Page 47 - Libro 2

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Venous Physiology
Ann Marie Kupinski
OBJECTIVES
KEY TERMS
calf muscle pump | edema | hydrostatics | resistance | transmural pressure | venous insufficiency
GLOSSARY
edema excessive accumulation of fluid in cells, tissues, or cavities of the body
hydrostatic pressure the pressure within the vascu- lar system due to the weight of a column of blood
The venous system is usually perceived to be rath- er passive. It is generally believed that the arterial system does all the work and the veins just return the blood back to the heart. However, changes in venous blood flow can cause various complications from varicose veins to pulmonary emboli. Venous disease affects a significant portion of the popula- tion. Several factors affect the movement of blood throughout the venous system. This chapter will present the major features of venous physiology and pathophysiology.
VENOUS CAPACITANCE
Veins are known as the capacitance vessels of the body. They serve an important role acting as a reser- voir. The venous side of the circulatory system holds approximately two-thirds of the total blood volume of the body (Fig. 3-1). The arterial side of the cir- culatory system typically holds about 30% of the blood volume with the remaining 3% to 4% within the capillaries. The cross-sectional area of a fully dis- tended vein can be three to four times that of the corresponding companion artery. Often, the veins are paired structures and this adds to their ability to hold blood.
transmural pressure the pressure on the walls of a vessel
venous insufficiency abnormal retrograde flow in veins
VENOUS RESISTANCE
By changing the cross-sectional area, veins can vary their resistance. When distended, they offer almost no resistance to blood flow. When partially empty, they assume an elliptical cross-sectional shape that offers a great deal of resistance to blood flow. When a vein is distended and takes on a more circular shape, it offers less resistance to blood flow. Remember the importance of the radius of a vessel to the resistance. Their ability to change shape permits veins to ac- commodate increases in blood flow without causing increases in the pressure gradient to the heart.
At several areas within the body, veins naturally offer resistance to flow. Veins tend to collapse as they enter the thorax. The subclavian veins are compressed by the first rib. The jugular veins collapse because of atmo- spheric pressure. Changes in resistance to blood flow within these upper extremity veins are usually minimal but can vary depending on the patient position and intravascular pressures. The inferior vena cava can be compressed by abdominal organs and intra-abdominal pressure, both of which will affect the venous return through the inferior vena cava. The effect of intra- abdominal pressure on resistance to venous flow will be discussed in more detail when describing resting venous dynamics later in this chapter.
List the hemodynamic factors which affect venous blood flow
Describe the components of hydrostatic pressure
Identify the forces that lead to edema formation
Define the changes that occur at rest and with exercise in the venous system
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