The distribution system
The distribution system consists of the eyelids and the tear meniscus along the lid margins in the open eye. Irregularities or disorders of the eyelids can interfere with lid function, resulting in ocular problems or discomfort.
The need to renew the tear film is caused by drying of the cornea. In a healthy eye, the tear film begins to evaporate after 15 to 30 seconds between blinks. This causes parts of the layer to break up, creating discomfort and stimulating blinking. The blinking reflex renews the tear film and its homogeneity, thus preventing dry spots. Blinking is an important factor in tear distribution (and also has a crucial role in tear drainage).
Mechanism of distribution
Frequent blinking plays a vital role in distributing tear fluid over the cornea. Each time the eye blinks, the components of the tear film are mixed together and a thin film is spread over the ocular surface. The tear film is completely replenished every 5 to 6 minutes.
|Blinking occurs approximately 5 to 20 times per minute and each blink takes about 0.3 to 0.5 seconds. The tear film (7.9 μl) is regenerated after each blink.
The tear film distribution process occurs in three steps:
- During closure of the eyelids
When the eyelids close, the superficial lipid layer is compressed and squeezed between the two lids, without going under them.
The lower lid has a horizontal, transverse motion (from 2 to 5 mm). This motion fulfils an important role in facilitating the removal of debris and ‘used’ tear fluid from the eye by pushing these substances towards the puncta. In addition, the resulting shear action across the thin aqueous layer between the moving lid and the ocular surface helps to redistribute the mucus layer. Any lipid-contaminated mucus is rolled up, usually in a thread-like shape, and dragged into the lower and upper fornix.
- During reopening of the eyelids
As the eyelids reopen, the aqueous layer is immediately redistributed across the surface of the eye and a thin layer of lipid spreads across it. In addition, the aqueous layer thickens. Meanwhile, the increasing negative pressure in the gradually thinning tear meniscus tends to limit the tear supply available for tear film formation.
- Between blinks
Just before blinking, excess or ‘used’ tears are drained from the eye. Between blinks, the tear film begins to thin in some areas and become unstable. This leads to the formation of dry spots. Unless a subsequent blink reverses this process by rewetting the eye, these areas enlarge and coalesce, so that eventually the entire corneal surface may become dry.
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