Show the model answerAttempt it first — that is what makes it stick
(a) Anatomical structure 3 marks
What earns the marks3 marks
| Introduce it | A physiological barrier between the cerebral capillaries and brain extracellular fluid |
|---|---|
| Name what forms it | Capillary endothelium, AND the specialised ependymal cells at the brain-CSF interface |
| Three endothelial features | Tight junctions, no fenestrations, high mitochondrial content |
| Beyond the basement membrane | Astrocyte foot processes with intercellular clefts |
The blood-brain barrier is a physiological barrier between the cerebral capillaries and the extracellular fluid of the brain, providing a favourable environment for nervous tissue by being selectively permeable to substances present in plasma.
| Component | Feature | Significance |
|---|---|---|
| Capillary endothelium | Tight junctions (zonulae occludentes) between adjacent cells | Reduce the intercellular pore from about 65 Å in other capillary beds to about 8 Å here |
| Capillary endothelium | Absence of fenestrations | No bulk paracellular route |
| Capillary endothelium | High content of mitochondria | Supplies ATP for the carriers and pumps that replace free diffusion |
| Brain-CSF interface | Specialised ependymal cells of the choroid plexus | The second morphological barrier — often omitted |
| Basement membrane | Surrounds the endothelium | Structural support; pericytes embedded within it |
| Astrocyte foot processes | Closely applied beyond the basement membrane, with intercellular clefts between them | Induce and maintain the endothelial tight junctions; carry the aquaporin channels handling water movement |
| Pericytes | Embedded in the basement membrane | Contribute to barrier induction and capillary tone |
Commonly lost: Marks were distributed across four elements — the introduction, the two morphological barriers, the three endothelial features, and the astrocyte foot processes. Missing the ependymal cells or the foot processes loses a whole element.
(b) Transfer of substances 4 marks
What earns the marks4 marks · 5 mechanisms
| What is permeable | O₂, CO₂, water, MW under ~30 kDa with high lipid solubility |
|---|---|
| What is not | Large, polar or lipid-insoluble substances |
| Five mechanisms | Name, explain in a clause, give an example — including pinocytosis |
| Two specific carriers | Glucose on GLUT1 (no ATP); Ca²⁺, Mg²⁺ and Cl⁻ by active transport |
| Mechanism | How it works | ATP | Example |
|---|---|---|---|
| Simple diffusion | Down a concentration gradient, directly through the endothelial cell membrane. Rate depends on lipid solubility, molecular size, ionisation and protein binding | No | O₂, CO₂, water, volatile agents, unionised lipid-soluble drugs such as thiopentone |
| Facilitated diffusion | Carrier-mediated, down a gradient, saturable and stereospecific | No | Glucose on GLUT1, which is insulin-independent; amino acids on the large neutral amino acid transporter |
| Primary active transport | Against a gradient, hydrolysing ATP | Yes | Na⁺/K⁺-ATPase at the abluminal membrane; P-glycoprotein efflux returning lipid-soluble drugs to blood |
| Secondary active transport | Uses the sodium gradient rather than ATP directly | Indirectly | Calcium, magnesium and chloride — how their concentrations in brain ECF and CSF are held independent of plasma |
| Vesicular transport | Pinocytosis and receptor-mediated transcytosis. Cerebral endothelium has far fewer vesicles than systemic, so the route is limited | Yes | Insulin, transferrin |
Commonly lost: Two classifications must be the right way round: GLUT1 glucose transport is facilitated diffusion — carrier-mediated and saturable, but down a gradient and needing no ATP — and ATPase transporters are active transport.
(c) Other functions 3 marks
Commonly lost: “Other” means beyond the selective transfer just described in part (b). Repeating transport here earns nothing.
| Function | Detail |
|---|---|
| Maintaining the constancy of brain extracellular fluid | So neuronal excitability is protected from swings in plasma composition |
| Enzymatic degradation | In the endothelial cells, effectively preventing entry — monoamine oxidase and dopa decarboxylase |
| A barrier to microorganisms | Preventing entry into the brain |
| Restricting peripheral immune access | Signalling molecules, antibodies and immune cells |
| Protection from circulating toxins | While allowing metabolic substrates free access |
| Preventing uncontrolled neurotransmitter and hormone passage | Circulating ones into the brain, or centrally released ones into the systemic circulation |