Systemic Capillary Pressure, Venous Pressure + Mechanisms of Venous Return

Systemic Capillary Pressure

Pressure continues to decline throughout the remainder of the systemic circuit

Pressure at the Arterial End of a Capillary Bed

At the arterial end of a capillary bed, the pressure is about 35 mm Hg

Pressure at the Venular End of a Capillary Bed

At the venular end of the capillary bed, the pressure has decreased to about 15 mm Hg

Decrease in Capillary Pressure

This decrease is largely due to the reduction in blood volume that takes place in capillaries

Systemic Venous Pressure

Pressure declines further in the venules and veins, dropping to only about 4 mm Hg in the inferior vena cava and as low as 0 mm Hg in the right atrium; Due largely to the high compliance of veins and declining resistance as these vessels merge and become larger

Venous Valves

Prevent backflow in the veins

Smooth Muscle in the Walls of Veins

May contract under sympathetic nervous system stimulation to increase the rate of venous return

Skeletal Muscle Pump

Skeletal muscles surrounding the deeper veins of the upper and lower limbs squeeze the blood in the veins and propel it toward the heart as they contract and relax

Respiratory Pump

Similar mechanism for veins of the thoracic and abdominopelvic cavities; Driven by the rhythmic changes in pressure in the cavities that occur with ventilation; During inspiration, high pressure in the abdominopelvic cavity pushes blood in the abdominal veins upward