The intrapleural pressure is less than atmospheric pressure by about 4 mm Hg. Due to greater atmospheric pressure outside, air rushes in. Expiration Is Usually Passive a. When respiratory center stops sending signals, diaphragm relaxes and returns to its original dome shape. Rib cage moves down and inward.
Lungs recoil due to elastic tissue within.
External and Internal Respiration External respiration is exchange of gases between air in alveoli and blood in pulmonary capillaries. Internal respiration is exchange of gases between blood in systemic capillaries and tissue fluid. Diffusion alone governs direction of gas flow. Oxygen diffuses into the capillary because there is a higher concentration of it in the alveolus. High altitude planes pressurize cabins to maintain this gas pressure.