The excretory system
The excretory system is made up of the lacrimal puncta, the lacrimal canals, the lacrimal sac, the nasolacrimal duct and the nasal cavity. It is estimated that under normal conditions, over 90% of the lacrimal fluid is removed by the excretory system – the remaining fluid evaporates between blinks.
Blinking has a vital role in tear drainage. Contraction of the orbicularis muscle during blinking causes alternate positive and negative pressures in the lacrimal sac, forming the lacrimal ‘pump’ which facilitates the drainage of excess lacrimal fluid.
|The lacrimal puncta are essential for proper physiological drainage of tears. Gravity suggests that the lower punctum is more important in each pair, but a functioning punctum may allow compensatory drainage if the other punctum becomes damaged.
Drainage of tears through the puncta, into the lacrimal sac and then into the nasal cavity takes place in a sequence of steps:
- With the start of a blink, the puncta close and the lacrimal canals and lacrimal sac are gradually squeezed until the lid completely closes. This action empties these structures of pre-existing tear fluid and creates a partial vacuum inside, especially when the lids start to open
- As the lids begin to open, the puncta are still closed, which creates more negative pressure inside the lacrimal canals
- When the eye is approximately one-third open, the puncta ‘pop’ open and tear fluid drains into the lacrimal canals
- The passage of the tear fluid down the wide-bored nasolacrimal duct into the nasal cavity occurs as a result of gravity. Tear fluid is then drained into the nasopharynx to be swallowed, or evaporates at the orifice of the nose, aided by the movement of air during inspiration and expiration
- This cycle repeats itself with each blink
Normal Tear Flow
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