The transport of oxygen between blood and tissue is limited by blood's capillary transit time: The time available for diffusion exchange before blood returns to the heart. If all capillaries had identical extraction properties, this physical limitation would render vasodilation and increased blood flow insufficient means to meet increased metabolic demands in the brain, heart, and other organs. In 1920, Danish physiologist August Krogh was awarded the Nobel Prize in Physiology or Medicine for his solution to this conceptual problem: Capillary recruitment, the opening of previously closed capillaries. Today, capillary recruitment mainly lives on i textbooks. When we suspect symptoms might represent hypoxia of a vascular origin, we search for relevant, flow-limiting conditions in our patients and rarely ascribe hypoxia or hypoxemia to short capillary transit times. This Primer describes how natural changes in capillary transit-time heterogeneity (CTH) and capillary hematocrit (across open capillaries) during increases in blood flow can match oxygen availability to metabolic demands in normal tissue, and how the assumption of negligible CTH lead us to underestimate capillaries' role in tissue oxygenation. CTH and capillary hematocrit depend on plasma viscosity, on the number, size and deformability of blood cells and their interactions with capillary endothelium, on the glycocalyx, endothelial cells, basement membrane, and pericytes that define the capillary lumen, and on any external compression. The Primer describes how risk factor- and disease-related changes in CTH and capillary hematocrit interfere with flow-metabolism coupling and tissue oxygenation, and discusses whether such capillary dysfunction contributes to vascular disease pathology.

中文翻译：

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血流，毛细血管通过时间和组织氧合。毛细血管新生一百周年

氧气在血液和组织之间的传输受到血液毛细血管通过时间的限制：在血液返回心脏之前，扩散扩散可用的时间。如果所有毛细血管都具有相同的提取特性，则这种物理限制将导致血管扩张和血流量增加，不足以满足脑，心脏和其他器官新陈代谢的需求。1920年，丹麦生理学家奥古斯特·克罗格（August Krogh）因其解决以下概念性问题而获得了诺贝尔生理学或医学奖：毛细血管募集，先前封闭的毛细血管的开放。如今，毛细血管招聘主要生活在教科书上。当我们怀疑症状可能代表血管源性缺氧时，我们会寻找相关的，我们患者的血流受限情况，很少将低氧血症或低氧血症归因于较短的毛细血管通过时间。本入门手册描述了在血流增加期间，毛细血管通过时间异质性（CTH）和毛细血管血细胞比容（跨开放毛细血管）的自然变化如何使氧气的可用性与正常组织的代谢需求相匹配，以及忽略的CTH假设如何导致我们低估毛细血管在组织充氧中的作用。CTH和毛细血管血细胞比容取决于血浆粘度，血细胞的数量，大小和可变形性以及它们与毛细血管内皮的相互作用，糖萼，内皮细胞，基底膜和构成毛细血管腔的周细胞以及任何外部压缩。