Expansion
Mean systemic filling pressure, right atrial pressure and venous resistance
Expansion
Venous return = (mean systemic filling pressure - right atrial pressure) / venous resistance
Contributing factors:
- Blood volume and venous tone, which set mean systemic filling pressure (normally about 7 mmHg)
- Skeletal muscle pump, with venous valves preventing backflow
- Respiratory pump, as inspiration lowers intrathoracic and raises intra-abdominal pressure
- Gravity and posture
- Right atrial pressure, which opposes return
About 70 per cent of blood volume sits in the venous system, which acts as a capacitance reservoir. Venoconstriction shifts blood from the unstressed to the stressed volume, raising filling pressure without any change in total volume.
This explains why positive pressure ventilation and PEEP reduce cardiac output by raising right atrial pressure, and why standing still causes syncope in a way that walking does not.