Muscles contract and relax during breathing in and out. Pressure changes occur in the thoracic and abdominal cavities. The changes in the pressure compress the nearby veins and assist blood returning to the heart is:

• A. Respiratory Pump
• B. Venous Pressure
• C. Pocket Valves
• D. Plasma

Answer: (A)

The main idea is that breathing movements create a respiratory pump that helps push blood back to the heart. When you inhale, the diaphragm and chest expand, lowering the pressure in the thorax while abdominal pressure rises as contents are shifted downward. This combination squeezes veins in the abdomen and chest and, along with the drawing in of venous blood by the lower intrathoracic pressure, pushes blood toward the heart. Veins have valves to prevent backflow, so the net effect is enhanced venous return with each breath. The other terms aren’t the mechanism: venous pressure is simply the pressure inside the veins, pocket valves are the valves inside veins that prevent backflow, and plasma is the fluid part of blood rather than a mechanism for moving blood back to the heart.