Phys Cerbral Circulation Flashcards
Three extracellular fluid compartments of the brain
1.) CSF Compartments (cerebral ventricles and subarachnoid space) 2.) Interstitial Compartment 3.) Vascular Compartment
Where is CSF formed
Chorid Plexuses within the lateral (left and right third and fourth cereral ventricles
What lines choroid plexuses
ependymal cell layer
Describe the path of CSF
Produced in the cerebral ventricles (lateral thrid and fouth) - flows into the subacrachnoid space surrounding the brain- flows from the subarachnoid space across the arachnoid villi and into the sagital venous sinus where it resturns to the venous circulation
significance of the arachnoid villi
act like one-way valves permitting flow of CSF into the venous sinus
Blood CSF Barrier (choroid plexus)
diffusion barrier in each choroid plexus - 1.) Leaky capillary endothelial cells 2.) Ependymal cells with tight junctions = prevention of free diffusion of substractes between blood and CSF
Modes of transport across choroid plexus
1.) Simple diffusion 2.) Facillitated diffusion 3.) Active Transport
Simple Diffusion across Choroid Plexus
CO2, O2, and H20 move down concentration gradients (lipid soluble substances diffuse more easily)
Facillitated Diffusion across Choroid Plexus
Glucose transport mediated by GLUT-1 (NON-INSULIN DEPENDENT)
Active transport across Choroid Plexus
Sodium (and chloried) actively pumped INTO CSF- generates and osmostic gradient that promotes the movement of water from the capillaries into the CSF
Discuss the composition CSF compared to plasma
Concentrations of Potassium, Calcium, glucose and proteins are lower and Magnesium is higher
How does pressure at the arachnoid villi (intracranial pressure) effect CSF formation
CSF formation is INDEPENDENT of intracranial pressure
What determines the rate of CSF absorption
CSF pressure - absorption does not occur until pressure at the villi exceeds 68 mm CSF (5mmHg) as it continues to increase absorption increases linearly
describe CSF absorption and formation under normal conditions
under normal conditions CSF absorption equals the rate of CSF formation
What is the function of CSF
1) Support (allows brain to float - due to similar specific gravity) 2.) Cushions the brain against jarring
What teathers the brain to minimize movement
arachnoid trabeculae
Countercoup
when hit in the face the brain quickly “snaps” back into place with such force that it can impact th opposite side of the cranium and cause injury ex: Seeing stars due to impact of occipitar region against the back of the cranium
Importance of CSF in supporting the brain (ex: Pneumoencepholagram)
When CSF is removed and replaced with air it causes stretch on the blood vessels and the arahnoid trabeculae causing tremensous pain
Hydrocephalus (definition)
excessive accumulation of CSF in the cranium
Non-communicating (internal) Hydrocephalus
Flow out of one or more cerebral ventricles is blocked and there is no communication with the subarachnoid space or the spinal cord . Increased CSF pressure flattens the brain from within against the cranium
Communicating (external) Hydrocephalus
Caused by blockage of CSF flow at the arachnoid villi or within the subarachnoid space and ther eis no CSF outflow. CSF production continues and the increased CSF pressure compresses the brain and reduced cerebral blood flow
Papilledma
Increase in CSF pressure that compresses the retinal veins and causes retinal edima which is visible as swelling of the optic disc. Visual inspection of the optic disc call allow estimation of CSF pressure
Pia mater
covers the surface of the brain and separtes the interstitial space from the CSF of the subarachnoid space
Ependymal lining
lines the cerebral ventricles and separates the interstitial spcae from the CSF
Discuss the composition of CSF compared to brain interstitial fluid? What mechanism is behind this?
