Cerebral Vasculature Flashcards
What percentage of blood pumped from the heart every minute does the brain receive?
14%
Does blood mix in the circle of willis?
NO; very little mixing means that if one artery is occluded, only the part of the brain that it supplies will be affected! (localized ischemia)
What is the effect of parasympathetics on cerebral circulation?
Vasodilation!
What are some neurotransmitters that parasympathetics use on cerebral circulation?
Ach, Vasoactive intestinal polypeptide (VIP), PHM-27
What do parasympathetics innervate for cerebral blood flow?
Larger blood vessels
What is the effect of sympathetics on cerebral circulation?
Vasoconstriction
When are sympathetics activated for cerebral circulation?
ONLY when blood pressure and cardiac output are high
What neurotransmitters does the sympathetic system use for cerebral circulation?
Norepinephrine and neuropeptide Y (NPY)
What receptors does the sympathetic system use for cerebral circulation?
Alpha 1 adrenergic
What is the result of the sensory innervation of distal blood vessels?
Vasodilation and PAIN (sensitive to torsion/manipulation)
What are some neurotransmitters used by sensory innervation of distal blood vessels?
Substance P, CGRP, NeurokininA
Describe what happens to the sensory innervation of distal blood vessels in the brain when the CSF is low
The brain moves more, so the sensory fibers sense more torsion = pain; AND they vasodilate to increase the intracranial volume closer to normal by passing more blood to brain
What controls cerebral circulation?
Oxygen consumption dictates where the brain blood will go! (Using more oxygen = receiving more blood)
What organ controls cerebral circulation?
The BRAIN itself = AUTOREGULATION!
Describe autoregulation
When the cerebral blood flow is NOT controlled by mean arterial blood pressure
Without sympathetics, describe the range of autoregulation
When the mean arterial blood pressure is below 60 mm HG or its above 140 mm HG, the cerebral blood flow will be dependent on it the mean arterial blood pressure! (low flow below 60 and high flow above 140, constant in between)
With sympathetics, describe the range of autoregulation
The range of autoregulation increases! After 60 mm HG is reached, autoregulation of blood flow is constant even for a time after 140 mmHG is reached!
Why is autoregulation important?
It protects the capillaries in the brain from damage from dangerously high blood pressures
What happens as the intracranial pressure increases?
Venous outflow decreases = decreased blood flow and decreased O2 to the brain which = a DRASTIC increase in system BP (trying to get blood to brain)
What are some causes of intracranial pressure increase?
Tumor, cerebral edema (swelling), intracranial bleeding, increased CSF
Where does Cerebrospinal fluid flow?
Through ventricles, around brain and spinal cord (black area of MRI)
Is CSF recirculated?
NO
Describe the ventricular system
2 lateral ventricles, 3rd ventricle, 4th ventricle
What connects the 2 lateral ventricles to the 3rd ventricle?
Intraventricular foramen
What connects the 3rd and 4th ventricles?
Cerebral aquaduct
What connects the 4th ventricle to the cisterna magna?
Median aperature
What connects the 4th ventricle to the arachnoid space?
Lateral aperature
Where is most of the CSF made?
Specialized tissue in choroid plexus in lateral ventricles
What are the 2 steps to the formation of CSF?
Passive filtration of serum
Modification of ion composition
What 2 pressures is the passive filtration of serum step dependent on?
Hydrostatic pressure and oncotic/osmotic pressure
Describe hydrostatic pressure in the capillary
= BP; large force that pushes fluid out of capillary into surrounding tissues because the surrounding tissues opposing pressure is low
Describe the oncotic/osmotic pressure
Pulls fluid into and out of capillary; forces are equal and opposite at choroid plexus
Where does fluid flow during formation of CSF?
From capillaries to the ventricles
What molecule controls the pH of CSF?
HCO3
What ions have low concentrations in CSF, but high in plasma?
K, Ca, protein, glucose
What ions have high concentrations in CSF, but low in plasma?
Mg, Cl, Co2
What ion is slightly of lower concentrations but almost equal in CSF compared to plasma?
Na
What is the role of CSF and how often is it produced?
Protects the brain (shock absorber); CONSTANT rate of production!
What 2 components of a capillary limit exchange in the brain?
Tight junctions and glial endfeet
What qualities of a molecule are needed in order to pass blood brain barrier?
Lipid soluble or have a channel
What is the purpose of the blood brain barrier?
Protect composition of CSF
How do the capillaries get glucose from there to the neurons?
Glut1 - 55K on capillaries and 45K on glia (2 forms)
Glut3 - used by neurons
Na/K/2Cl transporter
Moves these ions from CSF to blood
What controls the expression of the na/k/cl transporter?
Release of endothelin 1 and 3 from endothelial cells of blood vessels
Many drugs can cross blood brain barrier, but what moves them back across?
P-glycoprotein
Circumventricular organs
4 regions in brain that require exposure to blood-borne substances!
What are the 4 circumventricular organs?
Posterior pituitary, area postrema, organum vasculosum of lamina terminalis (OVLT), subfornical organ
Job of posterior pituitary
Release hormones into blood
Job of area postrema
Protect composition of blood by eliciting vomiting
Job of OVLT and subfornical organ
Control of body water/thirst/blood volume
