The document discusses aqueous humor, which is a fluid continuously secreted and drained in the eye. It is formed from plasma by epithelial cells in the ciliary processes and functions to deliver nutrients to the eye and maintain intraocular pressure. The rate of aqueous humor formation is normally around 2-3 μL/min. It is secreted into the posterior chamber and drained through the anterior chamber angle. Measurement of aqueous humor formation can be done using fluorophotometry with systemic or topical fluorescein administration.

2. Before We Start…..
• What is Aqueous humor ?
• What are the main
functions of Aqueous
humor?
• What are the methods that
used in measurement of
the rate of aqueous humor
formation ?
• What are the pathways of
Aqueous outflow ?
• What happened in the
absence of aqueous
circulation?

3. Introduction
• The aqueous humor is a transparent, colorless
solution continuously formed from plasma by the
epithelial cells of the ciliary processes.
• It is secreted into the posterior chamber, passes
from the posterior chamber through the pupil
into the anterior chamber, and is drained at the
anterior chamber angle.

4. Introduction
• Aqueous delivers oxygen and nutrients to, and
removes waste products from the posterior
cornea, lens, and perhaps the anterior vitreous.
• Continuous formation and drainage of the
aqueous helps maintain the IOP.
• The aqueous maintains a transparent and
colorless medium of lower refractive index
between the posterior cornea and the lens.

5. Circulation of the aqueous in the anterior chamber occurs via hydrostatic phenomena,
including mechanical forces caused by eyeball and head movements, thermal
currents resulting from the temperature differential between the warmer vascular iris
and the cooler avascular cornea, and the pressure gradients between the posterior
chamber, anterior chamber, and episcleral veins.

6. Outlines:
1. Anatomy of the Ciliary Body.
2. Physiology of Aqueous humor formation and secretion.
3. Biochemistry of aqueous humor formation.
4. Factors affecting Aqueous formation
5. Aqueous humor composition.
6. Secondary or plasmoid Aqueous.
7. Methods of measuring rate of Aqueous formation.
8. Aqueous outflow Pathways.
9. Measurement of Outflow Facility.

8. The pars plicata
exhibits 70 to 80 villus-
like structures on its
inner surface that are
arranged around the
circumference of the
crystalline lens( ciliary
process).
The processes are greatly convoluted, and divided into categories
of major and minor processes based on their relative height.
Major processes predominate, and they extend into the
posterior chamber of the eye approximately 1 mm.

10. Scanning electron micrograph of the inner surface of the iris and ciliary body
with the lens and zonules removed. The pars plicata region is distinguished
from the more posterior pars plana region (asterisk) by the presence of
major (M) and minor (m) ciliary processes.

12. The Ciliary Body
• Ciliary epithelium:
1. Non pigmented epithelium (NPE)
2. Pigmented epithelium
• Ciliary body stroma : This connective tissue matrix
extends into the core of each ciliary process.
• Ciliary Muscle.
• Supraciliary lamina: is the outermost layer of ciliary
body which lies adjacent to the sclera. acts as a potential
space. Thus it helps aqueous humor to exit by the
unconventional pathway.

16. Blood–Aqueous Barrier

17. Keep in your mind….
• To provide anatomic correlations for the physiology of
aqueous humor formation, it is most convenient to
describe the process in two steps:
1. elaboration of a plasma filtrate from which
aqueous humor is derived.
2. formation of aqueous humor from this filtrate.
• Although these steps are not independent, the first is
related primarily to the ciliary body
microvasculature and the second to the ciliary
epithelium.

18. The microvasculature of the ciliary processes arising from the short radial ciliary
arteries (arise from MAC)
The anterior arterioles supply the large diameter capillaries, whereas the posterior
arterioles supply the smaller caliber capillaries deep within each process.
Each short radial ciliary artery has many branches, providing an extensive capillary
network
The system of ciliary process venules in turn drains mainly through the vortex
system of the choroid.

