Torre-Bueno J R, Geiser J, Scheid P
Respir Physiol. 1980 Nov;42(2):109-22. doi: 10.1016/0034-5687(80)90108-5.
During normal breathing, the CO2 concentration in caudal air sacs of birds is higher, and the O2 concentration lower, than expected on the basis of the known air flow pattern. We have experimentally tested two hypotheses which could explain this finding: (1) Preferential shunting of re-inspired dead space gas into caudal air sacs; (2) Incomplete mixing of inspired and residential air sac gas. - Different portions of the inspired air in anesthetized ducks were labeled by injecting a small bolus of argon (Ar) into the trachea. The resulting Ar concentration was recorded continuously in the caudal thoracic air sac at the ostium and in deeper regions.-The amount of Ar entering the sac was found to be independent of the volume inspired prior to injection of the label, and hypothesis (1) thus dismissed. However, during inspiration and subsequent expiration the Ar bolus was found to be neither perfectly mixed within the inspired gas nor with the air sac residential gas. More than 10 sec of breath-hold were necessary for air sac gas to approach an equilibrium value. Gas layering (stratification) in caudal air sacs gas is proposed to cause the high CO2 and low O2 levels during steady state breathing, as air sac residential gas equilibrates with a layer of dead space gas that enters the air sac on each breath and contains a higher CO2 and lower O2 concentration than the mixed inspirate.
在正常呼吸过程中，鸟类尾气囊中的二氧化碳浓度较高，氧气浓度较低，这比根据已知气流模式所预期的情况要高。我们通过实验检验了两种可以解释这一发现的假设：（1）再吸入的死腔气体优先分流到尾气囊中；（2）吸入气体与驻留气囊气体混合不完全。——通过向麻醉鸭的气管中注入一小团氩气（Ar），对吸入气体的不同部分进行标记。然后连续记录在尾胸气囊开口处和更深区域的氩气浓度。——发现进入气囊的氩气量与注入标记前吸入的体积无关，因此假设（1）被排除。然而，在吸气及随后的呼气过程中，发现氩气团既没有在吸入气体中完全混合，也没有与气囊驻留气体完全混合。气囊气体需要超过10秒的屏气时间才能接近平衡值。有人提出，尾气囊气体中的气体分层（分层现象）会导致稳定呼吸过程中二氧化碳水平升高和氧气水平降低，因为气囊驻留气体与每一次呼吸进入气囊的一层死腔气体达到平衡，而这层死腔气体的二氧化碳浓度较高，氧气浓度较低，高于混合后的吸入气体。
