Answer:

Blood in humans passes from the post caval to the diastolic right atrium of the heart due to the pressure difference between the caval and atrium. The correct answer is option B.

Explanation:

Pressure difference:
The flow of blood from the post caval (inferior vena cava) to the diastolic right atrium of the heart is driven by the pressure difference between these two chambers. The pressure in the post caval is higher compared to the pressure in the right atrium. This pressure difference allows blood to flow from the post caval into the right atrium.

Post caval:
The post caval is also known as the inferior vena cava (IVC). It is a large vein that carries deoxygenated blood from the lower body (below the diaphragm) back to the heart. The blood from the lower limbs, pelvic organs, and abdominal organs is collected by smaller veins and ultimately drains into the post caval.

Diastolic right atrium:
The diastolic right atrium is the right upper chamber of the heart that receives deoxygenated blood from the post caval. It contracts during the diastolic phase of the cardiac cycle to pump blood into the right ventricle.

Flow of blood:
The pressure difference between the post caval and the diastolic right atrium drives the flow of blood. Blood moves from areas of higher pressure to areas of lower pressure. Therefore, when the pressure in the post caval is higher than the pressure in the right atrium, blood flows from the post caval into the right atrium.

Other options:
- Option A: Pushing open of the venous valves does not play a significant role in the blood flow from the post caval to the diastolic right atrium. Venous valves primarily prevent the backflow of blood and ensure unidirectional flow.
- Option C: Stimulation of the sinoatrial (SA) node is responsible for initiating the electrical signals that regulate the heart's rhythm but does not directly affect the flow of blood from the post caval to the right atrium.
- Option D: Suction pull is not a mechanism involved in the flow of blood from the post caval to the right atrium. Blood flow is primarily driven by pressure differences.

In conclusion, the flow of blood from the post caval to the diastolic right atrium of the heart is due to the pressure difference between these two chambers.