Thornley J H, France J, Dhanoa M S, Siddons R C
Institute of Terrestrial Ecology, Edinburgh, Penicuik.
Br J Nutr. 1995 Dec;74(6):747-63. doi: 10.1079/bjn19950003.
Using equations obtained in a previous analysis, results are computed numerically which illustrate the effects of diffusion and viscosity on faecal excretion patterns of markers in ruminants. Results are first given for plug flow, a velocity gradient produced by viscosity, and diffusion, each mechanism operating alone. Plug flow gives a period during which no marker appears in the faeces, then a sharp increase in faeces marker concentration, followed by rapid washout. A velocity gradient gives a more gradual appearance of marker in the faeces followed by a slower washout. Diffusion alone (although not realistic for ruminant marker kinetics) can give early appearance of marker followed by slow washout. Combining diffusion with a velocity gradient produced by viscosity can give a range of behaviour, depending on the effective diffusion coefficient, D'; an approximate method is used to compute these solutions. Because plug flow with no velocity gradient plus diffusion gives results similar to convective flow with a velocity gradient plus diffusion, we believe it will not be possible to determine the main mechanisms defining marker outflow patterns from observations of marker kinetics alone, and more detailed investigations will be needed. Although estimates of quantities such as mean transit time are unaffected by detailed mechanism, the interpretations of measures such as sigmoidicity, sharpness of the faecal marker concentration v. time curve, and length and nature of the washout tail are highly dependent on mechanism.
利用先前分析中得到的方程，通过数值计算得出结果，这些结果说明了扩散和黏度对反刍动物粪便中标记物排泄模式的影响。首先给出平推流、由黏度产生的速度梯度以及扩散单独作用时的结果，每种机制单独运行。平推流会出现一段粪便中无标记物的时期，然后粪便中标记物浓度急剧增加，随后快速排出。速度梯度会使标记物在粪便中出现得更为缓慢，随后排出速度也较慢。仅扩散作用（尽管对于反刍动物标记物动力学来说不太现实）会使标记物提前出现，随后缓慢排出。将扩散与由黏度产生的速度梯度相结合会产生一系列行为，这取决于有效扩散系数D'；使用一种近似方法来计算这些解。由于无速度梯度的平推流加上扩散所得到的结果与有速度梯度的对流加上扩散的结果相似，我们认为仅通过标记物动力学观测无法确定定义标记物流出模式的主要机制，还需要更详细的研究。尽管诸如平均转运时间等数量的估计不受详细机制的影响，但诸如乙状结肠性、粪便标记物浓度与时间曲线的尖锐程度以及排出尾段的长度和性质等测量结果的解释在很大程度上取决于机制。
