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Key points: A physiological response to increase microcirculatory oxygen extraction capacity at high altitude is to recruit capillaries.In the present study, we report that high altitude‐induced sublingual capillary recruitment is an intrinsic mechanism of the sublingual microcirculation that is independent of changes in cardiac output, arterial blood pressure or systemic vascular hindrance.Using a topical nitroglycerin challenge to the sublingual microcirculation, we show that high altitude‐related capillary recruitment is a functional response of the sublingual microcirculation as opposed to an anatomical response associated with angiogenesis.The concurrent presence of a low capillary density and high microvascular reactivity to topical nitroglycerin at sea level was found to be associated with a failure to reach the summit, whereas the presence of a high baseline capillary density with the ability to further increase maximum recruitable capillary density upon ascent to an extreme altitude was associated with summit success. A high altitude (HA) stay is associated with an increase in sublingual capillary total vessel density (TVD), suggesting microvascular recruitment. We hypothesized that microvascular recruitment occurs independent of cardiac output changes, that it relies on haemodynamic changes within the microcirculation as opposed to structural changes and that microcirculatory function is related to individual performance at HA. In 41 healthy subjects, sublingual handheld vital microscopy and echocardiography were performed at sea level (SL), as well as at 6022 m (C2) and 7042 m (C3), during ascent to 7126 m within 21 days. Sublingual topical nitroglycerin was applied to measure microvascular reactivity and maximum recruitable TVD (TVDNG). HA exposure decreased resting cardiac output, whereas TVD (mean ± SD) increased from 18.81 ± 3.92 to 20.92 ± 3.66 and 21.25 ± 2.27 mm mm−2 (P < 0.01). The difference between TVD and TVDNG was 2.28 ± 4.59 mm mm−2 at SL (P < 0.01) but remained undetectable at HA. Maximal TVDNG was observed at C3. Those who reached the summit (n = 15) demonstrated higher TVD at SL (P < 0.01), comparable to TVDNG in non‐summiters (n = 21) at SL and in both groups at C2. Recruitment of sublingual capillary TVD to increase microcirculatory oxygen extraction capacity at HA was found to be an intrinsic mechanism of the microcirculation independent of cardiac output changes. Microvascular reactivity to topical nitroglycerin demonstrated that HA‐related capillary recruitment is a functional response as opposed to a structural change. The performance of the vascular microcirculation needed to reach the summit was found to be associated with a higher TVD at SL and the ability to further increase TVDNG upon ascent to extreme altitude. Key points: A physiological response to increase microcirculatory oxygen extraction capacity at high altitude is to recruit capillaries.In the present study, we report that high altitude‐induced sublingual capillary recruitment is an intrinsic mechanism of the sublingual microcirculation that is independent of changes in cardiac output, arterial blood pressure or systemic vascular hindrance.Using a topical nitroglycerin challenge to the sublingual microcirculation, we show that high altitude‐related capillary recruitment is a functional response of the sublingual microcirculation as opposed to an anatomical response associated with angiogenesis.The concurrent presence of a low capillary density and high microvascular reactivity to topical nitroglycerin at sea level was found to be associated with a failure to reach the summit, whereas the presence of a high baseline capillary density with the ability to further increase maximum recruitable capillary density upon ascent to an extreme altitude was associated with summit success. [ABSTRACT FROM AUTHOR]