2. Cerebral blood flow
• the principal arterial inflow to the brain in human is via
four arteries: two internal carotids & two vertebrals
• blood flow to the brain is about 750 ml/min (15% of the
cardiac output)
• cerebral blood flow is 50ml/100g/min of brain tissue
• three metabolic factors have potent effects in controlling
cerebral blood flow:
• CO2 concentration
• H+ concentration
• O2 concentration
3. Autoregulation of cerebral blood flow
• cerebral blood flow is “autoregulated” extremely well
between arterial pressure limits of 60 & 140 mm Hg
4. Cerebral microcirculation
• the number of blood capillaries in the brain is greatest
where the metabolic needs are greatest
• the number of capillaries & rate of blood flow are about
four times as great as that of white matter in the gray
matter
• brain capillaries are much less “leaky” than the blood
capillaries in almost any other tissue of the body
• the capillaries are supported on all sides by “glial feet,” &
provide physical support to prevent overstretching of the
capillaries
5. Cerebrospinal fluid
• a clear, colorless, almost protein free filtrate of blood
• volume is about 150 ml & rate of production is 550ml/d
• 50-70% of the CSF is formed in the choroid plexuses & the
remainder is formed around blood vessel & along
ventricular walls
• is present around the brain & spinal cord (in the
subarachnoid space) as well as inside the brain & spinal
cord (in its ventricles & the central canal respectively)
• all these chambers are connected with one another, & the
pressure of the fluid is maintained at a constant level
6. Functions of CSF
1. Mechanical protection
• serves as a cushion between the CNS & surrounding bones, thus
protecting it against mechanical trauma
• also buoys the brain so that it “floats” in cranial cavity
2. Homeostatic function
• pH of the CSF affects pulmonary ventilation & cerebral blood flow
• CSF also serves as a transport system for polypeptide hormones
secreted by hypothalamic neurons
3. Circulation
• CSF is a medium for minor exchange of nutrients & waste products
between the blood & adjacent nervous tissue
7. Formation
CSF is formed mainly in the choroid plexuses of the lateral, third, &
fourth ventricles; some originates from the ependymal cells lining the
ventricles & from the brain substance through the perivascular spaces
8.
9. Absorption
some of the CSF is
absorbed directly into
the veins in the
subarachnoid space, &
some escapes through
the perivascular spaces
into the subarachnoid
spaces
10. • brain requires a very stable environment in order to
function normally
• barriers, called the blood-cerebrospinal fluid barrier & the
blood-brain barrier, exist between the blood &
cerebrospinal fluid & brain fluid, respectively (except in
some areas of the hypothalamus, pineal gland & area
postrema)
• barriers are highly permeable to water, carbon dioxide,
oxygen, & most lipid-soluble substances; slightly
permeable to electrolytes; & almost totally impermeable
to plasma proteins & most non–lipid-soluble large
organic molecules
