Center of Functionally Integrative Neuroscience, Department of Clinical Medicine, Aarhus University, Aarhus, Denmark.
Neuroradiology Research Unit, Section of Neuroradiology, Department of Radiology, Aarhus University Hospital, Aarhus, Denmark.
J Appl Physiol (1985). 2020 Dec 1;129(6):1413-1421. doi: 10.1152/japplphysiol.00537.2020. Epub 2020 Oct 8.
The transport of oxygen between blood and tissue is limited by blood's capillary transit time, understood as the time available for diffusion exchange before blood returns to the heart. If all capillaries contribute equally to tissue oxygenation at all times, this physical limitation would render vasodilation and increased blood flow insufficient means to meet increased metabolic demands in the heart, muscle, and other organs. In 1920, Danish physiologist August Krogh was awarded the Nobel Prize in Physiology or Medicine for his mathematical and quantitative, experimental demonstration of a solution to this conceptual problem: capillary recruitment, the active opening of previously closed capillaries to meet metabolic demands. Today, capillary recruitment is still mentioned in textbooks. When we suspect symptoms might represent hypoxia of a vascular origin, however, we search for relevant, flow-limiting conditions in our patients and rarely ascribe hypoxia or hypoxemia to short capillary transit times. This review describes how natural changes in capillary transit-time heterogeneity (CTH) and capillary hematocrit (HCT) across open capillaries during blood flow increases can account for a match of oxygen availability to metabolic demands in normal tissue. CTH and HCT depend on a number of factors: on blood properties, including plasma viscosity, the number, size, and deformability of blood cells, and blood cell interactions with capillary endothelium; on anatomical factors including glycocalyx, endothelial cells, basement membrane, and pericytes that affect the capillary diameter; and on any external compression. The review describes how risk factor- and disease-related changes in CTH and HCT interfere with flow-metabolism coupling and tissue oxygenation and discusses whether such contributes to vascular disease pathology.
血液与组织之间氧气的运输受到血液毛细血管通过时间的限制，该时间被理解为血液回流到心脏之前可用于扩散交换的时间。如果所有毛细血管在任何时候都平等地为组织供氧做出贡献，那么这种物理限制将使血管扩张和增加血流量不足以满足心脏、肌肉和其他器官增加的代谢需求。1920 年，丹麦生理学家奥古斯特·克罗（August Krogh）因其对这一概念问题的数学和定量实验证明而获得诺贝尔生理学或医学奖：毛细血管募集，即主动开放先前关闭的毛细血管以满足代谢需求。如今，毛细血管募集仍在教科书中提及。然而，当我们怀疑症状可能代表血管源性缺氧时，我们会在患者中寻找相关的、限制血流的条件，很少将缺氧或低氧血症归因于毛细血管通过时间短。这篇综述描述了血液流动增加时开放毛细血管中毛细血管通过时间异质性（CTH）和毛细血管血细胞比容（HCT）的自然变化如何解释正常组织中氧气供应与代谢需求的匹配。CTH 和 HCT 取决于许多因素：血液特性，包括血浆粘度、血细胞的数量、大小和变形性以及血细胞与毛细血管内皮的相互作用；解剖学因素，包括糖萼、内皮细胞、基底膜和周细胞，它们影响毛细血管直径；以及任何外部压缩。该综述描述了 CTH 和 HCT 与危险因素和疾病相关的变化如何干扰血流-代谢偶联和组织氧合，并讨论这种变化是否导致血管疾病的病理。
