• Can Ince, Anne-Marije van Kuijen, Dan M J Milstein, Koray Yürük, Lars P Folkow, Wytske J Fokkens, and Arnoldus S Blix.
• Department of Intensive Care Medicine, Erasmus Medical Center, Erasmus University Rotterdam, 's-Gravendijkwal 230, PO Box 2040, 3000 CA Rotterdam, Netherlands. c.ince@erasmusmc.nl
• BMJ. 2012 Jan 1;345:e8311.

ObjectiveTo characterise the functional morphology of the nasal microcirculation in humans in comparison with reindeer as a means of testing the hypothesis that the luminous red nose of Rudolph, one of the most well known reindeer pulling Santa Claus's sleigh, is due to the presence of a highly dense and rich nasal microcirculation.DesignObservational study.SettingTromsø, Norway (near the North Pole), and Amsterdam, the Netherlands.ParticipantsFive healthy human volunteers, two adult reindeer, and a patient with grade 3 nasal polyposis.Main Outcome MeasuresArchitecture of the microvasculature of the nasal septal mucosa and head of the inferior turbinates, kinetics of red blood cells, and real time reactivity of the microcirculation to topical medicines.ResultsSimilarities between human and reindeer nasal microcirculation were uncovered. Hairpin-like capillaries in the reindeers' nasal septal mucosa were rich in red blood cells, with a perfused vessel density of 20 (SD 0.7) mm/mm(2). Scattered crypt or gland-like structures surrounded by capillaries containing flowing red blood cells were found in human and reindeer noses. In a healthy volunteer, nasal microvascular reactivity was demonstrated by the application of a local anaesthetic with vasoconstrictor activity, which resulted in direct cessation of capillary blood flow. Abnormal microvasculature was observed in the patient with nasal polyposis.ConclusionsThe nasal microcirculation of reindeer is richly vascularised, with a vascular density 25% higher than that in humans. These results highlight the intrinsic physiological properties of Rudolph's legendary luminous red nose, which help to protect it from freezing during sleigh rides and to regulate the temperature of the reindeer's brain, factors essential for flying reindeer pulling Santa Claus's sleigh under extreme temperatures.

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