19. The capillaries of the ciliary processes are large, thin-walled, and highly
fenestrated. Thus, the capillary network of the ciliary processes provides a
large surface area of highly permeable vessels that readily leak fluids, ions,
and plasma proteins to provide the reservoir from which the ciliary epithelium
secretes aqueous humor.

20. Clinical Note….
• Scanning electron microscopic studies have revealed the
presence of localized constrictions in casts of afferent
arterioles that may reflect the presence of a sphincter-
like system for controlling blood flow.
• Through these mechanisms, alterations in blood
(under autonomic control) can influence aqueous
humor production by increasing or decreasing the
amount of filtrate made available to the ciliary epithelium.

21. Electron micrograph of the ciliary epithelium .The posterior chamber is at the top and
the ciliary body stroma, with its fenestrated capillaries (C), is at the bottom.
It appears that it is primarily the epithelia at the tips of the ciliary processes
that are involved in the production of aqueous humor.
In these areas, immunoelectron microscopic studies have clearly documented both
Na-K-ATPase activity and carbonic anhydrase activity.

24. a step-wise dissection
beginning from just beneath
the sclera and as seen in
meridional sections. The
longitudinal bundle (1) is seen
originating from epichoroidal
stars (f) that attach the muscle
to the inner surface of the
sclera and continue forward to
the scleral spur (d). Its tendons
continue beyond into the
trabecular meshwork (a).
Beneath the longitudinal
bundle, the bifurcating fibers
(g) of the radial bundle (2) are
seen, and, finally, the circular
bundle (3) is evident.

25. Keep in your mind….
• Contraction of the ciliary muscle, especially the
longitudinal fibers appears to pull on the scleral
spur. Doing so results in improved outflow of
aqueous humor through the trabecular
meshwork.
• The interstices of Ciliary muscle represent a part
of an increasing important alternate pathway for
the drainage of aqueous humor (Uveoscleral
Pathway).

27. Aqueous Humor Dynamics

29. Aqueous Humor Production
• Aqueous humor is produced by the ciliary processes
at an average rate of 2–3 μL/min.
• The ciliary body contains approximately 80 ciliary
processes, each of which is composed of a double
layer of epithelium over a core of stroma and a rich
supply of fenestrated capillaries.
• The inner nonpigmented epithelial cells, which
protrude into the posterior chamber, contain
numerous mitochondria and microvilli; these cells are
thought to be the actual site of aqueous production.

30. Keep in your mind…
The entire volume of the aqueous
humor is replaced every 90 to
100 minutes.

31. Aqueous Humor Production
• Until the early twentieth century, aqueous humor
was regarded as a stagnant fluid.
• Since that time, however, it has been shown to be
continuously formed and drained and the
associated anatomic drainage portals (Schlemm’s
canal, collector channels, aqueous veins, ciliary
muscle interstices) have been described.

32. Seidel's Procedure
• A cannula connected to a reservoir of indigo carmine dye
was inserted into the anterior chamber of the rabbit eye.
The reservoir was raised, thus creating a pressure of 15
mm Hg, and the dye was seen to enter the anterior
chamber and subsequently the episcleral veins. From this,
it was concluded that aqueous humor is continuously
formed and drained, and it is to a large extent from this
historic work that the modern study of aqueous humor
dynamics has developed.
• Seidël E: Weitre experimentelle Untersuchungen über die Quelle und den
Verlauf der introkulären Saftströmung. IX. Uber der Abfluss des
Kammerwassers aus der vorderen Augenkammer. Graefe's Arch Clin Exp
Ophthalmol 104:357, 1921

33. History…..
• Boerhaave first described the presence of the aqueous veins.
• Ascher observed a clear fluid in veins of the episclera and
demonstrated by means of external compression with a glass
rod that these veins were interconnected with veins containing
blood.
• Goldmann demonstrated that these vessels contained
aqueous humor by injecting fluorescein intravenously and
observing the dye entering the anterior chamber and
subsequently the aqueous veins.
• Ashton identified an aqueous vein in a living human eye, and
postmortem examination using a neoprene cast showed that
there was a direct passage between the vessel and Schlemm's
canal.

