Bharti School of Engineering and Computer Science, Laurentian University, 935 Ramsey Lake Road, Sudbury, Ontario, P3E 2C6, Canada.
Bull Math Biol. 2023 May 2;85(6):50. doi: 10.1007/s11538-023-01155-2.
Oxygen transfer in the microvasculature is a complex phenomenon that involves multiple physical and chemical processes and multiple media. Hematocrit, the volume fraction of red blood cells (RBCs) in blood, has direct influences on the blood flow as well as the oxygen supply in the microcirculation. On the one hand, a higher hematocrit means that more RBCs present in capillaries, and thus, more oxygen is available at the source end. On the other hand, the flow resistance increases with hematocrit, and therefore, the RBC motion becomes slower, which in turn reduces the influx of oxygen-rich RBCs entering capillaries. Such double roles of hematocrit have not been investigated adequately. Moreover, the oxygen-hemoglobin dissociation rate depends on the oxygen tension and hemoglobin saturation of the cytoplasm inside RBCs, and the dissociation kinetics exhibits a nonlinear fashion at different oxygen tensions. To understand how these factors and mechanisms interplay in the oxygen transport process, computational modeling and simulations are favorite since we have a good control of the system parameters and also we can access to the detailed information during the transport process. In this study, we conduct numerical simulations for the blood flow and RBC deformation along a capillary and the oxygen transfer from RBCs to the surrounding tissue. Different values for the hematocrit, arteriole oxygen tension, tissue metabolism rate and hemoglobin concentration and affinity are considered, and the simulated spatial and temporal variations of oxygen concentration are analyzed in conjunction with the nonlinear oxygen-hemoglobin reaction kinetics. Our results show that there are two competing mechanisms for the tissue oxygenation response to a hematocrit increases: the favorite effect of the higher RBC density and the negative effect of the slower RBC motion. Moreover, in the low oxygen situations with RBC oxygen tension less than 50 mmHg at capillary inlet, the reduced RBC velocity effect dominates, resulting in a decrease in tissue oxygenation at higher hematocrit. On the opposite, for RBC oxygen tension higher than 50 mmHg when entering the capillary, a higher hematocrit is beneficial to the tissue oxygenation. More interestingly, the pivoting arteriole oxygen tension at which the two competing mechanisms switch dominance on tissue oxygenation becomes lower for higher oxygen-hemoglobin affinity and lower hemoglobin concentration. This observation has also been analyzed based on the oxygen supply from RBCs and the oxygen-hemoglobin reaction kinetics. The results and discussions presented in this article could be helpful for a better understanding of oxygen transport in microcirculation.
微血管中的氧气传递是一个复杂的现象，涉及多个物理和化学过程以及多个介质。血细胞比容（血液中红细胞的体积分数）直接影响血流和微循环中的氧气供应。一方面，较高的血细胞比容意味着毛细血管中存在更多的红细胞，因此源端可获得更多的氧气。另一方面，随着血细胞比容的增加，流动阻力增大，因此红细胞的运动变得更慢，这反过来又减少了富含氧气的红细胞进入毛细血管的流入。血细胞比容的这种双重作用尚未得到充分研究。此外，氧合血红蛋白的离解速率取决于红细胞细胞质内的氧张力和血红蛋白饱和度，并且在不同的氧张力下离解动力学呈非线性方式。为了了解这些因素和机制在氧气传输过程中的相互作用，计算建模和模拟是首选，因为我们可以很好地控制系统参数，并且还可以在传输过程中访问详细信息。在这项研究中，我们对沿毛细血管的血流和红细胞变形以及从红细胞向周围组织的氧气传递进行了数值模拟。考虑了不同的血细胞比容、小动脉氧张力、组织代谢率和血红蛋白浓度和亲和力值，并结合非线性氧合血红蛋白反应动力学分析了氧气浓度的时空变化。我们的结果表明，组织氧合对血细胞比容增加有两种相互竞争的机制：更高的红细胞密度的有利影响和红细胞运动更慢的负面影响。此外，在毛细血管入口处 RBC 氧张力小于 50mmHg 的低氧情况下，红细胞速度降低的影响占主导地位，导致更高血细胞比容时组织氧合减少。相反，当 RBC 氧张力进入毛细血管时高于 50mmHg，更高的血细胞比容有利于组织氧合。更有趣的是，对于更高的氧合血红蛋白亲和力和更低的血红蛋白浓度，两种竞争机制在组织氧合中占据主导地位的枢轴小动脉氧张力变得更低。根据 RBC 提供的氧气和氧合血红蛋白反应动力学，也对这一观察结果进行了分析。本文提出的结果和讨论有助于更好地理解微循环中的氧气传输。
