VPB 121: Veterinary Physiology - II (2+1)

EXPIRATION

• Normal expiration is a passive process involving no active muscle contraction. Like inspiration, it is due to pressure gradients, but in an opposite direction. Because of the elastic recoil of the lungs and chest wall, there are two inwardly directed forces resulting 1) the elastic fibers that were stretched during inhalation, and 2) the inwardly directed force due to the surface tension arising from the alveolar fluid.
• As the neural signals to the diaphragm cease, it relaxes and this dome-shaped muscle moves rostrally, thus decreasing the volume of the thoracic cavity.
• The external intercostals also relax, and allow the ribs to move dorsally and caudally, thus further decreasing the volume of the thoracic cavity. This decreases lung volume and causes alveolar pressure to increase approximately 2 mmHg above atmospheric pressure. As a result, air flows out of the lungs to an area of lower pressure.
• During forceful exhalation, the abdominal and internal intercostals muscles contract. This causes the ribs to move caudally and dorsally compressing the abdominal viscera and decreasing the thoracic volume. This increases pressure inside the thoracic cavity and forces air outward.