2. Cerebral Vasculature and Brain Homeostasis Flashcards
what is the amount and flow of CSF
how is it made
Thru the ventricles and around the brain & spinal cord ==> flow is one way and reabsorbed by arachnoid villi at the end
~150 mL at any given time & not recirculated –> so make ~550 mL/day
made primarily by choroid plexus (esp in lateral & 4th ventricle) & rest by tissue that lines the ventricles
How are the 4 ventricles connected
Intraventricular Foramen –> Lateral ventricles - 3rd ventricle
Cerebral Aquaduct–> 3rd ventricle - 4th ventricle
Median Aperture –> 4th ventricle - cisterna magna
Lateral Apertures –> 4th ventricle - arachnoid space
What happens if the ….
- intraventricular foramen
- cerebral aquaduct
- median aperture
- lateral aperture
are blocked?
CSF build up in…
- lateral ventricles
- 3rd ventricle
- 4th ventricle
- 4th ventricle
and the ones prior?*?*?*?*?*
What is stage 1 of making CSF
Step 1: Passive filtration of serum (Dependent on two pressures)
- Hydrostatic pressure (BP in capillaries = tissue hydrostatic P)
- Oncotic Pressure (cancel each other out)
–> fluid moves from blood vessel into ventricle
*RMR: production of CSF is constant over wide rances of ICPs*
What is hydrostatic pressure
- In capillary = blood pressure –> Pushes fluid out. = large force on capillaries (= net Pressure bc oncotic cancel out!)
- Surrounding the capillary = Tissue hydrostatic pressure –> Pushes fluid into capillary = small force on brain/body
what is oncotic pressure
= osmotic pressure (dep. on solutes in blood)
- In capillary => “pulls” fluid into the capillary.
- Surrounding capillary =>“pulls” fluid out of the capillary
- At the choroid plexus, these two are equal and opposite.
What is step 2 of formation of CSF
Modification of ion composition:
- HCO3, Cl, and K concentrations controlled by channels on epithelial cells. •Aquaporin 1 allows H2O to cross.
Does production of CSF fluctuate with pressure?
NO! its CONSTANT!!!
= so even if a lot of CSF or increased ICP –> STILL make more & get build up
what molecules/ions in the CSF will be in similar in concentration as plasma
- Na+ (range overlaps with average concentration in plasma
- HCO3
CSF contains more ________ than plasma
Mg++ (interact w/ channels in brain)
Cl-
CO2 (brain metabolic rate makes it ==> CSF is 1st step of getting it out and into blood)
Plasma contains higher concentrations of which molecule/ions, compared to CSF?
K+ (bc metabolic rate, dont need much bc cells push it out)
Ca++ (can be toxic for cells)
Protein (low oncotic P –> no buffer for H+ ==> pH diff from plasma as well)
Glucose
How does absorption by arachnoid villi occur
endothelium of sinus & membrane on villi are fused => bulk flow of CSF (one way flow)
absorption proportional to ICP
-at ICP < 68 mm - no absorptions ==> BUILD UP
at normal pressure ~ 112 mm = normal absorption
*control of amount of CSF based on reabsorption NOT production*
What will occur if the arachnoid villi fibrose
decrease reabsorption –> increase pressure –> damage neurons!
primary role of CSF
protect brain!
brain weighs: 1400 g in air BUT w/ CSF it weighs 50 g
(bc high lipid concentration)
=bouyancy & protection
What is the blood brain barrier (BBB) and what is its fxn?
Capillaries that limit exchange bc tight jxn btn endothelial cells & Glial endfeet in contact with BVs (add lipid bilayers- restrict things coming)
Fxn = protect chemical composition of CSF, maintain electrolyte composition (esp K+ & Vm), protect from toxins