34. PHYSIOLOGY
• Three physiologic processes contribute to
the formation and chemical composition of the
aqueous humor:
1. Diffusion. 10 %
2. Ultrafiltration (and related dialysis). 20%
3. Active secretion. 70%

36. If a solution of protein and salt is separated from
either water or a less concentrated salt solution
by a membrane permeable to the salt and water
but not to the protein, then there will be a net
movement of water to the protein side by
diffusion, and a movement of salt away from the
protein side. The protein, of course, cannot
move across the membrane. This process is
called dialysis.
Ultrafiltration is similar to dialysis, but with the
addition of a hydrostatic pressure that increases
the rate of net movement of water and salt
molecules across the semipermeable membrane.
Ultrafiltration describe the bulk flow of blood
plasma across the fenestrated ciliary capillary
endothelia into the ciliary stroma, which can be
increased by augmentation of the hydrostatic
driving force.

37. However….
• The ultrafiltration component of aqueous humor
formation is sensitive to changes in IOP, decreasing
with increasing IOP.
• This phenomenon is quantifiable and is termed
facility of inflow or pseudofacility (Cps) =
0.06 μl × min × mmHg.
• Increased pseudofacility provides some protection
against a precipitous rise in IOP; as IOP rises, aqueous
inflow by ultrafiltration is partly suppressed, blunting
(but not completely suppressing) further IOP
elevation.

38. Active secretion
• Active secretion requires energy, normally
provided by the hydrolysis of (ATP). The energy is
used to move sodium, chloride, bicarbonate, and
other ions, against a concentration gradient.
• Active secretion is accounting for 80% to 90% of
total aqueous humor formation.
• Active secretion is essentially pressure-insensitive
at near-physiologic (IOP).

39. Steps of Aqueous formation
1. Formation of stromal pool.
2. Active transport of stromal filtrate.
3. Passive transport across non-pigmented
ciliary epithelium.

40. Active transport of stromal filtrate
• Selective transport of certain ions and substances
across the basolateral membrane of the NPE
against a concentration gradient.
• Two enzymes abundantly present in the NPE are
intimately involved in this process:
1. (Na+ -K+ -ATPase)
2. Carbonic anhydrase (CA).

42. Na+ -K+ -ATPase
• Na+ -K+ -ATPase provides the energy for the
metabolic pump, which transports sodium into
the posterior chamber, by catalyzing the reaction
ATP → ADP + Pi + energy.
• As a result of the primary active transport of Na+,
other ions and molecules are transported over the
epithelium by secondary active transport.
• Thus, aqueous humor in humans exhibits increased
levels of ascorbate, some amino acids, and certain
ions such as CI− as compared to plasma.
• There is also a passive transporter for HCO3.

43. Na+ -K+ -ATPase
• The primary active transport of Na+ is the primary
driving force for the secretion of aqueous humor. To
maintain electroneutrality, anions must accompany the
actively secreted Na+.
• Chloride can pass through chloride channels in the
basolateral membrane and HCO3 can enter aqueous via
exchange with chloride.
• The active transport of Na+ and the accompanying anions
create high osmolarity on the basolateral side of the NPE
cells, which causes diffusion of water out of the cells.
The movement of water is facilitated by aquaporins in NPE
cells (aquaporins 1 and 4).

44. Carbonic anhydrase
• Carbonic anhydrase (CA) is abundantly present in
the basal and lateral membranes and cytoplasm of
the pigmented epithelium and non-pigmented
epithelium of the ciliary processes.
• Isoenzymes of CA (II, IV and XII) are present in
the ciliary processes.
• The conversion of CO2 and H2O to carbonic acid
and its subsequent dissociation to H+ and HCO3
provides the HCO3, which is essential for the active
secretion of aqueous humor.

45. Reabsorption
• Sodium and chloride must continuously enter
the pigmented epithelial cells for the continuous
secretion of aqueous humor.
• This is achieved by:
1. Na+/H+ antiport
2. Cl−/HCO3 antiport
3. Na-K-2 Cl cotransporter.

