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The aqueous layer, which is about 8 mm thick, represents the bulk of the precorneal tear film. It is composed of 98% water, but also contains the following:
Water Oxygen (pO2) Carbon dioxide (pCO) Electrolytes: Na+ K+ Ca2+ Mg+ Cu2+ Cl– HCO3– HPO42– Ammonia Glucose
98% 55 mmHg 27.5 mmHg
134–170 mmol/l 20–40 mmol/l 0.61 mmol/l 0.5–1.1 mmol/l 1.35 mg/l 130 mmol 26 mmol/l 8–10 mmol/l 50 mg/l 0.5 mmol/l
Basal secretion of the aqueous layer is carried out by the accessory lacrimal glands of Krause and Wolfring, while reflex secretion is provided by the main lacrimal gland. The aqueous layer is slightly alkaline (pH between 7.14 to 7.82) and isotonic. The osmotic pressure of this layer is 305 mOsm/kg, while the osmolarity is approximately 300 mOsm/l.
The main function of the aqueous layer is to nourish and protect the eye. The aqueous layer carries nutrients and oxygen to the cornea and carries waste products away. It also hydrates the cornea and thereby prevents it from drying out and becoming opaque. In addition, the aqueous layer helps to lubricate the eye, so that the eyelids can open and close smoothly.
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