Water and Electrolytes

Under optimal conditions humans have been able to survive about 10 minutes without oxygen, up to 18 days without water, but nearly 60 days without food. Despite the fact that oxidation of nutrients produces water, there is a longer lasting supply of foodstuffs within the body than water. Part of the fatigue mechanism occurring during physical exercise can be attributed to fluid loss (dehydration) from the body and possibly to fluid-electrolyte shifts within the body. There is no adaptation to successive periods of dehydration and the performance of the strongest and fittest people will deteriorate rapidly with dehydration. When compared with the daily variability of many physico-chemical parameters, the least variability is found in body temperature, and plasma sodium, chloride, calcium, and osmolality ( 1 ). There is a close association between thermoregulation and the sodium, calcium, and osmotic concentration of the extracellular fluid; increases in plasma sodium concentration (hypernatremia) and plasma