HUMAN MEDICAL BIOLOGY, ANATOMY, PHYSIOLOGY AND PATHOLOGY - Y.I. Fedoniuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

SECTION 5. BLOOD AND LYMPH CIRCULATION. PATHOPHYSIOLOGY

PHYSIOLOGY OF BLOOD VESSELS

4. BLOOD CIRCULATION

4.2. VENOUS Blood FLOW

The Venous system contains 75-80% of the total circulating blood volume. Therefore, Veins are referred to as capacitance vessels.

The capacitance function of veins (blood pooling) is primarily due to The Structure of their walls. Compared to arterial walls, they are relatively thin and easily distensible. The venous beds of the Liver, Spleen, and subcutaneous tissue are particularly high in capacitance, serving as the primary blood reservoirs.

Blood pressure in the veins is quite low, ranging from 10-20 mmHg in small venules and dropping to 0 or even becoming negative (below atmospheric pressure) in the venae cavae near the right atrium.

Venous pressure in humans can be measured by inserting a hollow needle connected to a manometer into a superficial vein (such as the antecubital or subclavian vein). In healthy adults, the venous blood pressure in the antecubital vein is 50-100 mm of Water Column.

The primary driving force ensuring venous return to The Heart is the pressure gradient between the Origin of the venous system (venules – up to 20 mmHg) and the pressure in the venae cavae as they enter the right atrium (0 or less). During inspiration, the expansion of the thoracic cavity leads to the dilation of the intrathoracic veins; venous pressure drops below atmospheric pressure, thereby increasing venous return to the heart. This phenomenon is known as the thoracic suction pump mechanism.

An important factor facilitating venous blood flow is the contraction of skeletal Muscles. Venous return to the heart is supported by the valvular apparatus of the veins. Nearly all large veins contain semilunar Valves that open toward the heart. This structural feature allows blood to flow toward the heart while preventing backflow.