Page 50 - Libro 2

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PART 1 — INTRODUCTION TO THE VASCULAR SYSTEM
Arteriole Capillary Pc Pi
Venule
Inspiration
“Pcv” Pcv
πc πi
Ppv
Figure 3-6 The Starling forces governing the movement of fluid across the capillary bed.
occurs. Edema formation is limited by the action of the calf muscle pump. The contraction of the calf muscles acts to empty the veins and decrease the ve- nous pressure. With venous thrombosis, the venous pressure is increased. The increased venous pressure will be transmitted back through the venous system, into the smaller veins (venules) and finally, the capil- laries. This increase in pressure at the capillary level will result in edema formation. As described earlier, the use of compression stockings will increase the interstitial pressure, which will increase fluid reab- sorption thus decreasing edema. Elevating the legs will reduce the intracapillary pressure (by lowering the hydrostatic pressure), which also limits edema formation.
VENOUS DYNAMICS AT REST
Changes in intrathoracic and intra-abdominal pres- sure have a profound effect on the venous return to the heart. During inspiration, the diaphragm de- scends, which decreases the pressure with the chest cavity. This causes blood to pool into the pulmonary vascular bed as well as pulls air into the lungs. Also during inspiration, the descending movement of the diaphragm results in an increase in intra-abdomi- nal pressure. This collapses the inferior vena cava, which impedes venous return from the legs. Upon expiration, the diaphragm moves upward, which de- creases intra-abdominal pressures. This results in an increase in blood flow from the legs and a decrease in blood flow into the thorax (Fig. 3-7).
With a deep venous thrombosis, venous pressure is increased in the legs because of an increase in ve- nous resistance caused by the occluded or partially
Ppv
Expiration
“Pcv” Pcv
Lymphatic
Legs
Abdomen
Thorax
Arms/ Head
Figure 3-7 Resting venous flow and changes with respiration.
occluded veins (Fig. 3-8). Variations in abdominal pressure with respiration have little effect on the pressure gradient from the legs. Normal phasic ve- nous flow from the lower extremity may be reduced or absent. Venous flow from the legs may become continuous as the venous pressures in the legs ex- ceed the normal changes in intra-abdominal venous pressures.
VENOUS DYNAMICS WITH EXERCISE
The calf muscle pump aids in the return of blood from the legs against the force of gravity (hydrostatic pressure). The muscles act as the power source. The intramuscular sinusoids within the gastrocnemius and soleal muscles as well as the deep and super- ficial veins all play a part in this mechanism. The valves are necessary to ensure efficient action of the muscle pump (Fig. 3-9). Closure of the valves in the deep veins decreases the length of the column of blood, which helps reduce the venous pressure.