Phys Cerbral Circulation

Question 1

Three extracellular fluid compartments of the brain

Answer 1

1.) CSF Compartments (cerebral ventricles and subarachnoid space) 2.) Interstitial Compartment 3.) Vascular Compartment

Question 2

Where is CSF formed

Answer 2

Chorid Plexuses within the lateral (left and right third and fourth cereral ventricles

Question 3

What lines choroid plexuses

Answer 3

ependymal cell layer

Question 4

Describe the path of CSF

Answer 4

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

Question 5

significance of the arachnoid villi

Answer 5

act like one-way valves permitting flow of CSF into the venous sinus

Question 6

Blood CSF Barrier (choroid plexus)

Answer 6

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

Question 7

Modes of transport across choroid plexus

Answer 7

1.) Simple diffusion 2.) Facillitated diffusion 3.) Active Transport

Question 8

Simple Diffusion across Choroid Plexus

Answer 8

CO2, O2, and H20 move down concentration gradients (lipid soluble substances diffuse more easily)

Question 9

Facillitated Diffusion across Choroid Plexus

Answer 9

Glucose transport mediated by GLUT-1 (NON-INSULIN DEPENDENT)

Question 10

Active transport across Choroid Plexus

Answer 10

Sodium (and chloried) actively pumped INTO CSF- generates and osmostic gradient that promotes the movement of water from the capillaries into the CSF

Question 11

Discuss the composition CSF compared to plasma

Answer 11

Concentrations of Potassium, Calcium, glucose and proteins are lower and Magnesium is higher

Question 12

How does pressure at the arachnoid villi (intracranial pressure) effect CSF formation

Answer 12

CSF formation is INDEPENDENT of intracranial pressure

Question 13

What determines the rate of CSF absorption

Answer 13

CSF pressure - absorption does not occur until pressure at the villi exceeds 68 mm CSF (5mmHg) as it continues to increase absorption increases linearly

Question 14

describe CSF absorption and formation under normal conditions

Answer 14

under normal conditions CSF absorption equals the rate of CSF formation

Question 15

What is the function of CSF

Answer 15

1) Support (allows brain to float - due to similar specific gravity) 2.) Cushions the brain against jarring

Question 16

What teathers the brain to minimize movement

Answer 16

arachnoid trabeculae

Question 17

Countercoup

Answer 17

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

Question 18

Importance of CSF in supporting the brain (ex: Pneumoencepholagram)

Answer 18

When CSF is removed and replaced with air it causes stretch on the blood vessels and the arahnoid trabeculae causing tremensous pain

Question 19

Hydrocephalus (definition)

Answer 19

excessive accumulation of CSF in the cranium

Question 20

Non-communicating (internal) Hydrocephalus

Answer 20

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

Question 21

Communicating (external) Hydrocephalus

Answer 21

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

Question 22

Papilledma

Answer 22

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

Question 23

Pia mater

Answer 23

covers the surface of the brain and separtes the interstitial space from the CSF of the subarachnoid space

Question 24

Ependymal lining

Answer 24

lines the cerebral ventricles and separates the interstitial spcae from the CSF

Question 25

Discuss the composition of CSF compared to brain interstitial fluid? What mechanism is behind this?

Answer 25

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

Question 26

If there are no lymphatics in the brain, what takes over as the “lymphatic system” of the brain

Answer 26

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

Question 27

What is the Blood Brain Barrier (BBB)

Answer 27

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

Question 28

What areas of the brain lack BBB

Answer 28

Choroid Plexus and Circumventricular organs (posterior pituitary and Hypothalamus) involved in sampling the plasma or release of hormones into the blood

Question 29

what is the anatomical site of the BBB

Answer 29

Endothelial cells of cerebral capillaries (tight junctions)

Question 30

Describe the cellular make up of Circumventricular organs

Answer 30

Lack BBB - 1.) Fenestrated Capillaries (leaky) 2.) Ependymal cells with tight junctions

Question 31

Describe the cellular make up of areas in the brain that have BBB

Answer 31

1.) Capillaries with tight junctions 2.) Ependymal cells that are freelu permeable (allows CSF and interstitial space to have same composition)

Question 32

What is the significance of the blood brain barrier

Answer 32

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)

Question 33

Describe cerebral blood flow (general)

Answer 33

Total flow remains constant but regional distribution changes (depending on regional brain activity)

Question 34

Cerebral blood flow accounts for what percentage of the cardiac output

Answer 34

15% (750 ml/min)

Question 35

Cerbral blood flow equation

Answer 35

(P.arterial- P.cerbral.venous) /Cerebral Vascular Resistance

Question 36

Relationship between cerebral blood flow and intracranial pressure

Answer 36

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

Question 37

Cerebral O2 consumption

Answer 37

20% of systemic oxygen consumption: required to support neuronal activity

Question 38

What is the major regulator of cerebral flow

Answer 38

the coupling of local metabolic metabolism with blood flow

Question 39

Describe the effect of sympathetic stimulation on cerebral flow

Answer 39

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)

Question 40

What arterial factor has the greatest effect on cerebral blood flow

Answer 40

Arterial PCO2

Question 41

Factors involved in cerebral vasodilation

Answer 41

1.) Increased arterial H (decreased pH) - small effect 2.) Decreased arterial PO2 (<30 mmHG) - less important than CO2 3.) INCREAED ARTERIAL PCO2 - BIG EFFECT!!!

Question 42

Effect of hypoventilation on cerebral flow

Answer 42

increases arterial PCO2 (>40 mmHg) and causes cerebral vasodilation)

Question 43

Effect of hyerventilation on cerebral flow

Answer 43

decreases areterial PCO2 (<40 mmHg) and causes cerebral vasoconstriction. Hyperventilation can provide some relief to individuals with space filling lesions that increase intracranial pressure

Question 44

Effect of Hyercapnia on cerebral autoregulation

Answer 44

Hypercapnia (increased arterial CO2) abolishes cerebral autoregulation

Question 45

Define intracranial pressure

Answer 45

volume occupied by blood, CSF, and brain at any given time is relatively constant

Question 46

What promotes the circulation of CSF

Answer 46

arterial pulsations within the rigid cranium displace some of the CSF from the ventricles with each heart beat thereby promoting CSF circulation

Question 47

Cushing Reflex

Answer 47

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

Question 48

CNS ischemic response

Answer 48

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)