Читать книгу Understanding Anatomy and Physiology in Nursing - John Knight - Страница 112

Blood exiting the venous end of the capillary bed does so under very low pressure, entering the venules which are the smallest veins of the body. Venules from multiple capillary beds join up to form larger and larger veins. Most large- and medium-sized veins are equipped with semi-lunar valves to help prevent the backflow of blood under the influence of gravity. Since the pressure in veins is so low, physical movement of the body is essential to keep blood moving and avoid venous stasis, which can increase the risk of thrombosis. During bodily movement, contraction of the major muscle groups, such as those in the legs, will squeeze the thin-walled veins, ensuring blood is kept mobile, while the valves ensure the blood flows in the correct direction towards the heart.

This mechanism is termed the skeletal muscle pump and is particularly important for ensuring venous return from the lower regions of body. All veins ultimately drain into the superior and inferior vena cavae which deliver deoxygenated blood directly to the right atrium of the heart. Since veins are thin-walled vessels, they show a high degree of compliance (ability to distend) and many of the larger veins of the body act as capacitance vessels with around 60 per cent of the total blood volume found within the venous system.