From Fellrnr.com, Running tips
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Humans have 2-4 million sweat glands over nearly their whole body surface, and though each is tiny, together they weigh as much as a kidney (~100g)<ref name="Fitzpatrick"/>. Sweat glands are most numerous on the soles of the feet. Sweat is produced in two steps<ref name="Fitzpatrick"/>:
# A coil generates an ultra-filtrated fluid. This fluid has the same sodium concentration (isotonic) as the blood.
# The fluid travels from the coil up the sweat duct which reabsorbs sodium and chloride (but not [[Potassium|potassium]]). This reabsorption is via active transportation (i.e. it requires energy from ATP). The [[Glycogen]] stored in the sweat glands will only support the reabsorption for less than 10 min., so the energy is predominantly supplied by the blood. Glucose is the preferred energy source, though
lactate and pyruvate can also be used. Fatty acids, ketones, and amino acids are very poorly used. The reabsorption process also acidifies the final sweat.
The rate of sweat production depends on the local skin temperature and core body temperature. A rise in the localized skin temperature will produce an increased sweat rate within 60 seconds<ref name="Fitzpatrick"/>.
==Sodium Loss Table==