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

DEAD SPACE VENTILATION

• The tidal volume is used to ventilate not only the alveoli, but also the airways leading to the alveoli. Because there is little or no diffusion of oxygen and carbon dioxide through the membranes of most of the airways, they com­pose part of dead space.
• The other part of dead space ventilation is made up of alveoli with diminished capillary perfusion.
• Ventilating these alveoli is ineffec­tive in producing changes in the blood gases. Ventilation of nonperfused alveoli and the air­ways, because neither accomplishes exchange of the respiratory gases. is referred to as phys­iologic dead space.
• Physiologic dead space is defined as the volume of gas that is inspired but takes no part in gas exchange in the airways and alveoli. Therefore, the tidal volume has a dead space component and an alve­olar component.
• More ventilation of lower (with respect to gravity) regions of the lung than upper regions of the lung. Intrapleural (intrathoracic) pressure is less negative at the bottom of the lung than at the top because of the weight of the lung and the configuration of the chest wall.
• Therefore the transpulmonary pressure gradient is greater at the top of the lung than at the bottom.
• Therefore the alveoli at the top of the lung are at a higher, less compliant point on the pressure-volume curve than those at the bottom.
• Therefore the alveoli at the bottom of the lung increase their volume more with each inspiration and decrease their volume more with each expiration during eupnea (from FRC).
• Dead space ventilation is a necessary part of the process of ventilating the alveoli and is not totally wasted. It assists in humidifying inhaled air and in cooling the body under certain conditions, such as when panting is necessary.
• During panting, the respiratory frequency increases and the tidal volume decrease so that alveolar ventilation remains approximately constant.