The underwater environment: cardiopulmonary, thermal, and energetic demands

Abstract

Pendergast DR, Lundgren CE. The underwater environment: cardiopulmonary, thermal, and energetic demands. J Appl Physiol 106: 276 -283, 2009. First published November 26, 2008 doi:10.1152/japplphysiol.90984.2008.-Water covers over 75% of the earth, has a wide variety of depths and temperatures, and holds a great deal of the earth's resources. The challenges of the underwater environment are underappreciated and more short term compared with those of space travel. Immersion in water alters the cardio-endocrine-renal axis as there is an immediate translocation of blood to the heart and a slower autotransfusion of fluid from the cells to the vascular compartment. Both of these changes result in an increase in stroke volume and cardiac output. The stretch of the atrium and transient increase in blood pressure cause both endocrine and autonomic changes, which in the short term return plasma volume to control levels and decrease total peripheral resistance and thus regulate blood pressure. The reduced sympathetic nerve activity has effects on arteriolar resistance, resulting in hyperperfusion of some tissues, which for specific tissues is time dependent. The increased central blood volume results in increased pulmonary artery pressure and a decline in vital capacity. The effect of increased hydrostatic pressure due to the depth of submersion does not affect stroke volume; however, a bradycardia results in decreased cardiac output, which is further reduced during breath holding. Hydrostatic compression, however, leads to elastic loading of the chest wall and negative pressure breathing. The depthdependent increased work of breathing leads to augmented respiratory muscle blood flow. The blood flow is increased to all lung zones with some improvement in the ventilation-perfusion relationship. The cardiac-renal responses are time dependent; however, the increased stroke volume and cardiac output are, during head-out immersion, sustained for at least hours. Changes in water temperature do not affect resting cardiac output; however, maximal cardiac output is reduced, as is peripheral blood flow, which results in reduced maximal exercise performance. In the cold, maximal cardiac output is reduced and skin and muscle are vasoconstricted, resulting in a further reduction in exercise capacity. respiratory; renal; immersion; exercise; aging; sex differences; submersion; pressure; energy cost THE UNDERWATER ENVIRONMENT is unique, and while it is a magical place with great history and beauty and plant and animal life and holds a wealth of resources, it also imposes pronounced physiological stresses on humans and animals. This brief review offers an overview of how the major environmental challenges, i.e., the pressure, temperature extremes, and the unbreathable ambient medium, of the "Silent World" affect circulation, renal system and water balance, breathing, exercise, and thermal balance and how it imposes some threats due to pharmacological or toxic effects of respired gases at high pressure, i.e., great depths. Adaptations to ...

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