47. Clinical Notes
1. Reduction in intracellular pH inhibiting Na-K-
ATPase.
2. Decreased availability of H+ decreasing
H+/Na+ exchange and reducing the availability
of intracellular Na+ for transport into the
intercellular channel.
3. Inhibition of renal and erythrocyte CA leads to a
systemic acidosis which promotes inhibition of
aqueous humor formation.

49. Aqueous humor composition
• The greatest differences are the low protein
and high ascorbate concentrations in the
aqueous relative to plasma (200 times less and
20 times greater, respectively).
• Lactate is also normally in excess in the
aqueous, presumably as a result of glycolytic
activity of the lens, cornea, and other ocular
structures.

50. Aqueous humor composition
• Other compounds or ions in excess in the
aqueous relative to plasma are Cl− and certain
amino acids.
• Glucose, urea, and non-protein nitrogen
concentrations are slightly less than in plasma.
• Oxygen is also present in the aqueous humor, at
a tension determined to lie between 13 to 80
mmHg, depending upon the method of
measurement.

51. Aqueous humor composition
• Other components of aqueous humor include growth
factors, lysozyme, diamine oxidase, plasminogen
activator, dopamine β-hydroxylase, and
phospholipase A2; and prostaglandins, cyclic
adenosine monophosphate, catecholamines, steroid
hormones, and hyaluronic acid.
• Aqueous humor composition is altered as it flows
from the posterior chamber, through the pupil, and
into the anterior chamber.
• This alteration is secondary to other dilutional
exchanges and active processes.

52. Clinical Note…
• When the aqueous protein concentration
rises much above its normal 20 mg/100
mL, as in uveitis, the resultant light
scattering (Tyndall effect) makes the slit-
lamp beam visible as it traverses the
anterior chamber (a phenomenon known
as “flare”).

53. Secondary or plasmoid Aqueous
• After breakdown of the blood–aqueous barrier, the
resultant aqueous produced is known as secondary or
plasmoid aqueous.
• The most notable change is a marked increase in protein
concentration. In this situation, the ionic composition of
the aqueous approaches that of a simple dialysate of
plasma, and substances that are normally barred from
entering the aqueous now do so with ease.
• The unusually rapid rate of entry of substances such as
fluorescein, Evan's blue dye, albumin, or fibrinogen can
be used as a diagnostic indicator of barrier breakdown.

54. (Courtesy of RL Stamper, MD)
• Traumatic
▫ Mechanical
 Paracentesis
 Corneal abrasion
 Blunt trauma
 Intraocular surgery
▫ Physical
 X-ray
 Nuclear radiation
▫ Chemical
 Alkali
 Irritants (e.g., nitrogen mustard)
• Pathophysiologic
▫ Vasodilation
 Histamine
 Sympathectomy
▫ Corneal and intraocular infections
▫ Intraocular inflammation
▫ Prostaglandins
• Pharmacologic
▫ Melanocyte-stimulating hormone
▫ Cholinergic drugs, especially cholinesterase inhibitors
▫ Plasma hyperosmolality

55. Measurement of Aqueous Formation
• The most common method used to measure the
rate of aqueous formation is fluorophotometry.
• For this test, fluorescein is administered
systemically or topically, its gradual dilution in the
anterior chamber is measured optically, and
change in fluorescein concentration over time is
then used to calculate aqueous flow.
• The normal flow is approximately 2–3 μL/min, and
the aqueous volume is turned over at a rate of
approximately 1% per minute.

56. Principles of measurement of aqueous flow by ocular fluorophotometry. A: Optical axis of eye is
scanned for background fluorescence with a scanning ocular fluorophotometer. B: Topical
application of drops of fluorophore (2% fluorescein) applied to cornea. C: After a suitable delay
(approximately 15 hours), to allow fluorescein to diffuse from the corneal depot to the aqueous
humor, the eye is scanned once again. D: Repeated scans at 30-minute to 1-hour intervals over a
3- to 6-hour period facilitate monitoring of decline in fluorescence of aqueous humor with time.

