Phylogeny of gas exchange systems
Phylogeny of gas exchange systems
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DOI：
10.1055/s-2002-25080
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发表时间：
2002-04-01
影响因子：
0.4
通讯作者：
Gros, G
中科院分区：
文献类型：
--
作者：
Jürgens, KD;Gros, G
在进化过程中发展了几种气体运输系统，尽管环境和组织的单个细胞之间距离很远，但所有这些系统都能够为组织提供足够的氧气并消除代谢产生的CO2。几乎所有这些系统都利用对流和扩散步骤的组合。对流实现了气体在长距离上的有效运输，但需要能量并且不能跨越组织屏障发生。另一方面，扩散实现气体穿过屏障的传输，但需要优化扩散路径和扩散区域。当两个对流气流通过扩散屏障连接时（在禽肺的情况下为气体/流体，在鳃的情况下为流体/流体），各自的对流运动彼此通过的方向是气体交换效率的重要决定因素（顺流、逆流和横流系统）。在昆虫中发现的气管呼吸具有绕过血液中的对流气体运输步骤的优点，从而避免了循环系统的高能量消耗。这是通过一个气管系统实现的，该系统终止于气管，从身体表面到达身体内的每个细胞。在这些动物中，气体转移的最后一步是通过从气管（“毛细血管”）扩散到细胞的线粒体。缺点是气管系统占据身体体积的相当大的一部分，并且由于气管壁的有限的机械稳定性，该系统将不能在高流体静压的条件下操作，即在大型动物中。在“开放”系统中的呼吸，即扩散屏障直接暴露于环境空气，消除了通过对流将氧气带到屏障的问题，这在鸟类和哺乳动物的肺、昆虫的气管系统和鳃中是必需的。在通过皮肤的呼吸中发现了开放系统，这在某些两栖动物中是重要的，但受到皮肤厚度的限制，皮肤构成了O-2和CO2的实质性扩散路径。另一方面，厚厚的皮肤为动物的身体提供了机械保护和灵活性，并有助于避免通过体表大量水分流失。相反，鱼类的鳃表现出相当短的扩散距离，位于机械保护的空间中，并且不存在水分损失的问题。血液和水的流动发生在相反的方向上（逆流），并且这种情况使得动脉PO 2接近环境PO 2成为可能。一个主要的缺点是由环境介质构成的，因为与空气相比，水含有很少的O-2，为了补偿这一点，需要花费大量的能量来保持水通过鳃的高流速。在哺乳动物肺（“池系统”）中，死腔的存在和每次呼吸仅替换肺的气体体积的一小部分的节律性通气是不能到达呼气PO 2的动脉PO 2（大气PO 2的2/3）的原因。然而，该功能的一个优点是肺泡和动脉的PCO 2持续较高，这在整个身体中提供了高效的H+缓冲系统。哺乳动物肺的明显缺点被鸟类肺所避免，鸟类肺使用扩展的气道系统来建立毛细血管的一部分的连续平衡。在该系统中，动脉PO 2可以显著地超过呼气PO 2。这里的一个缺点是禽肺占据的空间巨大，在体重为1 kg的动物中，禽肺占据的空间是哺乳动物肺占据的空间的三倍。这里考虑的所有呼吸交换系统都表现出高度的优化-但遵循高度多样化的构造原理。不存在理想的气体交换系统。在每个物种中进化的系统在很大程度上取决于环境条件、体型和大小、动物的运动模式和能量消耗。
Several systems of gas transport have developed during evolution, all of which are able to sufficiently supply oxygen to the tissues and eliminate the CO2 produced by the metabolism, in spite of great distances between the environment and the individual cells of the tissues. Almost all these systems utilize a combination of convection and diffusion steps. Convection achieves an efficient transport of gas over large distances, but requires energy and cannot occur across tissue barriers. Diffusion, on the other hand, achieves gas transport across barriers, but requires optimization of diffusion paths and diffusion areas. When two convectional gas flows are linked via a diffusional barrier (gas/fluid in the case of the avian lung, fluid/fluid in the case of gills), the directions in which the respective convectional movements pass each other are important determinants of gas exchange efficiency (concurrent, countercurrent and cross-current systems). The tracheal respiration found in insects has the advantage of circumventing the convective gas transport step in the blood, thereby avoiding the high energy expenditure of circulatory systems. This is made possible by a system of tracheae, ending in tracheoles, that reaches from the body surface to every cell within the body. The last step of gas tranfer in these animals occurs by diffusion from the tracheoles ("air capillaries") to the mitochondria of cells. The disadvantage is that the tracheal system occupies a substantial fraction of body volume and that, due to limited mechanical stability of tracheal walls, this system would not be able to operate under conditions of high hydrostatic pressures, i.e. in large animals. Respiration in an "open" system, i.e. direct exposure of the diffusional barrier to the environmental air, eliminates the problem of bringing the oxygen to the barrier by convection, as is necessary in the avian and mammalian lung, in the insects' tracheal system and in the gills. An open system is found in the respiration via the skin, which is of significance in some amphibians, but is limited by the thickness of the skin that constitutes a Substantial diffusion path for O-2 and CO2. The thick skin, on the other hand, provides mechanical protection as well as flexibility for the animals' body and helps avoid massive water loss via the body Surface. The gills of fishes, in contrast, exhibit rather short diffusion distances, are located in a mechanically protected space, and the problem of water loss does not exist. The flows of blood and water occur in opposite direction (countercurrent flow) and this situation makes an arterial PO2 approaching the environmental PO2 possible. A major disadvantage is constituted by the environmental medium since water contains little O-2 compared to air and, to compensate this, much energy is expended to maintain a high flow rate of water through the gills. In the mammalian lung ("pool system"), the presence of a dead space and the rhythmic ventilation that replaces only a small fraction of the gas volume of the lung per breath, are responsible for an arterial PO2 (2/3 of the atmospheric PO2) that cannot reach the expiratory PO2. However, an advantage of this feature is the constantly high alveolar and arterial PCO2, which provides a highly effective H+ buffer system in the entire body. The apparent disadvantage of the mammalian lung is avoided by the avian lung, which uses an extended system of airways to establish continuous equilibration of a part of the capillary bloIn this system, arterial PO2 can significantly exceed expiratory PO2. A disadvantage here is the enormous amount of space taken up by the avian lung, in animals of 1 kg body weight three times as much as taken up by the mammalian lung. All respiratory exchange systems considered here exhibit high degrees of optimization - yet follow highly diverse construction principles. There is no such thing as an ideal gas exchange system. The system that has evolved in each species depends to an impressive extent on environmental conditions, on body build and size, on the animal's patterns of movement and on its energy consumption.