In emphysema , residual volume builds up and is no longer useful for gas exchange, reducing vital capacity. Inspiration Precedes Expiration a. Air passes from atmosphere through pharynx to alveoli. Lungs lie within a sealed-off thoracic cavity. 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.