57. Mathematical derivation of Aqueous formation
rate
• This method depend on the following algebraic
manipulation of the modified Goldmann equation
describing IOP in terms of episcleral venous pressure
(Pe), aqueous flow (Fin), trabecular outflow facility
(Ctrab), and uveoscleral outflow (Fu):
IOP = Pe + ((Fin − Fu)/Ctrab))
thus
IOP − Pe = ((Fin − Fu)/Ctrab))
and
Fin − Fu = Ctrab (IOP − Pe)
therefore
Fin = Ctrab (IOP − Pe) + Fu

59. Aqueous Outflow
• Trabecular or conventional route (Pressure-Sensitive)
Through the TM, across the inner wall of Schlemm's
canal into its lumen, and then into collector channels,
aqueous veins, and the episcleral venous circulation.
• Uveoscleral or unconventional route (Pressure-
insensitive )
Across the iris root, uveal meshwork, and the anterior
face of the ciliary muscle, through the connective tissue
between the muscle bundles, the suprachoroidal space,
and out through the sclera.

63. JCT
• The juxtacanalicular (JCT) region is an open
connective tissue matrix in which fibroblast-like cells,
rather than endothelial cells, are found.
• Detailed ultrastructural studies have documented that
tendons from the longitudinal bundle of the
ciliary muscle extend into the meshwork, culminating
in a system of elastic fibers that connect to the inner
wall of Schlemm's canal, called the cribriform
plexus.
• It is within this connective tissue matrix of the JCT
region that most of the resistance to aqueous
outflow is held to reside, but the actual source of this
resistance has remained elusive.

64. Scanning electron micrograph showing the lumenal surface of the inner wall of
Schlemm's canal. Mounds represent giant vacuoles, a small number of which manifest
pores (arrowhead). A pore (arrowhead) is seen at higher magnification in the inset.

65. THE FLOW PATHWAYS BEYOND SCHLEMM'S CANAL
• Approximately 30 external collector channels lead
from the outer wall of Schlemm's canal toward the
surface of the sclera.
• From the external collector channels aqueous passes
into a tortuous system of passages called the deep
scleral plexus that lead in turn to the deep scleral
veins and finally to the episcleral veins .
• Through this tortuous route the aqueous and blood are
mixed. But a smaller number of unique vessels called
aqueous veins (of Ascher) bypass this tortuous
pathway and connect directly to the episcleral veins.

66. Keep in your mind….
• The trabecular outflow pathway is
dynamic. With increasing IOP, the cross-
sectional area of the Schlemm canal
decreases, while the trabecular meshwork
expands.

69. Measurement of Outflow Facility
• The facility of outflow (C in the Goldmann
equation) is the mathematical inverse of outflow
resistance and varies widely in normal eyes, with
mean value ranging from 0.22 to 0.30
μL/min/mm Hg.
• Outflow facility decreases with age and is affected by
surgery, trauma, medications, and endocrine factors.
• Patients with glaucoma and elevated IOP typically
have decreased outflow facility.

70. Tonography
• Tonography is a method used to measure the
facility of aqueous outflow. With this technique, a
weighted Schiøtz tonometer or pneumatonometer is
placed on the cornea, acutely elevating the IOP.
• Outflow facility in μL/min/mm Hg can be computed
from the rate at which the pressure declines with time,
reflecting the ease with which aqueous leaves the eye.
• In general, tonography is best used as a research tool
for investigating mechanisms of action of IOP changes
and is rarely used clinically.

73. Resources
• Duane's Clinical Ophthalmology - VOLUME 3 - Chapter 43 , Anatomy of the
Ciliary Body and Outflow Pathways. THOMAS F. FREDDO and HAIYAN
GONG
• Duane's Clinical Ophthalmology- VOLUME 3 -Chapter 45- Aqueous Humor
Dynamics . J. CAMERON MILLAR, B'ANN TRUE GABELT and PAUL L.
KAUFMAN
• Adler’s physiology of the eye – 11th edition – section 4- chapter 11 -
Production and Flow of Aqueous Humor.
• American academy of ophthalmology – 10th edition – chapter 2- Intraocular
Pressure and Aqueous Humor Dynamics.