The Chest Wall and the Respiratory Pump

The design of the respiratory pump is different from the design of the heart. The ribs carry compressive stresses that balance the pressure difference across the chest wall, and the rib cage provides a mechanism for transforming muscle shortening into chest wall expansion. The external intercostal muscles in the cranial-dorsal quadrant and the parasternal muscles have inspiratory actions and the internal intercostals in the caudal-dorsal quadrant and the triangularis sterni have expiratory actions. The diaphragm separates the pleural cavity and the abdomen, and the combined effect of diaphragm muscle tension and curvature balances the difference between abdominal pressure and pleural pressure. The inspiratory action of the diaphragm is the result of the descent of the diaphragm that accompanies muscle shortening and of the inspiratory forces that the diaphragm exerts on the lower ribs. Compartmental models provide simplified descriptions of the mechanics of the chest wall. The work of breathing is small compared to the work of the heart.