The respiratory pump acts to enhance venous return to the heart, much like the skeletal muscle pump. During inspiration, the stroke volume of the right heart increases by about 20%. Intrapleural pressure, the prime mover in this mechanism, becomes increasingly negative. The exact magnitude of the changes depend on inspiratory depth and rate. At the same time the output of the left heart is decreased. During expiration, the output of the right heart decreases and the output of the left heart increases, while intrapleural pressure becomes less negative.
The reason for these cyclic cardiac changes has to do with the fact that blood is being pulled into the right heart by the negative intrathoracic pressure during inspiration, filling the right heart to a greater extent and therefore increasing its output. Just the opposite change occurs during expiration. In the left heart, there is a decrease in blood flow during inspiration because the lungs hold the blood, and also because the interventricular septum shifts toward the left side limiting diastolic filling.
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