At 2mm Hg in the inferior vena cava, what factors do youthink propel blood into the right heart from the lower partsof the body?

Several factors contribute to the propulsion of blood into the right heart from the lower parts of the body, even at a low pressure of 2mm Hg in the inferior vena cava:

1. Gravity: When a person is standing or sitting upright, gravity assists in the downward flow of blood from the lower parts of the body.

2. Muscular Contraction: The contraction of skeletal muscles, especially in the legs, helps to squeeze the veins and push blood upwards towards the heart. This is often referred to as the "muscle pump" or "skeletal muscle pump".

3. Respiratory Pump: During inhalation, the pressure in the thoracic cavity decreases and the pressure in the abdominal cavity increases. This pressure difference helps to push blood from the inferior vena cava into the right atrium of the heart.

4. Venous Valves: Veins, especially those in the legs, have one-way valves that prevent backflow of blood. This ensures that blood continues to flow towards the heart even when there is low pressure in the vena cava.

5. Vasoconstriction: The smooth muscles in the walls of veins can constrict to push blood towards the heart. This mechanism is regulated by the autonomic nervous system.

6. Cardiac Suction Effect: During the relaxation phase of the cardiac cycle (diastole), the right atrium and ventricle expand, creating a slight suction that draws blood into the heart.

These factors work together to ensure that blood continues to flow towards the heart, even when venous pressure is low.