Brain Circulation

Posted by e-Medical PPT Sunday, September 16, 2012
Total cessation of blood flow to the brain causes unconsciousness within 5 to 10 seconds.
Cerebral Blood Flow (adult)
Normal                     50-65 ml/100 g/minute
Ischemia (reversible)        8-23 ml/100 g/minute
Infarction (irreversible)     < 8 ml/100 g/minute

CBF (adult) depends on
Cerebral perfusion pressure (CPP)
Autoregulation (MAP, brain metabolism)
sympathetic: the large- and intermediate-sized brain arteries

Autoregulation
CBF is autoregulated with MAP between 60-140 mm Hg
vasodilitation
    - decrease MAP
    - increase in carbon dioxide concentration
    - increases hydrogen ion concentration (↓PH)
    - decrease in cerebral tissue Po2

vasoconstriction
    -Increase MAP, ↓ arterial Pco2,↑ PH


Cerebrospinal Fluid System
ventricles of the brain, cisterns around the outside of the brain, the subarachnoid space around both the brain and the spinal cord
Total amount at any time 150 milliliters
formation rate about 500 milliliters each day
Formation of CSF
2/3 as secretion from the choroid plexuses in the four ventricles but mainly in the two lateral ventricles
1/3 as secretion by the ependymal surfaces of the ventricles and by the arachnoidal membranes

Intracranial compensation
The brain is essentially non-compressible
Any increase in intracranial volume decreases CSF or CBV
CSF - primarily displaced into the spinal subarachnoid space
Blood - venoconstriction of CNS capacitance vessels displaces blood in jugular venous system

Exhaustion of compensation
Once these limited homeostatic mechanisms are exhausted additional small increases in intracranial volume produce marked elevations in ICP
Raised ICP may 1.decrease CBF  resulting in vicious cycle. 2. can cause brain herniation.
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