Introduction:
Blood circulation plays a vital role in the human body, ensuring the delivery of oxygen, nutrients, and removal of waste products. The flow of blood from the post caval to the diastolic right atrium of the heart is facilitated by several factors. Let's explore these factors in detail.

Factors facilitating blood flow:

1. Open venous valves:
- Venous valves are present in the veins to prevent backflow of blood.
- When the blood flows through the post caval (inferior vena cava), the venous valves open to allow the blood to pass through.
- The open valves ensure a unidirectional flow of blood, preventing any backward movement.

2. Suction pull:
- The diastolic right atrium of the heart experiences a slight suction pull due to its relaxation.
- This suction pull helps in drawing the blood from the post caval into the atrium.
- The pressure difference created by the relaxation of the atrium assists in the movement of blood.

3. Stimulation of the sinoatrial (SA) node:
- The sinoatrial node is a natural pacemaker located in the right atrium of the heart.
- It generates electrical impulses that regulate the heart's rhythm and contraction.
- The SA node initiates the electrical signals that cause the atrium to contract, facilitating the movement of blood into the ventricles.
- However, the stimulation of the SA node is not directly responsible for blood flow from the post caval to the right atrium.

4. Pressure difference between post caval and atrium:
- The primary driving force behind blood flow is the pressure difference between the post caval and the right atrium.
- Blood in the post caval has a higher pressure compared to the atrium during diastole (relaxation phase of the heart).
- This pressure gradient allows the blood to flow from the post caval to the diastolic right atrium.
- The blood continues its journey through the heart, entering the right ventricle and then being pumped to the lungs for oxygenation.

Conclusion:
In summary, blood flows from the post caval to the diastolic right atrium of the heart due to a combination of factors. The open venous valves, suction pull created by the relaxation of the atrium, and the pressure difference between the post caval and atrium all contribute to the facilitation of blood flow. Understanding these mechanisms is crucial for comprehending the intricate process of blood circulation in the human body.