Environmental heat stress is associated with a marked decrease in orthostatic tolerance (OT), which is defined as the ability to stand or sit upright without symptoms of dizziness, lightheadedness, presyncope, or fainting. In most healthy humans, the autonomic nervous system makes rapid and balanced adjustments to heart rate and peripheral blood flow, such that most people are able to stand up successfully most of the time, in most environments. The goal of this book is to discuss various aspects of the sympathetic neural response to heat stress, how the sympathetic nervous system coordinates the successful integrative physiological response to orthostasis, and what happens when it encounters both challenges simultaneously. We include overviews of mechanisms of thermoregulation and blood pressure regulation in humans, with particular focus on control of cardiac output and neurovascular control mechanisms during heat stress. We discuss the implications that these changes have for distribution of peripheral blood flow and, in particular, for blood flow to the cerebral circulation. The added stressor of dehydration is also discussed, as it so often goes hand in hand with heat stress. We end with a brief presentation of countermeasures against the decreases in OT with heat stress.
About the Author: Zachary Schlader received his Ph.D. in Sport and Exercise Science in 2011 from the Massey University (New Zealand). He subsequently completed a postdoctoral fellowship at the Institute for Exercise and Environmental Medicine at the University of Texas Southwestern Medical Center and Texas Health Presbyterian Hospital Dallas, which is located in Dallas, TX. In 2014, he joined the faculty of the Department of Exercise and Nutrition Sciences at the University at Buffalo (Buffalo, NY). He is a Fellow of the American College of Sports Medicine and currently serves on the editorial board for the American Journal of Physiology: Integrative, Regulatory, and Comparative Physiology. His research is focused on understanding the implications of thermal stress on integrative physiology in healthy and "at risk" (e.g., manual laborers, older adults, various disease states, etc.) populations. To date, his work has focused largely on the autonomic and cardiovascular responses elicited by changes in body temperature. The overall goal is to elucidate the underlying mechanisms and identify ways to mitigate the adverse effects of thermal stress.