Why do our tears and sweat have salt in them. Why aren’t they just be water?
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Your body struggles to move water across membranes. It’s easier for the body to pump salt from the cells in tear glands out into the ducts, and then let osmosis (the passive movement of water across a permeable membrane to create equal water potential on both sides) passively fill the tear duct with water to create a tear.
Common way to demonstrate this done in schools is get chunks of potato and weight them. Put one in pure water and one in very salty water. After a few hours, the one in salty water will weigh less, as water has moved from the relatively un-salty potato into the very salty water. The potato in the pure water will have gained weight – water has moved from the unsalty water into the relatively high salty tissue of the potato.
N.B. The body can move water via “aquaporin” channels, but these are only found on select cells, and as a rule in the body – if it wants to move water, it moves salt and the water will follow.
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