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v    Total cessation of BF to brain      à     ↓ in O₂ delivery    à     Shutdown of metabolic activity      à      Unconsciousness within 5-10 sec.

Brain Metabolism

q       Brain metabolism is ≈ 15% of total metabolism of body.

q       Brain has limited anaerobic capability (mostly aerobic) because:

Õ    ↑ Metabolic activity of neurons.

Õ    ↓↓ Amount of glycogen stored in neurons (only 2-min supply).

q       Therefore, most neuronal activity depends on second-by-second delivery of glucose & O₂ from blood.

q       Glucose transport to cell membranes of neurons is insulin-independent.

Cerebral BF

q       Brain receives ≈ 15% of total resting CO.

q       Cerebral BF is related to level of metabolism.

q       3 metabolic factors have potent effects on cerebral BF: CO₂, H⁺, O₂.

q       Act of making a fist with hand à immediate ↑ in BF in motor cortex of opposite cerebral hemisphere.

Explanation:

õ     ↑↑ Neuronal activity in particular area of brain.

õ     à ↑ CO₂:

o       Vasodilator in itself.

o       CO₂  +  H₂O   D   H₂CO₃   D   H⁺  +  HCO₃^-

Any substance ↑ acidity in brain (e.g. pyruvic, lactic acid) à ↑ H⁺ (vasodilator).

õ     à ↓ O₂ à Local vasodilator à ↑ Cerebral BF.

Cerebral BF is autoregulated:

q        Cerebral BF is nearly constant between limits of 60 & 140 mm Hg of mean arterial pressure (MAP).

õ     If arterial pressure < 60 mm Hg   à Cerebral BF becomes extremely compromised.

õ     If arterial pressure > 140 mm Hg à Overstretching / rupture of cerebral blood vessels.

    à Brain edema / Cerebral hemorrhage.

q        Sympathetic NS has a role in regulation of cerebral BF:

During strenuous exercise / states of enhanced circulatory activity     à Sympathetic impulses                                                   à

Vasoconstriction of large & intermediate-sized arteries           à

Prevent ↑ pressure from reaching small-sized blood vessels & thus hemorrhage.

Cerebral Microcirculation

q        Capillaries (hence BF) in gray matter (where neuronal cell bodies lie) are 4X greater than in white matter.

q        Capillaries are surrounded by "glial feet"    à Prevent overstretching of capillaries in case of ↑ pressure.

CSF System

q        CSF in brain ≈ 150 mL.

q        This fluid is found in: ventricles of brain, cisterns around brain, subarachnoid space around both brain & spinal cord. These chambers are interconnected & pressure of CSF is regulated at constant level.

q        A major function of CSF is to cushion brain.

q        Brain & CSF have same specific gravity. Therefore, brain essentially floats in CSF.

A blow to the head     à     Entire brain moves simultaneously with skull      à

No single portion of brain becomes momentarily contorted by blow.

Formation & Absorption of CSF

q        ≈ 500 mL of CSF is formed each day.

q        Most of this fluid originates from choroid plexuses of the four ventricles.  Additional amounts of fluid are secreted by ependymal surfaces of ventricles & arachnoidal membranes.

q        CSF is absorbed by multiple arachnoidal villi à Empties into venous blood.

q       Proteins that leak into interstitial spaces flows through perivascular spaces à Subarachnoid space à CSF à Absorbed through arachnoidal villi à Cerebral veins.

CSF Pressure

§        Normally is regulated by absorption of fluid through arachnoidal villi.

§        Arachnoidal villi function like one-way valves that allow CSF to flow into blood of venous sinuses, but prevent backward flow of blood into CSF.

§        Normal CSF pressure ≈ 10 mm Hg (120 mm H₂O).

§        Blockage of villi à ↑ CSF pressure (e.g. by infectious debris, blood cells from hemorrhage, fibrosis, tumors).

Hydrocephalus:

·        Obstruction to flow of CSF.

·        Obstructive (non-communicating) hydrocephalus:

o       Block of CSF before it reaches the subarachnoid space i.e. blockage within the ventricular system.

o       Usually congenital defect / tumor à blockade of aqueduct of Sylvius.

o       ↑ Fluid volume in the 2 lateral & 3^rd ventricles                  à Head swells tremendously in infants (since skull bones haven’t fused)      + Brain atrophy.

·        Communicating hydrocephalus:

Blockage of fluid flow into subarachnoid space around basal regions of brain / blockage of arachnoid villi themselves                                   à

Fluid collects inside ventricles & on outside brain                          à Head swells tremendously in infants (since skull bones haven’t fused).

Blood-CSF barrier & BBB:

◙       Exist at choroids plexus & at tissue capillary membranes in all areas of brain parenchyma except in some areas of hypothalamus & pineal gland.

◙       These barriers are:

o    Highly permeable to:

·        H₂O, CO₂, O₂, Lipophilic substances (e.g. alcohol, anesthetics).

o    Slightly permeable to: Electrolytes.

o    Totally impermeable to:

·        Plasma proteins + hydrophilic large organic molecules.

◙       The cause of low permeability is the presence of tight junctions between adjacent endothelial cells + absence of fenestrations.  

written by: Khalid Bin Yaroof. FMHS, UAE University.