cerebrospinal fluid (CSF)

Prof. Vajira Weerasinghe

Dept of Physiology

cerebral blood flow

• 750 ml/min (15% of cardiac output)

cerebrospinal fluid

• The cavity enclosing the brain & spinal cord, and the central canal: filled with CSF

• This fluid is in– ventricles– cisterns– subarachnoid space– central canal

cushioning function

• brain is floating in the fluid

• this provides a protective function

contrecoup injury

• when there is a severe blow on the head

• brain is lushed so that the opposite side is struk on the skull to cause an injury

• volume of CSF– 150 ml

• rate of production– 500 ml/day

• formed – mainly in the choroid plexuses of the ventricles– small amounts in the ventricles, arachnoid membranes & perivascular

spaces

formation• choroid plexus projects into

– horn of lateral ventricle– posterior portion of 3rd ventricle– roof of the 4th ventricle

• Mechanism:– active transport of Na through the epithelial cells, Cl follows

passively– osmotic outflow of water– glucose moves in to CSF– K and HCO3 moved out of CSF

absorption• arachnoid villi in the walls of venous

sinuses

circulation• fluid secreted in the lateral ventricles into the 3rd ventricle (secretes

here)

• pass along the aqueduct of Sylvius into the 4th ventricle

• through foramina of Luschka & Magendie into the cisterna magna (behind the medulla)

• subarachnoid spaces around the brain & spinal cord

• arachnoid villi in the venous sinuses

composition

• similar to plasma– CSF Plasma

– Na 147 (similar) 150 mmol/l

– K 2.9 (less) 4.6 mmol/l

– HCO3 25 24.8 mmol/l

– Cl 113 (more) 99 mmol/l

– Pco2 50 40 mmHg

– pH 7.33 7.4

– osmolality 289 (similar) 289 mosm

– protein 20 (less) 6000 mg/dl

– glucose 64 (less) 100 mg/dl

– urea 12 (less) 15 mg/dl

• some substances do not pass into CSF

blood brain barrier• tight junctions between capillary endothelial cells & epithelial cells in the choroid

prevent some substances entering CSF

• small molecules & lipid soluble substances pass through easily

• blood-brain barrier exists between blood & brain tissue

• blood-CSF barrier is present in choroid

• these barriers are– highly permeable to water, CO2, O2, lipid soluble substances (such as alcohol), most anaesthetics,

– slightly permeable to electrolytes– impermeable to proteins, large organic molecules– drugs (variable)

Blood brain barrier

blood brain barrier

• CO2 & O2 crosses easily

• H+ & HCO3- slow penetration• glucose

– passive: slow penetration– active transport system by glucose transporter GLUT

• Na-K-Cl transporter• transporters for other substances

blood brain barrier

– No blood brain barrier in the hypothalamus & posterior pituitary

• substances diffuses easily• these areas contain chemoreceptors for various

substances to detect changes in conc

CSF pressure

• lumbar CSF pressure: 70-180 mmH2O

• this is regulated by absorption through arachnoid villi

• rate of CSF formation is constant

CSF pressure rises

– if the arachnoid villi are blocked by a disease process

– brain tumour may compresses the brain and blocks the absorption

– haemorrhage into the brain tissue can block small channels in the arachnoid villi

– babies are born with high CSF (as in hydrocephalus) due to a defect before birth

• blocking the aqueduct of Sylvius

• blocking of arachnoid villi

cerebral oedema

– because brain is enclosed in a solid cranial vault, accumulation of fluid compresses brain tissue and have serious effects

– happens due to increased capillary pressure or damage to the capillaries

– causes• compresses vasculature, decreases blood flow, brain ischaemia,

arteriolar dilatation, increased capillary pressure, oedema worsens• decreases blood flow, deceases oxygen supply, increases capillary

permeability, more fluid leakage

Cerebral oedema• A 45-year-old man was brought to the emergency department by

his friends• because of a 1-day history of a severe headache and "bizarre

behavior.“• CT scan of his brain revealed

– acute intracranial hemorrhage– with cerebral edema– evidence of midline shift– increased intracranial pressure

• The patient was admitted to the intensive care unit (ICU)

• young patient •was found belatedly after a collapse •secondary to drug overdose•Note the extensive cerebral oedema •with loss of normal grey-white matter differentiation.•The patient never regained consciousness •eventually died with increased intracranial pressure.

brain metabolism• brain metabolism is 15% of total metabolism of the body (although brain mass is 2% of

total body mass)

• therefore brain has an increased metabolic rate

• this is due to increased activity of neurons (AP)

• requires oxygen

• brain is not capable of anaerobic metabolism

• energy supply is by glucose

• glucose entry is not controlled by insulin

Lumbar puncture

• This is the method of obtaining access to the subarachnoid spaceThis is done for the following purposes– To obtain CSF for examination – To estimate CSF pressure

• Patient lying on one side • LP needle is inserted between 3rd and 4th or 4th and

5th lumbar spinous processes• Fluid is withdrawn• Manometer is connected and pressure measured

Lumbar puncture