2

1. CNS Blood Flow, Blood-Brain
Barriers and CSF
Objectives
1. Describe main arteries and veins supplying the brain
2. Correlate regional function with arterial blood supply
3. Explain principles of cerebral blood flow regulation
4. Distinguish main components of BBB and their function
5. Describe formation, flow and absorption of CSF
6. Explain functions of CSF

9. Physiology of cerebral circulation
Normal rate of CBF : 50ml/100g/min ---750ml/brain/min
Normal oxygen consumption: 3.5 ml O2/100g/min
Regulation of cerebral blood flow
1. Regulation by CO2 and pH/H+ ion concentration
PaCO2 increases CBF via H+ from H2CO3…vasodilatation
Importance: H+ depresses neuronal activity…↑CBF removes H+
2. Regulation by hypoxia (oxygen deficiency)
PaO2 decrease results in brain vasodilatation, which…↑CBF
3. Regulation by activity of nervous tissue
Neural activity increases regional CBF within seconds (reading
loudly, clasping hands, looking in light, listening noisy music…)

10. Physiology of cerebral circulation
(2)
4. Autoregulation of CBF when BP changes
CBF does not change if BP increases from 60 to 140 mmHg
In hypertension: autoregulation is extended to 200 mmHg
Below 60 or above 200 mmHg: danger of brain hypoxia/bleeding
REMEMBER: Autoregulation keeps CBF constant
5. Role of autonomic nervous system
Only sympathetic nerves are present on large brain vessels but
they constrict vessels only when BP is very high (>200 mmHg)
No parasympathetic innervation of brain vessels

11. Blood-brain barrier
Specific permeability barriers exist between blood and brain
(blood-brain barrier) and blood and CSF (blood-CSF
barrier). Brain and CSF are selectively excluded by
above barriers and are thus normally protected against
fluctuations of blood constituents or drugs.
Three different mechanisms underlie Blood-Brain Barrier:
1. Tight junctions between capillary endothelial cells in the brain
which generally decrease passage of substances (in contrast to
fenestrated capillaries in other organs). In case of Blood-CSF
Barrier, tight junctions are located between epithelial cells of
choroid plexus, while its capillaries resemble those of systemic
circulation.

12. BBB - 2
2. Glial foot processes of astrocytes which surround and encase
brain capillaries controlling (additionally to capillary tight junctions)
the passage of substances between capillary lumen and brain
interstitial space.
3. Transport systems of brain capillaries which is supported by
more numerous energy-producing mitochondria. These systems
selectively and effectively transport glucose, amino acids and
some other blood constituents needed for brain function, including
some drugs. However, other drugs are not transported. As an
example: Dopamine is not transported into the brain but its
precursor L-DOPA is transported since it more closely resembles
amino acid phenylalanine.