The tear film is in direct contact with the conjunctiva and cornea. It is the second line of ocular surface defence, after the lids. This thin layer (approximately 7 mm) protects, nourishes and maintains the refractive power of the cornea.
The physicochemical characteristics of the human precorneal tear film:
| Volume | 7–9 μl |
| Basal secretion rate | 1-2 μl |
| Thickness | 6.5 μm |
| pH | 7.3 to 7.7 |
| Osmolarity | 296 to 308 mOsm/l |
| Refractive Index | 1.3361 to 1.3361 to 1.3379 |
| Viscosity | 1 to 100 cps |
| Evaporation rate | 10.1 x 10-7 gm/cm2/s |
The precorneal tear film is not secreted as a complete solution. Instead, its numerous components are secreted from various diverse glands and structures distributed around the eye. The tear film is typically made of three layers (lipid, aqueous and mucus) and it has now been demonstrated that there is no clear-cut barrier between these layers.
The inner mucus layer is diluted in the aqueous component and adheres tightly to the epithelial cells of the cornea, while the outer lipid layer limits tear evaporation. However, for the sake of simplicity the three layers will be described separately.
Structure of the precorneal tear film
The three-layered tear film consists of the inner mucus (or mucin) layer, which is in direct contact with the epithelial cells of the cornea and conjunctiva, the middle aqueous (or watery) layer and the outer lipid (or oily) layer.
Precorneal tear film
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The underlying mucin layer:
– consists of mucus that helps the tears to spread evenly and adhere to the epithelial surface of the cornea
– allows the eye to remain wet
– aids lubrication of the eye
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The middle aqueous layer:
– makes up the majority of the tear film and contains mostly water, nutrients and ions that are important to the health of the eye
– keeps the eye moist
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The outermost lipid layer:
– prevents the evaporation of tears from the surface of the eye
– maintains surface tension.
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