Aqueous humour

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Aqueous humour_table_infobox_0

Aqueous humourAqueous humour_header_cell_0_0_0
DetailsAqueous humour_header_cell_0_1_0
IdentifiersAqueous humour_header_cell_0_2_0
LatinAqueous humour_header_cell_0_3_0 humor aquosusAqueous humour_cell_0_3_1
MeSHAqueous humour_header_cell_0_4_0 Aqueous humour_cell_0_4_1
TA98Aqueous humour_header_cell_0_5_0 Aqueous humour_cell_0_5_1
TA2Aqueous humour_header_cell_0_6_0 Aqueous humour_cell_0_6_1
FMAAqueous humour_header_cell_0_7_0 Aqueous humour_cell_0_7_1

The aqueous humour is a transparent watery fluid similar to plasma, but containing low protein concentrations. Aqueous humour_sentence_0

It is secreted from the ciliary body, a structure supporting the lens. Aqueous humour_sentence_1

It fills both the anterior and the posterior chambers of the eye, and is not to be confused with the vitreous humour, which is located in the space between the lens and the retina, also known as the posterior cavity or vitreous chamber. Aqueous humour_sentence_2

Structure Aqueous humour_section_0

Composition Aqueous humour_section_1

Aqueous humour_unordered_list_0

  • Amino acids: transported by ciliary musclesAqueous humour_item_0_0
  • 98% waterAqueous humour_item_0_1
  • Electrolytes (pH = 7.4 -one source gives 7.1)Aqueous humour_item_0_2
    • Sodium = 142.09Aqueous humour_item_0_3
    • Potassium = 2.2 - 4.0Aqueous humour_item_0_4
    • Calcium = 1.8Aqueous humour_item_0_5
    • Magnesium = 1.1Aqueous humour_item_0_6
    • Chloride = 131.6Aqueous humour_item_0_7
    • HCO3- = 20.15Aqueous humour_item_0_8
    • Phosphate = 0.62Aqueous humour_item_0_9
    • Osm = 304Aqueous humour_item_0_10
  • Ascorbic acidAqueous humour_item_0_11
  • GlutathioneAqueous humour_item_0_12
  • ImmunoglobulinsAqueous humour_item_0_13

Function Aqueous humour_section_2

Aqueous humour_unordered_list_1

  • Maintains the intraocular pressure and inflates the globe of the eye. It is this hydrostatic pressure which keeps the eyeball in a roughly spherical shape and keeps the walls of the eyeball taut.Aqueous humour_item_1_14
  • Provides nutrition (e.g. amino acids and glucose) for the avascular ocular tissues; posterior cornea, trabecular meshwork, lens, and anterior vitreous.Aqueous humour_item_1_15
  • May serve to transport ascorbate in the anterior segment to act as an antioxidant agent.Aqueous humour_item_1_16
  • Presence of immunoglobulins indicate a role in immune response to defend against pathogens.Aqueous humour_item_1_17
  • Provides inflation for expansion of the cornea and thus increased protection against dust, wind, pollen grains and some pathogens.Aqueous humour_item_1_18
  • For refractive index.Aqueous humour_item_1_19
  • Prevents eye drynessAqueous humour_item_1_20

Production Aqueous humour_section_3

Aqueous humour is secreted into the posterior chamber by the ciliary body, specifically the non-pigmented epithelium of the ciliary body (pars plicata). Aqueous humour_sentence_3

5 alpha-dihydrocortisol, an enzyme inhibited by 5-alpha reductase inhibitors, may be involved in production of aqueous humour. Aqueous humour_sentence_4

Drainage Aqueous humour_section_4

Aqueous humour is continually produced by the ciliary processes and this rate of production must be balanced by an equal rate of aqueous humour drainage. Aqueous humour_sentence_5

Small variations in the production or outflow of aqueous humour will have a large influence on the intraocular pressure. Aqueous humour_sentence_6

The drainage route for aqueous humour flow is first through the posterior chamber, then the narrow space between the posterior iris and the anterior lens (contributes to small resistance), through the pupil to enter the anterior chamber. Aqueous humour_sentence_7

From there, the aqueous humour exits the eye through the trabecular meshwork into Schlemm's canal (a channel at the limbus, i.e., the joining point of the cornea and sclera, which encircles the cornea) It flows through 25–30 collector canals into the episcleral veins. Aqueous humour_sentence_8

The greatest resistance to aqueous flow is provided by the trabecular meshwork (esp. Aqueous humour_sentence_9

the juxtacanalicular part), and this is where most of the aqueous outflow occurs. Aqueous humour_sentence_10

The internal wall of the canal is very delicate and allows the fluid to filter due to high pressure of the fluid within the eye. Aqueous humour_sentence_11

The secondary route is the uveoscleral drainage, and is independent of the intraocular pressure, the aqueous flows through here, but to a lesser extent than through the trabecular meshwork (approx. Aqueous humour_sentence_12

10% of the total drainage whereas by trabecular meshwork 90% of the total drainage). Aqueous humour_sentence_13

The fluid is normally 15 mmHg (0.6 inHg) above atmospheric pressure, so when a syringe is injected the fluid flows easily. Aqueous humour_sentence_14

If the fluid is leaking, the hardness of the normal eye is compromised, leading to collapse and wilting of the cornea. Aqueous humour_sentence_15

Clinical significance Aqueous humour_section_5

Main article: Glaucoma Aqueous humour_sentence_16

Glaucoma is a progressive optic neuropathy where retinal ganglion cells and their axons die causing a corresponding visual field defect. Aqueous humour_sentence_17

An important risk factor is increased intraocular pressure (pressure within the eye) either through increased production or decreased outflow of aqueous humour. Aqueous humour_sentence_18

Increased resistance to outflow of aqueous humour may occur due to an abnormal trabecular meshwork or due to obliteration of the meshwork resulting from injury or disease of the iris. Aqueous humour_sentence_19

However, increased interocular pressure is neither sufficient nor necessary for development of primary open angle glaucoma, although it is a major risk factor. Aqueous humour_sentence_20

Uncontrolled glaucoma typically leads to visual field loss and ultimately blindness. Aqueous humour_sentence_21

Uveoscleral outflow of aqueous humour can be increased with prostaglandin agonists, while trabecular outflow is increased by M3 agonists. Aqueous humour_sentence_22

Fluid production can be decreased by beta blockers, alpha2-agonists, and carbonic anhydrase inhibitors. Aqueous humour_sentence_23

Additional Images Aqueous humour_section_6

Aqueous humour_unordered_list_2

  • Aqueous humour_item_2_21
  • Aqueous humour_item_2_22
  • Aqueous humour_item_2_23

See also Aqueous humour_section_7

Aqueous humour_unordered_list_3

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: humour.