Ch 61 Cerebral blood flow, cerebrospinal fluid, and brain metabolism Flashcards
Which four metabolic factors contribute to cerebral blood flow regulation?
CO2 conc, H+ conc, O2 conc, substances released by astrocytes
How does CO2 increase cerebral blood flow? How does it return to normal after such increases?
Combines with water and then dissociates so H+ ions are released. H+ acts directly on cerebral vessels to cause vasodilation directly proportional to the increase in [H+]. Other acidic metabolic byproducts have the same effect
vasodilation = increased blood flow = carries H+, CO2, and byproducts away from brain tissues = remove carbonic acid from tissues = return {H+} to normal
What is the effect of reduced cerebral tissue PO2?
When dropped below 30 mmHg, cerebral blood flow immediately increases due to vasodilation
What are astrocytes and what is their function?
non-neuronal cells that support and protect neurons and provide them nutrition. they release vasoactive metabolites such as nitric oxide, K+, arachidonic acid metabolites, and adenosine to cause adjacent vasodilation
Their ‘feet’ also support brain capillaries and prevent overstretching
How does cerebral blood flow change in accordance with peripheral blood pressure changes?
It stays constant as long as arterial pressure stays between 60-140 mmHg
How does the SNS regulate brain blood pressure?
When arterial pressure rises acutely to high levels, it constricts large and mid-sized brain arteries enough to prevent this pressure change from reaching small brain vessels
How does the metabolic rate, blood flow, and number of capillaries of gray matter compare to that of white matter?
Gray matter MR is about 4x that of white matter, and the number of capillaries and rate of blood flow are 4x that of WM
Where is CSF fluid present?
The ventricles, cisterns around the outside of the brain, and the subarachnoid space around bother the brain and spinal cord
How does a countrecoup injury occur?
A blow struck to the head makes the incompressible CSF and brain to move in unison with the skull, but on the other side of the skull, the sudden movement makes the skull pull away due to brain’s inertia. There is a momentary vacuum in the cranial area opposite the blow, and then when the skull stops accelerating, the vacuum collapses and the brain hits the opposite side
Where is CSF formed?
2/3s is secreted from the choroid plexuses in the four ventricles, especially the two lateral ventricles . The rest is secreted by ependymal surfaces of all ventricles and the arachnoidal membranes
Where is the cisterna magna?
Behind the medulla and beneath the cerebellum
CSF flows first from the lateral ventricles to ____
then to the third ventricle, then along the aqueduct of sylvius, into the fourth ventricle
What is the choroid plexus and where is it located?
Projections of blood vessels covered in epithelial cells in the temporal horn of each lateral ventricle, the posterior portion of the third ventricle, and the roof of the fourth ventricle.
It produces CSF
How does the choroid plexus produce CSF?
- Active transport of Na+ through the epithelium
- Na+ pulls Cl- with it (+ attracts -)
- NaCl now increases osmotically active sodium chloride in the CSF = osmotic flow of water with it
A small amount of glucose moves in as bicarb and K+ move out of the CSF as well
How does normal CSF composition compare to that of blood plasma?
about equal: osmotic pressure, Na+
Chloride: 15% greater than plasma
K+ 40% less
glucose is 30% less
How is CSF absorbed?
Endothelial cells covering the arachnoidal villi (and their granulations) have vesicular passages directly through the bodies of cells that allow free flow of CSF, dissolved protein molecules, and blood cells into venous blood
How is the lymphatics system modified/adapted for the brain’s unique structure and needs?
The perivascular spaces are a specialized lymphactic system: on reaching the subarachnoid spaces, excess protein that has left the brain tissue flows with the CSF to be absorbed through the arachnoidal villi into the large cerebral veins
How do the arachnoidal villi help regulate csf pressure?
They function like valves to allow CSF to flow into the blood of venous sinuses but not backward. When CSF pressure is 1.5mm Hg greater than blood pressure in the venous sinuses, it flows out.
In disease states, villi get blocked by particulates, fibrin, or cells that leaked and allow CSF pressure to rise
What is papilledema and how does it develop?
Swelling of the optic disc, observable by ophthalmoscope.
This occurs when pressure rises in the CSF and is communicated into the optic nerve sheath. Then the fluid is pressed in along the spaces between the optic nerve fibers to the interior of the eyeball, decreases outflow of fluid in the optic nerves, and fluid accumulates in the optic disc at the center of the retina
What are the types of hydrocephalus?
- Communicating: caused by fluid flow being blocked in the subarachnoid spaces around the basal regions of the brain or blockage of the arachnooial villi where the fluid is normally absorbed into the venous sinuses. This causes head swelling if the skull is pliable
- Noncommunicating: caused by a block in the aqueduct of Sylvius, resulting from atreisa before birth or tumor blockage. The ventricular volume expands, pressing the brain into a thin shell against the skull
Where does the blood brain barrier exist? Where does it NOT exist?
Exists at: the choroid plexus and tissue capillary membranes in all areas of brain parenchyma except
NOT at: some areas of hypothalamus, pineal gland, and area postrema
Why is the BBB so impermeable?
The endothelial cells of brain tissue capillaries are joined by tight junctions
What are the ‘vicious circles’ that ensue with development of brain edema?
- Edma compresses vasculature, decreasing blood flow and causing ischemia that causes arteriolar dilation, which increases capillary pressure and causes more edema
- Decreased blood flow decreases O2 delivery, which makes capillaries more permeable and allows more fluid leaking. And turns off the Na pumps of neuronal cells, which allows them to swell
Why is the brain’s metabolic need so high?
THe neurons need energy to pump ions through their membranes to transport Na+ and Ca++ to the outside and K+ inside. Each action potential increases this need
Why is the brain so reliant upon oxygen?
High neuronal metabolic rate makes it reliant on oxygen, and the neurons are not capable of much anaerobic metabolism. Lose blood flow = lose consciousness in 5-10 s
How much glucose is stored in neurons, by amount of time?
2 minutes
What is special about glucose delivery to brain tissue?
Glucose transport into neurons is NOT dependent on insulin! But overdosing insulin rapidly depletes the brain of glucose by shuttling the valuable glucose away from the brain, into the insulin-sensitive neurons
