1. APPLIED PHYSIOLOGY
DR. EL-BASHIR EL-SAEED EL-BASHIR
GAR-ELNABI
Makka Faculty of optometry

4. Physiology
• A contact lens effectively occludes the cornea
from its normal environment of oxygen, tears
and ocular secretions. The effect depends
upon lens thickness, size, method of fitting
and material.
• In this context, the following definitions are
used:
• Anoxia occurs where no oxygen is present.

5. Hypoxia
• Occurs where there is reduced oxygen supply
to the ocular tissues.
• Hypercapnia: is accumulation of carbon
dioxide (CO2).

6. Corneal metabolism
• Constant metabolic activity in the cornea
maintains transparency, temperature, cell
reproductive and the transport of tissue
materials.
• The main nutrients needed for these functions
are glucose, amino acid and oxygen. Glucose
and amino acids are provided by the aqueous
humour, whereas oxygen is mainly derived
from tears.

7. Corneal temperature
• The normal corneal temperature of 33-36 C.
may alter during contact lens wear. The effect
becomes more significant under closed eye
conditions. The change may be only 3 degree
C.

8. Stromal acidosis
• A drop in stromal pH induces a state of acidosis in
contact lens wearers as a result of corneal
hypoxia and hypercapnia.
• It appears that that hypercapnia accounts for
about 30% of the total pH drop which can occur
even without a change in corneal thickness.
• Chronic acidosis may explain some of the
alterations seen in both corneal structure and
function following contact lens wear.

9. Tear osmolarity
• Corneal thickness is also affected by the
osmolarity of the tears. In normal, open eye, the
salt content of the tear film is about 10% greater
than that of freshly produced tears due to
evaporation.
• When the eye is closed during sleep, it is bathed
by fresh isotonic tears.
• The cornea responds to the less concentrated
solution by drawing water into the stroma faster
than it can be pumped out by the endothelium.

10. Hence, on wakening, the cornea is
found to have increased thickness by
About 5%. Deswelling occurs rapidly during the first
2 hours the eyes are open.
• Corneal sensitivity: one of the first, important
effects of hypoxia, of which the patient is
unaware, is a drop in corneal sensitivity.
• Tissue fragility: reduce oxygen supply to corneal
epithelium, for example with extended wear,
causes a decrease in the level of metabolic
activity, including the rate of cell mitosis. The
thickness of the epithelium reduces(thinning).

11. Closed eyelid conditions during sleep
The following changes are induced:
• Increase in temperature.
• Hypotonic shift in tear osmolarity (osmolality)
as a result of increased evaporation.
• Slight acidic shift in tear pH as a result of
retardation of carbon dioxide efflux from the
cornea.
• Corneal oxygenation reduced from 155 mmHg
(open eye) to 55 mmHg (closed eye).