If you exercise hard enough to soak your clothes with sweat, you should feel thirsty. The perceived need for water lags behind actual losses, and if you drink only enough to satisfy thirst you don't complete restore lost water. You could also feel thirsty because of mineral imbalances in your body. The sensation of thirst depends upon internal sensory organs that monitor blood volume, mineral content and other complex factors.
Fluid Volume
Fluid inside your cells accounts for about two-thirds of your total body fluids. Blood volume accounts for only 8 percent of your total body fluids. Loss of blood volume through perspiration, injury or illness such as diarrhea triggers thirst, but changes in cell fluid levels also cause a demand for water. If you eat salty foods that alter your electrolyte balance, you upset the osmotic system that shifts fluid through cell walls. Eating too much salt could cause thirst even without water loss, because higher sodium levels outside cell walls cause cells to lose water.
Osmotic Pressure
Electrically charged minerals bond to water ions and maintain the correct fluid balance inside and outside cell walls. Potassium and magnesium work from within cell walls, while sodium and chloride remain in solution outside. Correct mineral levels keep water flowing in and out of cells to deliver food and remove waste products. Drinking more water allows your kidneys to excrete excess salt in urine, but plain water won't replace lost sodium. You could sweat away between 2.25 and 3.4 g of salt every hour during an intense workout, according to SportsMedWeb of Rice University.
Internal Sensors
Your body depends on several internal sensors to trigger thirst and regulate internal water volume. Your brain's hypothalamus senses rising mineral concentrations in your blood and secretes hormones that slow your kidneys and conserve fluid. The hypothalamus also signals your brain cortex to stimulate thirst. Other sensory cells in your heart and major blood vessels increase the production of fluid-regulating hormones when blood pressure falls and slow their release when blood volume rises. Balancing water retention and water intake prevents hyponatremia, an electrolyte imbalance caused by low blood sodium levels. Either hyponatremia or dehydration could have fatal consequences.
Hydrating Sensibly
Your body's fluid regulation systems sometimes falter. If you're 65 or older, you run a greater risk of dehydration because you may not feel thirsty when fluid volume drops. As you age, the production of essential hormones that regulate thirst declines. No matter what your age, you dehydrate faster in cold, dry weather. Your body's regulating sensors react to fluid levels at your body's core. As blood flow to cold extremities drops, blood volume could decrease without triggering thirst. Drink more water than you feel you need if you work in the cold.
References
- The National Academies Press; "Fluid Replacement and Heat Stress"; Roger W. Hubbard, et al.; 1994
- University of New Hampshire Media Relations; Cold Weather Increases Risk of Dehydration; Sharon Keeler; January 2005
- University of California Department of Molecular & Cell Biology: Fluid and Electrolyte Balance
- SportsMedWeb; Salt and the Ultraendurance Athlete; February 2005
- MissouriFamilies.org; Dehydratiuon in the Winter -- Elderly at Risk; Melinda Hemmelgarn; January 2010
