Klocke R A, Schünemann H J, Grant B J
Department of Medicine, State University of New York at Buffalo, USA.
Am J Respir Crit Care Med. 1995 Dec;152(6 Pt 1):2014-20. doi: 10.1164/ajrccm.152.6.8520770.
The length of time that blood remains in the pulmonary capillary is an important variable in gas exchange. We have investigated the distribution of capillary transit times in isolated rabbit lungs perfused with a bicarbonate-free buffer. The time course of gas exchange was monitored by enclosing the lungs in a plethysmograph. A bolus of buffer containing dissolved acetylene was injected into the perfusion system. Exchange of this inert gas occurred as soon as the bolus reached the capillary bed, thereby describing the input function into the bed. A separate bolus injection of bicarbonate solution resulted in production and excretion of CO2 as long as the bolus remained in the capillary bed. The rate of CO2 production was adjusted by partial inhibition of endothelial carbonic anhydrase. The distribution of capillary transit times was computed from a model of CO2 production in the capillary bed and the observed rates and volumes of acetylene and carbon dioxide excretion. The recovered distributions indicate that there is a fairly wide distribution of capillary transit times (relative dispersion, 0.45) around the mean value of 1.71 s (+/- 0.53 [SD]). Only 10% of capillary transit times are less than one half of mean transit time. It is likely that gas exchange reaches equilibrium in the capillary bed except possibly during strenuous exercise or exposure to high altitude or in disease.
血液在肺毛细血管中停留的时间是气体交换中的一个重要变量。我们研究了用无碳酸氢盐缓冲液灌注的离体兔肺中毛细血管通过时间的分布情况。通过将肺置于体积描记器中来监测气体交换的时间进程。将含有溶解乙炔的缓冲液团注入灌注系统。一旦缓冲液团到达毛细血管床，这种惰性气体就会发生交换，从而描述进入该床的输入函数。单独注入碳酸氢盐溶液会导致只要缓冲液团留在毛细血管床内就会产生和排出二氧化碳。通过部分抑制内皮碳酸酐酶来调节二氧化碳的产生速率。根据毛细血管床中二氧化碳产生的模型以及观察到的乙炔和二氧化碳的排出速率及体积来计算毛细血管通过时间的分布。恢复的分布表明，在1.71秒（±0.53[标准差]）的平均值周围，毛细血管通过时间存在相当广泛的分布（相对离散度为0.45）。只有10%的毛细血管通过时间小于平均通过时间的一半。除了在剧烈运动、暴露于高海拔环境或患病期间，气体交换很可能在毛细血管床中达到平衡。
