Final answer:
Exhalation results in increased intra-thoracic pressure which increases venous return to the heart due to the pressure on the veins in the chest.
Step-by-step explanation:
The subject of this question is the respiratory pump and its effect on venous return during the respiratory cycle. When you exhale, the volume of the thorax decreases, leading to an increase in intra-thoracic pressure. The respiratory pump aids the blood flow through the veins of the thorax and abdomen. During exhalation, the pressure within the thoracic cavity increases, which increases the pressure in the thoracic veins. This increased pressure speeds up the blood flow into the heart. However, the valves in the veins prevent the blood from flowing backward from the thoracic and abdominal veins. This pressure pushes on the veins in the chest, which, along with the help of the valves in these veins, results in increased venous return to the heart. Therefore, the answer to the student's question is: a) Increases due to increased intra-thoracic pressure during exhalation.