Big fat blood flow

Main Choroidal Function

serve outer retina

Ophthalmic Artery Branches

central retinal artery, posterior ciliary arteries, anterior ciliary arteries

Central Retinal Artery

does not anastomose; blood cannot leak out, and-arterial system

Central Retina Capillaries (layers)

3–4 dense layers

Peripheral Retina Capillaries (layers)

one layer

Posterior Ciliary Artery

branches into 10–20 short and 2 long ciliary arteries

Short Posterior Ciliary Arteries

form choriocapillaris

Choroidal Drainage

via vortex veins, one per quadrant

Anterior Uvea Drainage

vortex veins + minor communication with anterior episcleral vessels

Retinal Artery Occlusion

causes irreparable retinal damage

Choroidal Artery Occlusion

destroys outer retina

Anterior Segment Occlusion

minimal damage due to anastomosis

Blood–Retinal Barrier

tight junctions of retinal capillary endothelium & RPE

O₂/CO₂/Water Permeability

high in retinal vascular beds

Continuous Capillaries

most impermeable; in retina and iris

Fenestrated Capillaries

porous membrane; in ciliary processes & choriocapillaris, not much leakage because hydrostatic and colloid pressures are balanced

Choriocapillaris Function

maintains high glucose in RPE; delivers vitamin A–binding proteins

BRB defect

Optic disc, low permeability of retinal capillaries prevents most leakage in this are

Choroidal & Ciliary Capillaries

fenestrated and very leaky

Systemic vs Pulmonary Circulation Pressure

~120 mmHg systemic, 22 mmHg pulmonary

Resistance comparison

Arterioles/capillaries > arteries > veins

Viscosity

resistance of flow, depends on hematocrit

RBCs, percentages

97% of cellular elements; deformable; 40–45% of blood volume

Hematocrit, percentages

% blood volume of cells (42% men, 38% women) higher= higher viscosity, blood viscosity= 3x water

Plasma Proteins

include albumin, globulins, fibrinogen

Albumin

creates oncotic pressure preventing plasma leakage

α & β Globulins

transport proteins

γ Globulin

antibody

Plasma Viscosity

1.5–2× water

Fahraeus–Lindquist Effect

in vessels <1.5 mm, viscosity decreases because RBC align single file

Low Velocity Effect

very low flow increases viscosity 10× due to cell adherence

Flow Equation

resistance= change in pressure/flow rate

Left ventricle/aortic flow

aorta pulsatile (laminar and turbulent qualities); turbulent at peak velocity

Small Arteries Flow

becomes laminar

Capillary Flow

no turbulence/viscosity effects; RBC single file

Venule/Arteriole Velocity

decreases locally, can occur in reduced perfusion pressure, increased viscosity, or increased venous pressure

Transmural Pressure

intravascular – extravascular pressure, walls of blood vessels exert a tension in response to this pressure

Passive Tone

tension from mechanical stretch of vessel wall

Autoregulation

maintains constant blood flow over a range of pressures

Arteries, arterioles

strong walls, low resistance, 15% of blood in systemic circulation, arterioles= control valve for capillaries, strong muscular walls that can dilate or constrict

Veins

low-pressure return pathway, thin walls, 64% of blood in systemic circulation because take up 4x more cross sectional area than arteries

Velocity vs Cross Section

inversely proportional

Aortic Pressure

100 mmHg (120/80)

Systemic Arterial Pressure

95–97 mmHg

Systemic Arteriolar Pressure

85→55 mmHg; highest resistance

Capillary Pressure

30→10 mmHg (venous end)

Systemic Venule Pressure

10→0 mmHg (right atrium)

Fluorescein Angiography

qualitative retinal/choroidal flow assessment under the assumption there is a constant relationship between fluorescence and fluorescein concentration

Fluorescein Angiography Process/Limits

Inject -> enters retinal and choroidal circulation (mean transit time= blood volume/blood flow), visualized with blue light (465 nm), fluorescence emission= 525 nm) early arterial phase- retinal artery fills first, arteriovenous phase- dye in veins, peak phase= when dye concentration in retina and choroid is maximum, not suitable to study choroidal circulation because bount to albumin and leaks from fenestrated choriocapillaris

ICG (Indocyanin green) Angiography, specific protein binding

ideal for choroid, not blocked by RPE because fluorescence near infrared region, completely bound to proteins/does not pass through walls of choriocapillaris, usually bound to globulin such as alpha lipoproteins, choroidal artery to vortex vein mean time= 5 sec

Laser Doppler Velocimetry

measures retinal blood velocity in large vessels, can be used to estimate blood volume

Retinal blood flow rate, mean retinal circulation time

35–80 μL/min, 4-5 seconds (retinal artery to retinal vein)

Choroid Flow Rate

200× retinal artery flow, require highest amount of oxygen to nourish retina and avascular fovea

Uveal Blood Flow Ranking

choroid > ciliary processes > ciliary muscle > iris

O₂ Extraction (retinal vein vs artery)

retinal vein O₂ is 38% lower than artery

Blood Flow Determinants

vascular pressure, neural tone, vasoactive substances, metabolic activity

Two Control Mechanisms

direct smooth muscle action; endothelial mediator release

Endothelin-1

vasoconstrictor peptide

Local Control of Capillary Flow

via myogenic tone (vascular muscles always contracted without innervation) & autoregulation

Myogenic Response

local control- stretch → dilation → pacemaker activation → constriction

Autoregulation

local control present in central retinal artery, not choroid

Metabolic Vasodilation

Extrinsic control of arterioles, ↓pO₂, ↑pCO₂, ↑K⁺, ↑H⁺, ↑adenosine

Parasympathetic Effects

ACH/VIP → vasodilation

Sympathetic Effects

Nor-epi → vasoconstriction, act directly on smooth muscle, seen in choroidal circulation

Perfusion Pressure

arterial pressure – venous pressure

Blood Flow Formula

= perfusion pressure (arterial pressure - venous pressure) / resistance

Ophthalmic Artery Pressure

~80 mmHg

Arteries Entering Eye Pressure

60–70 mmHg, increased laying down

Venous Perfusion Pressure

~50 mmHg (when IOP = 15)

Episcleral & Vortex Vein Pressure

7–8 mmHg, may increase with increased IOP

Scleral Passage Pressure Drop

5–10 mmHg

Intraocular Venous Congestion

from ↑ extraocular vein pressure

Transmural Pressure

intravascular – extravascular, small in intraocular veins and venous capillaries (around 2 mmHg above IOP)

Retinal Autoregulation Limit

~30 mmHg

High IOP Effects

retinal & ciliary body stable blood flow; choroid falls drastically

Choroidal Autoregulation

absent

Autoregulation Mechanisms

myogenic (variation in transmural pressure, stimulus) + metabolic (stimuli= accumulations of CO2 and hypoxia)

Lack of Choroidal Autoregulation

no pacemaker cells; weak CO₂ response

Choroidal Flow Drop

compensated by ↑O₂ extraction (for moderate IOP)

Chronic Hyperoxia

retinal oxygen tension increases and normalizes over time

Photoreceptor O₂, Glucose Usage (under different lighting conditions)

higher in dark, flickering light= increases glucose uptake by ganglion cell layer of inner retina