Fedde M R
Department of Anatomy and Physiology, College of Veterinary Medicine, Kansas State University, Manhattan 66506-5602, USA.
Poult Sci. 1998 Aug;77(8):1130-8. doi: 10.1093/ps/77.8.1130.
The avian respiratory system exchanges oxygen and carbon dioxide between the gas and the blood utilizing a relatively small, rigid, flow-through lung, and a system of air sacs that act as bellows to move the gas through the lung. Gas movement through the paleopulmonic parabronchi, the main gas exchanging bronchi, in the lung is in the same direction during both inspiration and expiration, i.e., from the mediodorsal secondary bronchi to the medioventral secondary bronchi. During inspiration, acceleration of the gas at the segmentum accelerans of the primary bronchus increases gas velocity so it does not enter the medioventral secondary bronchi. During expiration, airway resistance is increased in he intrapulmonary primary bronchus because of dynamic compression causing gas to enter the mediodorsal secondary bronchi. Reduction in air flow velocity may decrease the efficiency of this aerodynamic valving and thereby decrease the efficiency of gas exchange. The convective gas flow in the avian parabronchus is orientated at a 90 degree angle with respect to the parabronchial blood flow; hence, the cross-current designation of this gas exchanger. With this design, the partial pressure of oxygen in the blood leaving the parabronchus can be higher than that in the gas exiting this structure, giving the avian lung a high gas exchange efficacy. The relationship of the partial pressure of oxygen in the moist inspired gas to that in the blood leaving the lung is dependent on he rate of ventilation. A low ventilation rate may produce a ow oxygen partial pressure in part of the parabronchus, thereby inducing hypoxic vasoconstriction in the pulmonary arterioles supplying this region. Inhaled foreign particles are removed by nasal mucociliary action, by escalator in the trachea, primary bronchi, and secondary bronchi. Small particles that enter parabronchi appear to be phagocytized by the epithelial cells in eh atria and infundibulum. These particles can e transported to interstitial macrophages but the disposition of the particles from this site is unknown. The predominant site of respiratory infections in the caudal air sacs, compared to other parts of the respiratory system, can be explained by the gas flow pathway and the mechanisms present in the parabronchi for particle removal.
鸟类呼吸系统利用相对较小、坚硬且气流通过式的肺，以及作为风箱以推动气体通过肺部的气囊系统，在气体与血液之间交换氧气和二氧化碳。气体在肺部主要的气体交换支气管——古肺副支气管中的流动方向在吸气和呼气时相同，即从中背侧次级支气管流向中腹侧次级支气管。在吸气过程中，初级支气管加速段处气体的加速增加了气体速度，因此气体不会进入中腹侧次级支气管。在呼气过程中，由于动态压缩，肺内初级支气管的气道阻力增加，导致气体进入中背侧次级支气管。气流速度的降低可能会降低这种空气动力学瓣膜的效率，从而降低气体交换的效率。鸟类副支气管中的对流气流相对于副支气管血流呈90度角定向；因此，这种气体交换器被称为交叉流。采用这种设计，离开副支气管的血液中的氧分压可以高于离开该结构的气体中的氧分压，这使得鸟类肺部具有较高的气体交换效率。潮湿吸入气体中的氧分压与离开肺部的血液中的氧分压之间的关系取决于通气速率。低通气速率可能会在部分副支气管中产生低氧分压，从而在供应该区域的肺小动脉中诱导缺氧性血管收缩。吸入的外来颗粒通过鼻黏液纤毛作用、气管、初级支气管和次级支气管中的“清洁器”清除。进入副支气管的小颗粒似乎被心房和漏斗中的上皮细胞吞噬。这些颗粒可以被转运到间质巨噬细胞，但颗粒从该部位的处置情况尚不清楚。与呼吸系统的其他部分相比，尾气囊是呼吸道感染的主要部位，这可以通过气流途径以及副支气管中存在的颗粒清除机制来解释。
