Internal carotid artery and External carotid artery
The internal carotid artery supplies
Structures of the cranium and Eye and related structures
(supplies blood to the eye)
The external carotid artery supplies:
Superficial areas of the head and neck
Small portion of the circulation to the ocular adnexa
Internal carotid artery- 3 things
1- Enters the skull through the carotid canal
2- Then immediatly enters the cavernous sinus
3- As it emerges, it branches to the opthalmic artery (fist major branch)
The first branch of the Internal Carotid Artery:
The Opthalmic Artery
Opthalmic Artery - 4 things
1- Enters the orbit within the dural sheath of the optic nerve
2- Passes through the optic canal (below and lateral to the nerve)
3- In the orbit it runs inferolateral to the optic nerve for a short distance
4- Then crosses either above or below the nerve (20% below)
9 Opthalmic Artery Branches
1- Central Retinal Artery
2- Lacrimal Artery
3- Ciliary Arteries - 2
4- Ethmoid arteries - 2
5- Supraorbital Artery
6- Muscular Arteries - 2
7- Sup and Inf Medial Palpebral arteries
8- Supratrochlear artery
9- Dorsonasal artery
Central Retinal Artery - 4 things
1- The smallest branch
2- Leaves the opthalmic artery as it lies below the optic nerve
3- Enters the meningeal sheath of the nerve about 10-12 mm behind the globe
4- Supplies the inner retina
Central Retina Artery info
In the optic nerve, the CRA provides the collateral branches ( to the nerve and pia mater)
The CRA passes through the lamina cribose and enters the optic disk nasal to center branching into the Superior and Inferior.
The S and I branches divide further into nasal and temporal branches
These vessels continue to bifurcate
2 capillary networks are formed, the deep capillary plexus and the Superficial capillary plexus
No blood vessels are found in the macula or fovea
Orgion of Posterior Ciliary Arteries
The Opthalmic Artery from the Internal Carotid Artery
Posterior Ciliary Artery Branches
Short Ciliary Arteries (10-20)
Long Ciliary Arteries (2)
Short Posterior ciliary arteries info:
Arise as one or two branches, then form 10-20 branches
Enter the scelera in a ring around the optic nerve and from the arterial network within the choroidal stroma
Divide to form posterior choriocapillaris
Other branches form the Circle of Zinn
Supplies the Optic nerve of the head (external part of retina)
Job of posterior choriocapillaries
Nourishes retina as far anteriorly as equator
Circle of Zinn
Encircles the optic nerve at the choriod
Where does almost the entire blood supply of the eye (minus the inner retina) come from:
The uveal vessels
Long Posterior Cilliary Arteries
One enters nasally one temporally
Near the optic nerve
Give 3-5 branches at the ora serrata and passes directly back to form the anterior choriocapillaris.
These capillaries nourish retina from the equator forward.
Run between the sclera and choroid anterior globe.
Enter the ciliary body and branch superiorly and inferiorly
Anastomose with each other and with the anterior ciliary arteries and form the major arterial circle of the iris.
Major arterial circle of the iris
Located in the ciliary stroma
Is the source of the radial vessels found in the iris
Long Anterior Ciliary Arteries
Branch from the vessels that supply the rectus muscles
Exit the muscles near the insertions, run along the tendons
Then loop inward to pierce the sclera (just outer to the limbus)
Before entering the scelera, they send branches into the conjunctiva, forming a network of vessels in the limbal conjunctiva.
Other branches enter the episcelera to form a network of vessels before entering the uvea, then enter the ciliary body and anastomose with the branches of the long posterior ciliary arteries (major circle of the iris)
Generally, 2 emanate from each of the rectus muscles, wih exception of the lateral muscle (only 1)
Anterior Ciliary Arteries supply blood to
The conjunctiva, episclera, ciliary body, and iris
Venous drainage by the Radial episcleral collecting veins
Receive blood from the anterior conjuntiva, limbal archades, anterior episcleral veins, and perforating scleral veins.
Then form the anterior ciliary veins (leave the globe anteriorly over the rectus muscles)
Vortex veins drainage
Posteriorly 4-7 vortex veins drain the venous system ( the choriod, cilary body, and iris) into the superior and inferior opthalmic veins
The Central Retinal Vein:
33% larger then CRA
Drains into the superior opthalmic vein and/or the cavernous sinus
Blood flow through the blood vessel depends on:
The perfusion pressure (PP), the pressure that drives blood through the vessel, and the resistance (R) generated by the vessels.
Ocular pressure can be reduced by either:
Reduction of arterial pressure or increase in intraocular pressure.
