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In humans, blood passes from the post caval to the diastolic right atrium of heart due to:

1. pushing open of the venous valves
2. suction pull
3. stimulation of the sino auricular node
4. pressure difference between the post caval and atrium

Correct answer: pressure difference between the post caval and atrium

Solution

Blood flows from the post caval vein into the right atrium because the atrial pressure is lower than the venous pressure during atrial diastole. This pressure gradient, not valve action or electrical stimulation, is the direct cause of flow.

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