The CSF and the interstitial fluid have similar compositions due to the fact that there is no diffusion barrier between the brain interstitial fluid and the CSF - pial and ependymal membranes are freely permeable to all substances within the CSF
If there are no lymphatics in the brain, what takes over as the “lymphatic system” of the brain
CSF- fluid, protein and cellular debris are removed from the interstitium via 1.) bulk flow of CSF through arachnoid villi OR 2.) by membrane pumps at the choroid plexus
What is the Blood Brain Barrier (BBB)
diffusion barrier between capillary blood and the interstitium in almost all areas of the brain EXCEPT: choid plexus and circumventricular organs involved in plasma sampling or hormone release into blood
What areas of the brain lack BBB
Choroid Plexus and Circumventricular organs (posterior pituitary and Hypothalamus) involved in sampling the plasma or release of hormones into the blood
what is the anatomical site of the BBB
Endothelial cells of cerebral capillaries (tight junctions)
Describe the cellular make up of Circumventricular organs
Lack BBB - 1.) Fenestrated Capillaries (leaky) 2.) Ependymal cells with tight junctions
Describe the cellular make up of areas in the brain that have BBB
1.) Capillaries with tight junctions 2.) Ependymal cells that are freelu permeable (allows CSF and interstitial space to have same composition)
What is the significance of the blood brain barrier
1.) Maintains contant neuronal environment 2.) Protects the brain from exogenous and endogenous substances 3.) Prevents escape of neurotransmitters into systemic circulation 4.) Selective permeability (ex: Dopamine cannot cross but L-DOPA can)
Describe cerebral blood flow (general)
Total flow remains constant but regional distribution changes (depending on regional brain activity)
Cerebral blood flow accounts for what percentage of the cardiac output
15% (750 ml/min)
Cerbral blood flow equation
(P.arterial- P.cerbral.venous) /Cerebral Vascular Resistance
Relationship between cerebral blood flow and intracranial pressure
Because the brain and circulation are contains in a ridgid cranium, cerebral blood flow in influenced by intracranial pressure (“surround pressure”) Increases in intracranial pressure can severely reduced cerebral blood flow
Cerebral O2 consumption
20% of systemic oxygen consumption: required to support neuronal activity
What is the major regulator of cerebral flow
the coupling of local metabolic metabolism with blood flow
Describe the effect of sympathetic stimulation on cerebral flow
NO DIRECT ROLE. 1.) Extends the autoregulatory plateau to accommodate higher pressures 2.) Blunts the effects of sudden increases in cerebral arterial pressure (accentuates the normal myogenic response that keeps flow from increasing too much)
What arterial factor has the greatest effect on cerebral blood flow
Arterial PCO2
Factors involved in cerebral vasodilation
1.) Increased arterial H (decreased pH) - small effect 2.) Decreased arterial PO2 (<30 mmHG) - less important than CO2 3.) INCREAED ARTERIAL PCO2 - BIG EFFECT!!!
Effect of hypoventilation on cerebral flow
increases arterial PCO2 (>40 mmHg) and causes cerebral vasodilation)
Effect of hyerventilation on cerebral flow
decreases areterial PCO2 (<40 mmHg) and causes cerebral vasoconstriction. Hyperventilation can provide some relief to individuals with space filling lesions that increase intracranial pressure
Effect of Hyercapnia on cerebral autoregulation
Hypercapnia (increased arterial CO2) abolishes cerebral autoregulation
Define intracranial pressure
volume occupied by blood, CSF, and brain at any given time is relatively constant
What promotes the circulation of CSF
arterial pulsations within the rigid cranium displace some of the CSF from the ventricles with each heart beat thereby promoting CSF circulation
Cushing Reflex
If CSF increases to 33 mmHg the increased intracranial pressure begins to compress incoming arteries and reduce cerebral flow (leads to cerebral ischemia) and compress cerebral veins restricting outflow (increased cerebral vascular resistace). Associated with a profound sympathetic vasoconstriction in the periphery and an increased MAP in an attempt to maintain cerebral perfusion
CNS ischemic response
Cerebral hypoxia and ischemia cause reflex systemic vasoconstriction that is accompanied by bradycardia (due to activation of the baroreceptor reflex in response to increased MAP) Cerbral vessels DO NOT CONSTRICT. Can be thought of as a “last ditch effort” by the brain to maintain blood flow (arterial pressure can rise up to 300 mmHg)
