This aqueous layer loses water to the atmosphere, however, the existing electrolytes of potassium, chloride, bicarbonate, sodium, calcium and magnesium, etc stay in the remaining fluid as it becomes more concentrated. The concentration of molecules in a fluid is called osmolarity, and a normal physiological concentration is iso-osmolar or isotonic. An increase in concentration, or a hypertonic tear film, is an inciting factor in ocular surface inflammation, and it is this inflammation that is very largely responsible for dry eye symptoms.
We know now that the old-fashioned analogy for dry eye – that it is like a car engine getting low on oil with the answer being to pour more oil (artificial tears) on it – is not accurate.
We are actually dealing with a vicious cycle of inflammation.
The rather sensitive conjunctival goblet cells, responsible for mucin production, are adversely affected by this inflammation, so inefficiency of the lipid layer leads to a higher concentration of electrolytes, which leads to inflammation, which in turn leads to mucin production being adversely affected.