The acqueous layer

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 soluble gases: oxygen (O2) and carbon dioxide (CO2)
  • Electrolytes (which help to maintain the normal fluid balance within the eye): sodium, chloride, magnesium, calcium, potassium, copper, zinc, manganese, bicarbonates and phosphate ions
  • Hormones: prostaglandins, thyroid hormone, insulin, prolactin
  • Organic molecules: glucose (which seems to be the only sugar), urea, pyruvate
  • Proteins: for example, lysozyme and lactoferrin (which fight bacteria), interferon (which fights viral infection) and antibodies such as immunoglobulins A, G and E. In normal conditions, 99% of these proteins are synthesised by the main lacrimal gland.

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 tonicity of the tears is equivalent to 0.9% sodium chloride. Most eye formulations try to set the tonicity of the product equivalent to 0.9% saline in order to avoid eye irritation. 

If tears are isotonic, there is no change in water volume in the cornea and vision remains normal. If tears are hypotonic, water will flow into the cornea and, in swelling, will become more myopic (e.g. prolonged crying). If tears are hypertonic, water will flow out of the cornea and it will shrink, becoming more hyperopic (e.g. swimming in the ocean). 

Role of the aqueous layer

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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