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An Outline of Cardiovascular Structure and Function
Published in Joseph D. Bronzino, Donald R. Peterson, Biomedical Engineering Fundamentals, 2019
In addition to prioritizing and controlling the distribution of blood, the physiologic regulation of cardiovascular function is directed mainly at four other variables: CO, blood pressure, blood volume, and blood composition. From Equation 1.1, we see that CO can be increased by increasing the HR (a chronotropic eect), increasing the EDV (allowing the heart to ll longer by delaying the onset of systole), decreasing the ESV (an inotropic eect), or doing all three things at once. Indeed, under the extrinsic inuence of the sympathetic nervous system and the adrenal glands, HR can triple-to some 240 beats/min if necessary-EDV can increase by as much as 50%—to around 200 mL or more of blood-and ESV can decrease a comparable amount (the cardiac reserve)—to about 30-35 mL or less. e combined result of all three eects can lead to over a sevenfold increase in CO-from the normal 5 to 5.5 L/min to as much as 40 to 41 L/min or more for very brief periods of strenuous exertion.
Post-exercise, passive heat acclimation with sauna or hot-water immersion provide comparable adaptations to performance in the heat in a military context
Published in Ergonomics, 2023
Edward Ashworth, James Cotter, Andrew Kilding
Regarding military operations, reductions in core and skin temperature allows more work to be completed before heat tolerance limits work capacity (Shen and Zhu 2015), while simultaneously promoting safety (Faulkner et al. 2016). In a group of highly motivated individuals, both internal and external cues to keep moving can override self-preservation signals to limit activity, thereby placing the individual in danger (Epstein, Druyan, and Heled 2012). By lowering core temperature there is a reduced, but not eliminated, likelihood of reaching critical limits where organs are damaged, thereby minimising casualties (Armstrong et al. 2010). Similarly, lowering heart rate increases the cardiac reserve, allowing work output to increase as muscular blood flow can be facilitated while maintaining blood flow to cutaneous circulations for cooling (Horowitz 2003). Therefore, the observed cardiovascular adaptations help to directly improve performance and safety by increasing the capacity for heat loss. However, caution should be advised when interpreting the improved safety of soldiers following HA programmes. Adaptations that provide a safer working environment may only do so when matched for exercise intensity. Therefore, if soldiers are asked to move as quickly as possible to a new location, they may achieve the task faster, but in doing so the extra demand consumes the HA-induced increase in thermoregulatory capacity, thereby exposing soldiers to heat illness.