Questions: Blood flow back to the heart is due to pressure generated by contractions of the heart. the respiratory pump. the skeletal muscle pump. All of the choices are correct.

The answer is the last one: All of the choices are correct.

Explanation for each option:

1. Contractions of the heart: The heart's contractions generate the primary force that propels blood through the circulatory system, including the return of blood to the heart.
2. The respiratory pump: During inhalation, the pressure in the thoracic cavity decreases, which helps draw blood back to the heart from the veins.
3. The skeletal muscle pump: When skeletal muscles contract, they compress the veins running through them, helping to push blood back toward the heart.

All these mechanisms work together to ensure efficient blood flow back to the heart.