Incorporating Variable Vascular Heat Exchangers Into Models Of Human Thermoregulation

Models of human thermoregulatory function generally assume that heat transfer across the skin surface is uniform. However, only glabrous skin regions contain unique vascular structures that enable a large volume of blood to flow immediately below the skin surface. These are the body’s radiators. We are constructing a novel model of thermoregulatory function that incorporates heat transfer across the glabrous skin regions as separate from heat transfer across the general skin surface. Modeling of human temperature regulation Many mathematical models of thermoregulation in humans have been developed over the last 50 years. The first models were developed in the 1960’s and became the accepted standard for human thermoregulatory function modeling. These early models were based on cylindrical representations of the body (e.g., three cylinders representing the head, trunk, and extremities, or six cylinders representing the head, trunk, two arms, and two legs). Key assumptions of these early models were that conduction within the cylindrical elements was purely radial, that heat transfer across the skin was uniform, and that different body segments are homogeneous with respect to temperature. While the models have grown in complexity and diversity over the years, the assumption that heat transfer across the skin surface is uniform has persisted (for examples see Wissler, 2008, Nelson 2009, Yokota et al 2008). Unfortunately, when these original models were generated the primary mammalian thermoregulatory effector mechanism – the body’s radiators – had yet to be appreciated. In fact hands and feet were excluded from the models. Copyright © 2011, Association for the Advancement of Artificial Intelligence (www.aaai.org). All rights reserved. A majority of the internally generated heat is dissipated from the body across the skin surface (less the portion of the internally produced heat that is dissipated through respiration). Heat is moved around the body by the circulating blood. Heat is produced as a byproduct of cellular metabolism. The greater the metabolic effort of a cell, the greater is the heat production. The tissues of body can be sorted according to their resting metabolic activity. The thermal core is comprised of the metabolically active visceral organs (kidneys, heart, lungs, brain, and splanchnic organs). These core organs comprise less that 10% of the total body mass but account for 70-75% of the total resting heat production. Peripheral tissues are comprised of those tissues with very low resting metabolic activity (the skin, skeletal muscles, bone, and connective tissues). During rest the peripheral tissues are metabolically quiescent and, although comprising more than 90% of the total body mass, account for less than 30% of the resting metabolic heat production. Generally, blood flow to the tissues is determined by metabolic demand. In a human resting in a thermoneutral environment 65-70% of the total cardiac output goes to the core organs and 30-35% goes to the peripheral tissues with less than 3% flowing to the skin (excluding thermoregulatory blood flow). Capillary blood flow to all skin regions is uniform and relatively stable. There are two conditions in which the distribution of cardiac output changes: exercise and heat stress. During exercise the metabolic demand of the active muscles results in increased blood flow to those tissues. During heat stress large volumes of blood are delivered to the skin to facilitate heat transfer. Both conditions result in an increase in cardiac output (up to 14 L/min with heat stress alone and up to 20 L/min during exercise). During heat stress the metabolic demands of the core organs and peripheral tissues do not change and, thus, the blood flow serving the 19 Computational Physiology — Papers from the AAAI 2011 Spring Symposium (SS-11-04)