HOW HEALTHY DOGS BREATHE: RESPIRATORY PHYSIOLOGY DETERMINES PHYSICAL EXAM FINDINGS

摘要

Respiration requires an integrated system of control (CNS), movement of a bellows (ribs and associated muscle) to draw air into the lungs, exchange of gas within the lung, and a feedback system (chemoreceptors, stretch receptors) to close the loop. This lecture will present some basic elements of the bellows part of this system, providing a basis for evaluation of respiratory patterns in the dog. Lungs are a viscoelastic tissue suspended within the pleural cavity. If the chest is opened to the atmosphere the lungs collapse until small airway occlusion prevents further loss of gas. The collapse won't stop until the lung volume is quite a bit smaller than the normal functional residual capacity (FRC) in the intact animal. The lung volume in a normal animal is maintained by support from the thoracic wall. Because there is an airtight seal within the pleural space and the lungs are coupled to the chest wall with a viscous fluid, the tendency of the lungs to collapse is countered by the tendency of theribs to spring out. Between breaths, these opposing forces yield a pleural pressure of-5 cm H_2O at the 8th-9th interspace in an average size dog. Breathing takes place around this equilibrium, with most of the energy of breathing expended to increase the thoracic volume above the equilibrium point during inspiration.