23A12: Exam Report

Describe the physiology of cerebrospinal fluid (CSF)

88% of candidates passed this question.

This question has been repeated multiple times. Better answers demonstrated a structured approach to discussing the physiology of CSF dividing concepts into; Formation and Composition, Regulation and Circulation, and Functions of CSF with the appropriate level of detail.

20A16: Exam Report

Outline the formation, circulation and functions of cerebrospinal fluid

81% of candidates passed this question.

This is a three-part question and was marked as such.

The circulation and functions of CSF was generally well answered. Formation of CSF, however, was answered poorly, with many candidates listing its composition instead.

The examiners were looking for an understanding of the physiological processes of formation not the composition

20A16: Exam Report

Outline the formation, circulation and functions of cerebrospinal fluid

81% of candidates passed this question.

This is a three-part question and was marked as such.

The circulation and functions of CSF was generally well answered. Formation of CSF, however, was answered poorly, with many candidates listing its composition instead.

The examiners were looking for an understanding of the physiological processes of formation not the composition

19A11: Exam Report

Describe the physiology of cerebrospinal fluid (CSF) (60% of marks). Describe the anatomy relevant to performing a lumbar puncture (40% of marks).

86% of candidates passed this question.

Better answers had a structure with headings such as function, formation, circulation, absorption and composition with dot point facts under each heading. The second part of the question lent itself to a diagram with labelling which scored well. Precise surface anatomy and mentioning all layers from the skin to the sub-arachnoid space scored well.

18B15: Exam Report

Outline the production / absorption (30% of marks), composition (30% of marks) and function of cerebrospinal fluid (CSF) (40% of marks).

71% of candidates passed this question.

This question was generally well answered. Better answers noted production including an amount, site and mechanism. Similarly, absorption included the site, the rate and factors which affect the rate. The electrolyte and pH and how they compare to extracellular fluid should have been included in the section on composition.

17B09: Exam Report

Briefly outline the formation, absorption, distribution, role and composition of cerebrospinal fluid.

44% of candidates passed this question.

The question spelt out very specific areas of CSF physiology to outline and the marks were evenly distributed among these areas. The candidates who did not pass this question usually did not provide enough detailed information.

Details of the production and absorption of CSF were commonly lacking. The majority of candidates correctly described the composition of CSF; indicating whether a particular variable was higher or lower than in plasma, scored less marks than more specific information.

17A24: Exam Report

Outline the physiology of cerebral spinal fluid (CSF).

67% of candidates passed this question.

Better answers included details on CSF production (amount, site), reabsorption and factors which influences CSF and its circulation.

15A16: Exam Report

Describe the physiology of cerebrospinal fluid.

83% of candidates passed this question.

Most candidates answered the question well. The most common mistake was incorrect CSF composition. Better answers also discussed raised ICP and CSF’s role in the compensation for raised ICP.

K1iv / 23A12 / 20A16 / 19A11 / 18B15 / 17B09 / 17A24 / 15A16 : Outline the formation, absorption, distribution, role & composition of CSF

Definition & Normal Values

  • Clear body fluid that occupies subarachnoid space & ventricular system
    • Total volume = 150mL
    • Daily turnover = x4 = 550mL/day

Formation

  • Formed by EPITHELIAL CELLS
  • In 2 places:
    1. 70% CHOROID PLEXUS of 2 lateral ventricles
    2. 30% EXTRACHOROIDAL – endothelial cells lining brain capillaries
  • CHOROID PLEXUS = network of blood vessels covered by epithelial cells which project from ventricles like cauliflowers
    • Choroid plexus epithelium is impermeable due to TIGHT JUNCTIONS

∴ secretion via pumps

    • Hydrostatic P and Bulk Flow promote fluid entry into clefts b/w epithelial cells of choroid plexus
Formation

Flow

  • Ependymal cells lining ventricles have cilia
  • Ciliary movement propels CSF towards 4th ventricle
  • Through Foramen Magendie & Luschka
  • Into Subarachnoid Space of brain & spinal cord
  • Flow determined by pressure in lateral ventricles
  • Flow also affected by respiration & arterial pulsations

Absorption

  • CSF reabsorbed across ARACHNITID VILLI
    • 90% arach villi intracranial
    • 10% arach villi spinal
  • Then passes into Dural Sinuses
  • Due to pressure difference between CSF & venous sinuses
  • When CSF P > Venous sinus P = CSF is absorbed across Arachnoid villi
  • Normally, formation = reabsorption

BUT

  • ↑ICP = ↓CPP <70mmHg = ↓CSF formation
  • ↓ICP <7mmHg = minimal CSF reabsorption (remember its role)

Factors Affecting Formation

  • CPP
  • Body temp → ↓temp = ↓CMRO2 = ↓CSF
  • ↑serum osmolality (mannitol) = ↓CSF
  • Drugs: acetazolamide, steroids, spironolactone, frusemide → ↓CSF

Composition & Normal Values

  • Clear, colourless
  • No cells
  • No immunoglobulins
  • Low protein (not much buffering)
  • Low glucose (not much nutrient supply)

CSF

Plasma

Osmolality

289

289

mOsm/kg H2O

Protein

20 ↓↓↓

6000

mg/dL

Glucose

64 ↓

100

mg/dL

Na

147 ↓

150

mol/L

K

2.9 ↓↓

4.6

mol/L

Ca

2.3 ↓↓

4.7

mol/L

HCO3

25.1 ↑

24.7

mol/L

PCO2

50.2 ↑

39.5

mmHg

pH

7.33↓

7.40

Role of CSF

1. Brain Cushioning

  • Br weight = 1500g → in CSF = 50g!
  • Otherwise would cut off its own blood supply

2. Pressure Buffer

  • Monroe-Kellie doctrine
  • ↑ICP = ↓CSF formation to ↓ICP

3. Acid Base Control of Respiration

  • Central ChemoR on ventral floor of Medulla & 4th ventricle
  • Bathed in ISF → communicates with CSF
  • BBB permeable to CO2 but not H+
  • PaCO2 equilibrates with CSF PCO2
  • CO2 diffuses past epithelial cells → converted H+
  • ↑H+ detected by ChemoR → hyperventilation
  • ↓protein of CSF → very sensitive to pH ∆

∴ pCO2 tightly regulated

4. Maintains Constant Ionic Environment

  • Neurons very sensitive to ionic ∆
  • Regulates [ ] Ca2+, Mg2+, K+
  • These are very low [ ] in CSF ∴able to rapidly buffer

5. Provide Nutrients

6. Remove Waste

7. Transport: Neurotransmitters, Hormones