Dr. Shawn Dolan at First Endurance has an updated article about electrolytes that spelled it out quite thoroughly. Here's a sample that taught me something about how the body triggers thirst:
"During exercise, the body loses fluids and sodium through sweating. This causes a decrease in blood volume, thereby increasing sodium and chloride concentrations in the blood. The increased concentration of electrolytes in the blood through decreased blood volume is what triggers the thirst mechanism. By the time an athlete becomes thirsty; the electrolytes are already out of balance, so restoration of blood volume is critical for the prevention of dehydration. Water consumption is effective in increasing blood volume; however, there is a consequential dilution of sodium in the blood due to the increased blood volume and excessive sodium losses in sweat, so electrolyte replenishment is key."Electrolyte concentration triggers thirst...who knew? Go here to read the whole article. It's worth checking out.