Cerebral Circulation
Anatomy:
Arterial inflow through 2 internal carotids + 2 vertebrals. Circle of Willis.
Venous drainage by way of the deep veins and dural sinuses – into internal
jugular veins.
Capillaries:
Number of capillaries of the brain gray matter is about 4x as great as that of
white matter. Capillaries are supported on alal sides by „glial feet“ – providing
physical support to prevent overstretching of the capillaries in case of high
pressure and to prevent transudation of fluid into the brain – against brain edema.
Difficult penetration – the blood – brain barrier
(except of some areas of the
hypothalamus, the pineal gland and the area postrema).
Function of the B-B barrier
Barrier is highly permeable to H
2
O, CO
2
, O
2
and lipid soluble substances (alcohol,
anesthetics).
Slightly permeable to the electrolytes (Na
+
, Cl
-
, K
+
)
Totally impermeable to plasma proteins and large organic molecules.
Importance: The BB barrier – maintains the constancy of the environment of the
neurons in CNS.
- protects of the brain from toxins in the blood
- prevents neurotransmitters against the escape into the circulation
Ontogeny: BB barrier develops postnatal (jaundiced newborns – penetration of the bile pigments
into CNS – kernicterus).
Clinical implication: Application of the drugs (ATB) penetrating in the BB barrier for treatment
of cerebral diseases.
Cerebral Blood Flow
CBF = Adults: 750 ml/min = 54 ml/min/100 g of brain w. = approx. 15% of CO
Children: 105 ml/min/100 g
A decrease of CBF to „adult value“ in puabertal age (sex hormones)
BF in various parts of the brain:
BF in gray matter is about 6 times that in white matter.
A marked fluctuation in regional BF with changes in activity (the movements – motor
area, the speech – sensory + motor area).
Regulation of CBF
Monro-Kellie doctrine: The sum of the volume of blood (75 ml), cerebrospinal fluid (75 ml)
and brain (1400 g) in the cranium must be relatively constant.
1) Autoregulation:
The intracranial pressure (ICP – CSF pressure) = 10 mm Hg. When ICP > 33 mm Hg – CBF
is reduced – ischemia – stimulation of the vasomotor and cardioinhibitory centers –
hypertension, bradycardia = Cushing reflex – helps to maintain CBF and to preserve
O
2
for
brain and coronary circulation.
The myogenic autoregulation – in the range 65 – 140 mm Hg.