Test 115  (Ocular physiology)

1. With regard to the choroid: 

a. the choroid receives 85% of blood flow to the eye

b. the blood vessels in the choroid contains tight junction

c. the choroid vessels are embedded in a matrix made up
    of type III collagen

d. in the presence of high partial pressure of carbon 
   dioxide the choroidal vessels increase in diameter

e. more than 50% of oxygen is extracted from blood 
    passing through the choroid

Answera.T  b.F  c.T  d.T  e.F.
The blood vessels in the choroid are highly fenestrated. Only 5 to 10% of oxygen is extracted from the blood passing through the choroid.
 

2. True statements about the pupil include:

a. mydriasis decreases optical aberration

b. it is controlled mainly by the autonomic system

c. miosis increases the depth of focus for near vision

d. a change in the pupil diameter from 2 to 8 mm 
    increases the amount of light entering the eye 
    by 16-fold

e. miosis occurs when the short ciliary nerve is stimulated

Answera.F   b.T   c.T  d.T  e.T
Miosis decreases the optical aberration. The short ciliary nerve contains parasympathetic nerve that supplies the sphincter pupillae and is involved in the light reflex.

 
3. The following are true about iris:

a. it receives 5% of total blood flow

b. it contains blood vessels with radial coils

c. it contains fenestration in the blood vessels

d. the sympathetic activity in the iris dilator muscles  
    is mainly mediated by b receptors

e. the dilator muscles of the iris are continuous with the 
    outer layer of ciliary body pigmented epithelial cells

Answera.T   b.T  c.F  d.F  e.T
The iris vessels have tight junctions and lack fenestration. The sympathetic activity in the iris dilator muscles is mediated mainly by alpha receptors.
.

4. True statements about the retinal blood flow include:

a. the retina receives 5% of total ocular blood flow

b. the retinal blood flow is mainly under the sympathetic
    control

c. the retina blood vessels are impermeable to ascorbate

d. the pericytes control the contractile activity of the retinal 
    blood vessels 

e. the flow rate in the retinal blood vessels is about 
   150mm/s

Answera.T  b.F  c.F  d.T  e.F
Retinal blood flow is mainly autoregulated. The blood vessels contain tight junctions but smaller molecules such as glucose and ascorbate can penetrate them by facilitated diffusion. The retinal circulation has a low flow rate of 25 mm/s and high oxygen exchange whereas choroidal circulation has a high flow rate of 150mm/s but low oxygen exchange.

5. The following are true about the aqueous:

a. the protein content of aqueous is about 1/5 that of 
    the plasma

b. the main type of protein found in the aqueous is 
    transferrin 

c. Ig G is found in the aqueous

d. stimulation of a2 receptors reduces aqueous 
    production

e. stimulation of b2 receptors of the ciliary body increases 
    aqueous secretion

Answera.F   b.F  c.T  d.T  e.T
The protein content within the aqueous is 1/500 that of the plasma. The main protein in aqueous is albumin.

 
 

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