<prompt> Normal intrapleural pressure is1-2 mm Hg2-4 mm Hg9.5-10 mm Hg-3 to -4 mm Hg  <response>: The normal external pressure is equal to the normal intrapleural pressure the pressure in the chest cavity, which is 4 mm Hg. Note in the figure that a rise in intrapleural pressure, to 2 mm Hg, shifts the entire cardiac output curve to the right by the same amount. This shift occurs because to fill the cardiac chambers with blood requires an extra 2 mm Hg right atrial pressure to overcome the increased pressure on the outside of the heart. Likewise, an increase in intrapleural pressure to 2 mm Hg requires a 6 mm Hg increase in right atrial pressure from the normal 4 mm Hg, which shifts the entire cardiac output curve 6 mm Hg to the right. 