Tissue oxidative metabolism can increase the difference between local temperature and arterial blood temperature by up to 1.3^oC: Implications for brain, brown adipose tissue, and muscle physiology

Abstract

Tissue temperature increases, when oxidative metabolism is boosted. The source of nutrients and oxygen for this metabolism is the blood. The blood also cools down the tissue, and this is the only cooling mechanism, when direct dissipation of heat from the tissue to the environment is insignificant, , in the brain. While this concept is relatively simple, it has not been described quantitatively. The purpose of the present work was to answer two questions: 1) to what extent can oxidative metabolism make the organ tissue warmer than the body core, and, 2) how quickly are changes in the local metabolism reflected in the temperature of the tissue? Our theoretical analysis demonstrates that, at equilibrium, given that heat exchange with the organ is provided by the blood, the temperature difference between the organ tissue and the arterial blood is proportional to the arteriovenous difference in oxygen content, does not depend on the blood flow, and cannot exceed 1.3C. Unlike the equilibrium temperature difference, the rate of change of the local temperature, with respect to time, does depend on the blood flow. In organs with high perfusion rates, such as the brain and muscles, temperature changes occur on a time scale of a few minutes. In organs with low perfusion rates, such changes may have characteristic time constants of tens or hundreds of minutes. Our analysis explains, why arterial blood temperature is the main determinant of the temperature of tissues with limited heat exchange, such as the brain.

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“…The lower tissue temperature also impacts enzyme's activity for the energy metabolic pathways. If energy production is significantly impaired, cell signaling process is expected to be impaired [259]. Low air temperature may lower local temperature along the respiratory track even if the body's core temperature is not lowered or even if the person does not feel cold.…”

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“…While the rise of temperature differential between the brain site and arterial blood or temporal muscle consistently induced by arousing stimuli could provide a measure of brain metabolic activation, a similar change occurs in the brown fat, another factor contributing to body thermogenesis [47,48]. Under normothermic conditions, temperature in brain fat tissue is slightly lower than in body core, but it is consistently increased during environmental cooling and exposure to pyrogens, becoming up to one degree higher than in body core ( [49,50].…”

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“…) requires a high thermal sensitivity from the thermal imaging method in order to detect small variations. Arterial blood temperature is the main determinant of temperature in tissues with limited heat exchange and according to Zaretsky et al, temperature difference between tissue and arterial blood is proportional to arteriovenous difference in oxygen content, is independent of blood flow, and cannot exceed 1.3°C [5]. Thus, internal temperature measurement may be used to estimate tissue oxygen uptake but the temperature sensitivity in measurement of physiology should be around 1.0°C or below.…”