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.

This presentation describes various aspects of the regulation of tissue oxygenation, including the roles of the circulatory system, respiratory system, and blood, the carrier of oxygen within these components of the cardiorespiratory system. The respiratory system takes oxygen from the atmosphere and transports it by diffusion from the air in the alveoli to the blood flowing through the pulmonary capillaries. The cardiovascular system then moves the oxygenated blood from the heart to the microcirculation of the various organs by convection, where oxygen is released from hemoglobin in the red blood cells and moves to the parenchymal cells of each tissue by diffusion. Oxygen that has diffused into cells is then utilized in the mitochondria to produce adenosine triphosphate (ATP), the energy currency of all cells. The mitochondria are able to produce ATP until the oxygen tension or PO2 on the cell surface falls to a critical level of about 4–5 mm Hg. Thus, in order to meet the energetic needs of cells, it is important to maintain a continuous supply of oxygen to the mitochondria at or above the critical PO2 . In order to accomplish this desired outcome, the cardiorespiratory system, including the blood, must be capable of regulation to ensure survival of all tissues under a wide range of circumstances. The purpose of this presentation is to provide basic information about the operation and regulation of the cardiovascular and respiratory systems, as well as the properties of the blood and parenchymal cells, so that a fundamental understanding of the regulation of tissue oxygenation is achieved.

Nerve endings in the walls of the carotid sinuses and the aortic arch transduce arterial pressure changes and provide the central nervous system with a steady stream of encoded information. On the basis of this information, efferent autonomic neural activity is modulated finely, and the neurohumoral milieu of the heart and the blood vessels is adjusted on a second-to-second basis. The arterial baroreflex may be the most important of the cardiovascular control mechanisms, because the baroreflex, above all other reflex mechanisms, is the one whose speed is most adequate to respond rapidly to the abrupt transients of arterial pressure that occur in daily life. This volume presents the many experimental methods available for use in humans that have been recently developed. Some are ingenious and yield results that earlier might have been thought impossible to obtain from human volunteers. Development of these new methods has increased the scientific credibility of human baroreflex research, and this work discusses the advances made in these studies. It clearly describes the existing deficiencies in the understanding of baroreflex mechanisms, and suggests methods for future research in this developing field.

MANY aspects of physiology are best understood in terms of bodily reactions to environmental stress, and temperature is one of the most often encountered stress factors in the environment. The responses to temperature can involve practically all of the organ systems of the body and it is for this reason that the study of the regulation of body temperatures represents one of the finest examples of complex reaction integrated by the nervous and endocrine systems, and hence of the principles of biological control. Thus, while thermoregulation offers an abundance of opportunities for the individual who likes to specialize in depth, it is an ideal type of physiology for those who prefer to think of the functioning of the body as whole. This book is written primarily for the undergraduate, but I hope also that some students may find time to read it, before embarking on a university course, as an introduction to some of the ideas that will be encountered in the more detailed study of the biological sciences, including medicine. I have tried to discuss the evidence for important ideas, since this is fundamental to the scientific method, and have been particularly concerned to avoid the use of the sort of technical jargon that gives a spurious impression of authority while in reality creating confusion out of what is in essence simple.

From 11 to 15 July 1977 about 60 physiologists, endo crinologists, ecologists and other biologists from 14 countries convened at the University Montpellier for a symposium on Environmental Endocrinology. This meet ing was organized as a Satellite Symposium of the 27th International Congress of Physiological Sciences, Paris, 18-23 July 1977. This volume is a record of the com munications presented at the symposium. The objectives of the program were to examine the role of the endocrine system in a wide spectrum of adjustments and adaptations to changes in environmental conditions by various spe cies of animals, including man, and to promote an ex change of ideas among investigators who have approached these functions from diverse aspects. The diversity of the information and ideas communicated is great. Of necessity, they represent only an extremely modest se lection of the many facets of endocrine function in the interaction of animals with their environments. Be yond the usefulness of the communications individually, we hope that they collectively demonstrate the substan tial heuristic value of the concept of environmental endocrinology as it was perceived by the participants. We acknowledge gratefully the kindness and sympathy of Professor Jaques ROUZAUD, President of the University of Montpellier II, for his generous extension of the hospitality of the University to the Symposium. We are most grateful to Mrs. Monique VIEU who effected so well the secretarial organization of the Sympos.

Here is the first single-volume work to analyze in a comprehensive manner the mechanisms by which the human cardiovascular system adjusts to physical stresses such as exercise, heat, upright posture, and hypoxia. Emphasizing the regulation of regional blood flow and blood volume, the work delineates the unique character of the human cardiovascular system and the regulatory problems it faces. It represents a timely response to the growing interest in cardiovascular health, exercise, and physical conditioning. In describing the basic mechanisms of vasomotor control, the author focuses on the interaction between the arterial and venous systems. He indicates how control of regional circulations interacts with control of venous volume and cardiac performance, showing how vasomotor and venomotor regulation serves the entire organism. This treatment redresses an imbalance in contemporary teaching of cardiovascular physiology which has focused more on the heart than on the vasculature. The book utilizes graphics imaginatively throughout to clarify concepts in a fresh and effective way. Features: * Analytical, quantitative, and problem oriented * Uniquely comprehensive; coverage extends from small blood vessel functioning to the entire system * Imaginatively illustrated, with exceptionally clear graphics that explain basic principles and controversial ideas * Original, new ideas on how peripheral circulation influences heart performance * Probes the upper limits of human cardiovascular function, their causes and how they change

The book is designed to provide a flowing description of the physiology of heat stress, the illnesses associated with heat exposure, recommendations on optimising health and performance, and an examination of Olympic sports played in potentially hot environmental conditions. In the first section the book examines how heat stress effects performance by outlining the basics of thermoregulation and how these responses impact on cardiovascular, central nervous system, and skeletal muscle function. It also outlines the pathophysiology and treatment of exertional heat illness, as well as the role of hydration status during exercise in the heat. Thereafter, countermeasures (e.g. cooling and heat acclimation) are covered and an explanation as to how they may aid in decreasing the incidence of heat illness and minimise the impairment in performance is provided. A novel and particular feature of the book is its inclusion of sport-specific chapters in which the influence of heat stress on performance and health is described, as well as strategies and policies adopted by the governing bodies in trying to offset the deleterious role of thermal strain. Given the breadth and scope of the sections, the book will be a reference guide for clinicians, practitioners, coaches, athletes, researchers, and students.