That property of a vascular bed that permits constant or nearly constant blood flow throughout a wide range of perfusion pressures
Blood flow in the retina appears to be controlled by:
Primary metabolic needs, especially the need for oxygen
When O2 levels are low it leads to:
More blood production
When there is less O2, what happens
When there is too much O2
Ones that maintain the body in rest
Used to make body aware for emergencies
- Lowers the blood flow
Blood flow formula
P arteries (intering tissue) - P veins (leaving tissue)/ Resistance
What is the mean arterial pressure of the arteries entering the eye?
What is the pressure of the episcleral veins leaving the eye?
The numeric value that indicates how easily blood can pass through a tissue.
Is approx 50 mmHg (diff between A and veins)
What value is being used in glaucoma research?
- Research is showing that glaucoma patients with low OPP are 1.5 x more likely to develop progressive nerve damage from ischemia.
Ocular perfusion pressure
Diastolic bp-intraocular bp
Acute angle closure attack
High IOP will infringe on CRA which leads to:
reduction in blood supply to the retinal tissue and:
retinal vessels increase their vessel diameter to allow more blood flow (autoregulation).
If IOP is high enough for a long time:
CRA will obtain its critical closing pressure and shut down.
Pressure at which a blood vessel collapses and blood flow stops:
Critical closing pressure
What happens in acute closure attack?
IOP increases- which reduces the artieral pressure entering the retina
If IOP remains elevated for a long enough perior of time:
CRAO occurs, PP decreases = hypoxia retina
What is the primary threat to vision loss in an acute angle closure attack?
CRAO- Central Retinal Artery Occlusion
Pressure in the ocular veins (outside the eye) is lower than what?
To drain into the trabecular meshwork and into the venous system.
IOP is lower then what?
The perfusion pressure of the retinal and uveal arteries.
Allows nutrients to be delivered from the choriocapillaris to the retina.
Capillaries that contain pores that span the endothelial lining,
These pores permit the rapid exchange of water and solutes
The majority of blood flow in the ocular vessels is always in the choriocapillaris (60%)
Capillaries have huge fenestration- they provide the outer retina with oxygen, glucose, vitamen A, etc.
Has Fenestrated capillaries
Iris and renal capillaries are non-fenestrated
Arise either from the vessels entering the choriod or from the Circle of Zinn.
Arise from the ciliary circulaton and not from the retinal supply
Occur 15-50% of the population
Supply the macular area
ORA direct blood supply to the macular area will be maintained in those individuals with such a cilioretinal artery
Retinal blood flow has what 2 things?
Dual blood supply
Blood retinal barrier
Dual blood supply
Central retinal artery= inner retina
Choriocapilaris (choriod)= outer retina
Blood Retinal Barrier
Barrier formed by tight junctions between endothelial cells lining the retinal vessels as well as between RPE cells.
What does keeping the blood of the choriocapillaris out of the retina do?
Prevents damage cause blood is toxic to the retina
Retinal blood supply
Vessel walls are transparent (colomn of blood within the vessel can be seen)
Lighter colored blood is the oxygenated blood of the artery.
Venous deoxyenated blood is slightly darker.
The artery generally lies superficial to the vein
What predispose thrombus formation in the CRV?
This anatomical position predisposes to thrombus formation in the CRV by various factors, including slowing the blood stream, changes in the vessel wall, and changes in the blood.
Inflammations in the Anterior Ciliary Artery generate:
An increase of the blood supply to the affected areas, causing hyperemia.
Anterior Ciliary Artery Inflammations
The superficial blood vessels are injected, giving the conjunctiva a bright red color that often increases toward the fornix.
The vessels move with conjunctivial movement and can be blanched with a topical vasoconstrictor.
The deeper scleral and episcleral vessels are injected, giving the circumlimbial area a pirplish or rose-pink color
These vessels fo not move with the conjunctiva and are not blanched with a topical vasoconstrictor.
ANS system control
By Sympathetic and Parasympathetic Innervation
Vasoconstriction of the blood vessels
Maintaining a resonable blood flow through the uvea during sudden spikes in blood pressure.
Helps autoregulation to maintain the intraocular flow and constant volume.
Vasodilation of the blood vessels
Maitains high ocular blood flow during loss of systemic blood pressure.
Mediates increase blood flow in response of increase retinal activity.
The Cranial Nerves
The nucleus are found in the brain stem, except 1st and 2nd pair.
Some are Sensoral, some Motor, some Both
The Brain stem
The midbrain, the pons, and the medulla oblongata
Lies over the basilar portion of the occipital bone, with slight inclination