Mechanisms of Tear Drainage

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Drainage of tears involves a number of different mechanisms (Table 1.3). Physical factors, such as gravity, respiration, and evaporation, have been suggested. A decisive role is played by capillary attraction, aided by contraction of the lacrimal portion of the orbicularis muscle with blinking, as well as distension of the lacrimal sac by the action of the orbicularis muscle [19].

The mucin diversity of the epithelium of the naso-lacrimal ducts together with TFF peptides that are able to influence the rheological properties of tear fluid have already been mentioned. Besides antimicrobial defense, these components are necessary epithelial secretion products to enhance tear transport [12]. Disorders in the balance of single mucins and TFF peptides are described in Chap. 2.

The lamina propria of the lacrimal sac and nasolacrimal duct consist of two strata: underneath the epithelium, loose connective tissue containing a thin layer of elastic fibers and many lymphatic cells, sometimes arranged in follicles, as well as a rich venous plexus situated under the loose connective tissue that is connected caudally with the cavernous body of the nasal inferior turbinate. Collagen bundles and elastic and reticular fibers between the blood vessels of the rich venous plexus are arranged in a helical pattern

Active lacrimal pump mechanism aided by contraction of the lacrimal portion of the orbicularis muscle

Distension of the lacrimal sac by the action of the lacrimal portion of the orbicularis muscle

Epithelial secretion products (mucins and TFF peptides) of the epithelium of the lacrimal sac and nasolacrimal duct

"Wringing-out" mechanism governed by a system of helically arranged fibrillar structures

Opening and closing of the lumen of the lacrimal passage effected by the bulging and subsiding of the cavernous body

Capillarity

Respiration

Evaporation

Absorption of tear fluid through the lining epithelium of the lacrimal sac and nasolacrimal duct and run spirally from the fornix of the lacrimal sac to the outlet of the nasolacrimal duct, where they contribute biomechanically to tear outflow during blinking (Fig. 1.9) [25]. Specialized types of blood vessels are distinguishable inside the vascular tissue and are comparable to those found in a cavernous body [20].

The blood vessels are specialized arteries (barrier arteries), venous lacunae (capacitance veins), veins (throttle veins), and arteriovenous anastomoses. They facilitate closure and opening of the lumen of the lacrimal passage by swelling and shrinkage of the cavernous body. Swelling occurs when the barrier arteries (arteries with an additional muscular layer) are opened and the throttle veins (veins whose tunica media contains a muscle layer of helically arranged smooth muscle cells) are closed. Filling of the capacitance veins (widely convoluted venous lacunae) occurs at the same time as closure of the lumen of the lacrimal passage. In contrast, closure of the barrier arteries and opening of the throttle veins reduces the blood flow to the capacitance veins, simultaneously allowing blood outflow from these veins with resultant shrinkage of the cavernous body and dilatation of the lumen of the lacrimal passage [20]. Arteriove-nous anastomoses enable for direct blood flow between arteries and venous lacunae; thus, the subepi-thelially located capillary network can be avoided, and rapid filling of capacitance veins is possible when the shunts of the arteriovenous anastomoses are open. While regulating the blood flow, the specialized blood

Collagen Arrangement Fibril

Fig. 1.9. Fibrillar structures of the nasolacrimal ducts. a Scanning electron microscopic photograph of the outer surface of a nasolacrimal duct revealing the helical arrangement of collagen fibrils (arrows). Bar=550 ^m. b-d The arrangement of collagen and elastic fibers in a schematic view. If the nasolacrimal duct distends while blinking, it will be "wrung out" due to the screw-shaped arrangement of the collagen bundles. (From [10])

Fig. 1.9. Fibrillar structures of the nasolacrimal ducts. a Scanning electron microscopic photograph of the outer surface of a nasolacrimal duct revealing the helical arrangement of collagen fibrils (arrows). Bar=550 ^m. b-d The arrangement of collagen and elastic fibers in a schematic view. If the nasolacrimal duct distends while blinking, it will be "wrung out" due to the screw-shaped arrangement of the collagen bundles. (From [10])

vessels permit opening and closing of the lumen of the lacrimal passage, effected by the bulging and subsiding of the cavernous body, and, at the same time, regulate tear outflow [20].

The presence of the cavernous body is lacking in nearly all textbooks of anatomy (Fig. 1.1) and is therefore unknown to most nasolacrimal surgeons as well as radiologists; however, it is densely innervated [14]. Epiphora related to emotions such as sorrow or happiness occur not only by increased tear secretion from the lacrimal gland and accessory lacrimal glands, but also by closure of the lacrimal passage. This mechanism acts, for example, to provide protection against foreign bodies that have entered the conjunctival sac: Not only is tear fluid production increased, but tear outflow is also interrupted by the swelling of the cavernous body to flush out the foreign body and protect the efferent tear ducts themselves [2, 14, 20]. Moreover, it can be assumed that the valves in the lacrimal sac and nasolacrimal duct described in the past by Rosenmuller, Hanske, Aubaret, Beraud, Krause, and Taillefer could be caused by different swelling states of the cavernous body and must therefore be considered as speculation [20].

The cavernous body of the efferent tear ducts actually plays an important role in the physiology of tear outflow regulation and can be influenced pharmacologically (Fig. 1.10) [2]. Interestingly, administration of a decongestant drug or insertion of a foreign body at the ocular surface both prolong the tear transit time significantly, but by different mechanisms. Application of a decongestant drug simultaneously with insertion of a foreign body shortens the tear transit time significantly compared to the effect of the de-congestant drug alone, but there is no significant difference compared with application of a foreign body alone. The tear transit time is independent of the side (right or left) and gender, and whether the eyeglasses

Lacrimal Pump

Fig. 1.10. Schematic/anatomical model of the state of the cavernous body and lacrimal passage in the resting state (1) and under different experimental conditions (2-4) indicating the specific swelling and compression of the cavernous body and how it permits or restricts tear drainage. (From [19])

Fig. 1.10. Schematic/anatomical model of the state of the cavernous body and lacrimal passage in the resting state (1) and under different experimental conditions (2-4) indicating the specific swelling and compression of the cavernous body and how it permits or restricts tear drainage. (From [19])

are worn or not, or whether the person is suffering from a common cold or not [2].

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  • fnan
    What is the mechanism for the flowing of tears?
    2 years ago
  • sophie
    What are the mechanism of tearing.?
    2 years ago

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