Page 100 - Pharmaceutics IV (02-06-01305)

Ophthalmic DD: Introduction

Anatomy & physiology

Nasolacrimal system: tear/lacrimal fluid
Tear fluid secretion & volume:
• The normal secretion rate is about 0.5–2.2 µL/min. Under reflex

tearing the secretion rate may increase to as high as 300 to 400
µcL/min.
• The normal volume of tear fluid in the cul-de-sac is about 6 to 8 µcL, it
can hold about 30 µcL. With greater fluid additions, excess fluid will
overflow at the lacrimal lake or be splashed into the eyelashes by
reflex blinking.

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Ophthalmic DD: Introduction

Anatomy & physiology

Nasolacrimal system: tear/lacrimal fluid
Tear fluid pH and buffer capacity:
• The pH of normal tear fluid is 7.4. It varies in certain cases: it is more

acidic in contact lens wearers.
• The buffering capacity of the tear fluid is not symmetric around the

mean pH. It has a higher buffer capacity to resist decrease in pH
than an increase in pH.

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Salma E. EL-Habashy