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Blood flow, capillary transit times, and tissue oxygenation: the centennial of capillary recruitment.
Østergaard, Leif.
  • Østergaard L; Center of Functionally Integrative Neuroscience, Department of Clinical Medicine, Aarhus University, Aarhus, Denmark.
J Appl Physiol (1985) ; 129(6): 1413-1421, 2020 12 01.
Article in English | MEDLINE | ID: covidwho-1064196
ABSTRACT
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 capillary dysfunction contributes to vascular disease pathology.
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Full text: Available Collection: International databases Database: MEDLINE Main subject: Oxygen / Oxygen Consumption / Capillaries / Microcirculation / Models, Cardiovascular Type of study: Prognostic study Topics: Long Covid Limits: Animals / Humans Language: English Journal: J Appl Physiol (1985) Journal subject: Physiology Year: 2020 Document Type: Article Affiliation country: Japplphysiol.00537.2020

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Full text: Available Collection: International databases Database: MEDLINE Main subject: Oxygen / Oxygen Consumption / Capillaries / Microcirculation / Models, Cardiovascular Type of study: Prognostic study Topics: Long Covid Limits: Animals / Humans Language: English Journal: J Appl Physiol (1985) Journal subject: Physiology Year: 2020 Document Type: Article Affiliation country: Japplphysiol.00537.2020