assumes a more circular cross-section and its walls become stretched this reduces compliance decreases slope . Point A is the control pressure and volume. Point B is the pressure and volume resulting from increased tone decreased compliance brought about for example by sympathetic stimulation of the vein. that an increase in venous volume will increase venous pressure. The amount by which the pressure increases for a given change in volume depends on the slope of the relationship between the volume and pressure . the compliance . As with arterial vessels see Fig. 5-4 the relationship between venous volume and pressure is not linear see Fig. 5-10 . The slope of the compliance curve AV AP is greater at low pressures and volumes than at higher pressures and volumes. The reason for this is that at very low pressures a large vein collapses. As the pressure increases the collapsed vein assumes a more cylindrical shape with a circular cross-section. Until a cylindrical shape is attained the walls of the vein are not stretched appreciably. Therefore small changes in pressure can result in a large change in volume by changes in vessel geometry rather than by stretching the vessel wall. At higher pressures when the vein is cylindrical in shape increased pressure can increase the volume only by stretching the vessel wall which is resisted by the structure and composition of the wall particularly by collagen smooth muscle and elastin components . Therefore at higher volumes and pressures the change in volume for a given change in pressure . compliance is less. The smooth muscle within veins is ordinarily under some degree of tonic contraction. Like arteries and arterioles a major factor determining venous smooth muscle contraction is sympathetic adrenergic .