37 Questions
What is the thickness of the tear film?
7 to 10 μm
Which layer of the tear film retards evaporation and provides lubrication for smooth eyelid movement?
Lipid layer
What is the primary source of atmospheric oxygen for the cornea?
Tear film
Which substances are contained in the tear film for antibacterial purposes?
Lysozyme, beta-lysyn, lactoferrin, IgA
Which glands contribute to the lipid layer of the tear film?
Meibomian and Zeiss glands
What is the function of the lipid layer in the tear film?
Retards evaporation and provides stability
Which layer of the tear film helps maintain corneal hydration?
Aqueous layer
What percentage of tear fluid passes through the nasolacrimal drainage system?
75%
Where is the lacrimal punctum located?
At the junction of the lacrimal and ciliary portions of the eyelid margin
What prevents retrograde movement of fluid up from the nasal cavity?
Valve of Hasner
What causes tears to be driven into the nasolacrimal duct?
Lacrima sac collapse
What is the length of the interosseous part of the nasolacrimal duct?
12 mm
What is the function of the canaliculi in tear drainage?
Act as siphons to pull tears in through the punctum
Where does the lacrimal sac empty into?
Nasolacrimal duct
Which of the following is a symptom of canaliculitis?
Pain, swelling, erythema, and discharge
What is the normal range for Tear Break Up Time (TBUT)?
Between 15 and 45 seconds
What is the most common congenital lacrimal abnormality?
Congenital Nasolacrimal Duct Obstruction
What is the primary cause of Dacryocystitis?
Obstruction of the nasolacrimal duct
What is the characteristic feature of Congenital Dacryostenosis?
Lacrimation and discharge
What is the recommended action if Congenital Nasolacrimal Duct Obstruction is not relieved 4-8 weeks after birth?
Probing
What is the surgical procedure for acute canaliculitis with a pouting puncta?
A posterior cut down of the canaliculus and curettage of the granules
What is the characteristic feature of Dacryoadenitis?
Pain, edema, and erythema
What is the measure of the stability of the tear film?
Tear Break Up Time
Which component of the tear film is responsible for facilitating adhesion of the aqueous layer to the ocular surface?
Mucous layer
What is the normal total volume of tears on the ocular surface?
7 to 10 μl
What is the average pH of tears?
7.45
Which gland is responsible for reflex secretion of tears in response to a stimulus?
Main lacrimal gland
What is the blood supply for the lacrimal gland?
Lacrimal artery
Which nerve provides sensory innervation to the lacrimal gland?
Trigeminal nerve
How is lacrimal gland fluid distributed over the ocular surface?
Through contraction of the orbicularis muscle
What is the average osmolarity of the tear film?
315 mOsm/kg
Where is the main lacrimal gland located?
Temporal fossa of the orbital plate of the frontal bone
What is the average pH of tears?
7.45
What is the normal tear production rate?
1 μl/min
What is the composition of the main lacrimal gland fluid?
Water, electrolytes, and antibacterial agents
What is the total volume of tears on the ocular surface?
7 to 10 μl
What is the function of the mucous layer in the tear film?
Facilitates adhesion of the aqueous layer to the ocular surface
Study Notes
Anatomy and Physiology of the Tear Film
- The aqueous layer makes up 60-70% of the total tear film and contains water, electrolytes, inorganic salts, glucose, urea, enzymes, immunoglobulins, cytokines, proteins, peptide growth factors, glycoproteins, and antibacterial substances.
- The mucous layer, derived from conjunctival goblet cells, comprises 30-40% of the tear film and facilitates adhesion of the aqueous layer to the ocular surface, as well as binding and entrapping bacteria and viruses.
- Tear film stability is achieved through the even spread of the mucous layer over the corneal epithelium by the eyelids and the absorption of mucous by the glycocalyx to create a hydrophilic surface.
- The total volume of tears on the ocular surface is 7 to 10 μl, with normal tear production at about 1 μl/min and the eye's capacity to hold 20-30 μl at once.
- The tear film is normally isotonic and has an average osmolarity of 315 mOsm/kg, which can vary based on factors such as blink rate, humidity, ocular pathology, and contact lens use.
- The average pH of tears is 7.45, with good buffering ability, allowing the eye to tolerate ophthalmic solutions with different pH levels.
- The lacrimal secretory system consists of the main lacrimal gland, accessory lacrimal glands (Krause and Wolfring), Meibomian glands, and conjunctival goblet cells, serving the dual function of tear formation and drainage.
- The main lacrimal gland, located in the temporal fossa of the orbital plate of the frontal bone, has orbital and palpebral lobes and secretes fluid composed of water, electrolytes, and antibacterial agents.
- The blood supply to the lacrimal gland is through the lacrimal artery, while sensory innervation is provided by the lacrimal nerve, and reflex tearing occurs in response to external stimuli.
- The tear film is distributed across the ocular surface through the secretion of lacrimal gland fluid into the upper fornix, which descends across the anterior surface of the globe and is spread and reformed with each blink.
- Both upper and lower eyelid margins have a tear meniscus, with the upper tear meniscus continuous with the lower meniscus and leading directly to punctum in the medial canthus for drainage.
Test your knowledge of the nasolacrimal drainage system with this quiz. Learn about the components and functions of the system, including the lacrimal punctum, canaliculi, lacrimal sac, and nasolacrimal duct. Understand how tear fluid passes through this system and its importance in maintaining eye health.
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