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CSF – Definition
• The CSF is a specialized extracellular fluid in the ventricles and the subarachnoid space.
– Power and Kam, Principles of physiology for the anaesthetist
CSF – Function
• Mechanical protection by buoyancy– Effective weight reduced from 1400 g to 47 g
• Maintenance of constant ionic environment• Acid-base regulation– Esp. via control of respiration
• Nutritional
CSF – Production (1)
• Formed:– 70% in choroid plexus– 30% around blood vessels and ventricular walls
• Volume of CSF: 150 ml• Rate of formation– 550 ml/day– 0.4 ml/min– Turnover 4 times per day
CSF – Production (2)
• Fenestrated capillaries– ultrafiltrate
• Transport– Hydrostatic pressure and bulk flow: H2O– Active transport: Na+, K+, H+
– Facilitated transport: glucose– Passage through junctions and vesicles: protein
CSF – Circulation
• From lateral ventricles to 3rd ventricle:– Foramina of Monro
• From 3rd to 4th ventricles:– Aqueduct of Sylvius
• Leaves 4th ventricle to cisterna magna:– Foreman of Magendie– Foramen of Luschka
• From cisterna magna– Superiorly: subarachnoid space around cerebellum– Caudally: spinal subarachnoid space– Cephalad to basilar cisterns
CSF – Absorption
• Absorbed into venous blood• Arachnoid villi:– Dural walls of sagittal and sigmoid sinuses (85-90
%)– Dural walls of dural sinusoids on dorsal nerve
roots (10-15 %)
• Mean CSF pressure: 15 cmH2O
• Pressure in superior sagittal sinus: 9 cmH2O
CSF – Normal constituentsSubstance Unit CSF Plasma Ratio
CSF/PlasmaNa+ meq/kg H2O 147.0 150.0 .98
K+ meq/kg H2O 2.9 4.6 .62
Cl- meq/kg H2O 113.0 99.0 1.14
HCO3- meq/L 25.1 24.8 1.01
PCO2 mmHg 50.2 39.5 1.28
pH 7.33 7.4
Protein mg/dL 20.0 6000.0 0.003
Glucose mg/dL 64.0 100.0 .64
BBB – Definition
• A physiological boundary between the bloodstream and central nervous system, preventing transfer of substances from plasma to brain.– Yentis et al, Anaesthesia and Intensive Care A-Z
BBB – Morphology
• Capillary endothelial cells have:– Tight junctions between adjacent cells– Absence of fenestrations– High content of mitochondria
• Perivascular area of closely applied foot processes of astrocytes
BBB – Function
• Provides favourable environment for nervous tissue function– Protects brain from potentially toxic substances– Allows free access of metabolic substrates
• Prevention of escape of NT into general circulation
BBB – Factors affecting rate of transfer
• Ion channels• Facilitated diffusion• Active transport• Pinocytosis• Factors influencing rate of diffusion:– Molecular size– Concentration gradient– Ionization– Lipid solubility– Protein binding
Cerebral Blood Flow
• What is it in ml/min, ml/100g/min, as % of CO• Measurement• What factors affect it?• Autoregulation
CBF – Measurement
• Applying the Fick principle– Kety-Schmidt technique• N2O 10% breathed for 10-15 min• Jugular venous concentration assumed to be same as
brain concentration
• Detection of radioactive decay• Regional flow measured by Doppler
CBF – Factors affecting
• Arterial PCO2
• Arterial PO2
• MAP• Cerebral metabolic rate for oxygen• Drugs
CBF – Autoregulation
• The phenomenon where CBF is kept constant over a MAP range of 50-150 mmHg.• Power and Kam, Principles of physiology for the anaesthetist
• Thought to be through myogenic response in vascular smooth muscle.– As pressure rises smooth muscle constricts
• Causing reduced flow
– As pressure fall smooth muscle relaxes• Causing increased flow
CBF – coupled to metabolism
• Regional CBF varies with local metabolic rates• Local metabolic factors:– H+
– K+
– Adenosine– Phospholipid metabolites– Glycolytic metabolites– NO
ICP – Definition
• Pressure exerted by the CSF in the frontal horns of the lateral ventricles of the brain.• Yentis et al, Anaesthesia and Intensive Care A-Z
• Normally 7-17 mmHg supine.
• CPP=MAP – ICP
Monro-Kellie doctrine
• Skull is a rigid closed container.• Its contents is incompressible.• Contents made up of:– Blood 50-70 ml (5-7 %)– CSF 50-120 ml (5-12 %)– Brain 1.4 kg (80-85 %)
• ICP depends on volume of intracranial contents
Brain metabolism
• Cerebral function is totally dependent on oxidative phosphorylation of glucose to provide ATP.
• Uses 20% of resting total body O2 consumption (brain 2 % total body weight)
• Sensitivity to hypoxia:– Lack of storage– High metabolic rate