Eight ångström
— the gap that decides what reaches the brain.

The minimum high-yield framework

1. Definition. A physiological barrier between the cerebral capillaries and the extracellular fluid of the brain, which provides a favourable environment for nervous tissue by being selectively permeable.
2. Three structural elements. Capillary endothelium with tight junctions, no fenestrations and abundant mitochondria; a continuous basement membrane; and the closely applied astrocyte foot processes with intercellular clefts in the perivascular area.
3. Pore size. About 65 Å between endothelial cells in most capillary beds; about 8 Å in the brain.
4. Crosses freely: oxygen, carbon dioxide, water, lipid-soluble unionised substances of molecular weight below about 30 kDa. Excluded: large, polar or lipid-insoluble substances, and protein-bound drug.
5. Five transport mechanisms: simple diffusion, facilitated diffusion (GLUT1 for glucose — not active transport), primary and secondary active transport (calcium, magnesium, chloride; efflux pumps), and pinocytosis or receptor-mediated transcytosis.
6. Functions: a constant brain extracellular environment; protection from toxins and from circulating neurotransmitters and hormones; enzymatic degradation of substances at the endothelium; a barrier to microorganisms; and restriction of peripheral immune access.
7. Not everywhere. The circumventricular organs lack it, on purpose.
8. Carbon dioxide crosses freely; hydrogen ion and bicarbonate do not. That single asymmetry explains central chemoreception, the effect of hyperventilation on cerebral blood flow, and why CSF pH adapts over hours.

Two further modifiers are worth stating. Active efflux by P-glycoprotein can make a lipid-soluble drug behave as though it were excluded — loperamide is the standard example, an opioid that is lipid-soluble but pumped straight back out and therefore has no central effect at normal doses. And the barrier is not uniform: it is absent at the circumventricular organs, and it is broken in disease.

Previously examinedApril 2022 · April 2025 — the anatomical structure of the barrier, the mechanisms of transport across it, and its other functions. Worked answers in the library

1. Maintaining the constancy of the brain extracellular fluid, so that neuronal function is not disturbed by changes in plasma composition. A patient’s plasma potassium can move considerably without altering neuronal excitability.
2. Protection from potentially harmful substances circulating in plasma — toxins, drugs and metabolic waste.
3. Preventing the uncontrolled passage of circulating neurotransmitters and hormones into the brain, and equally of centrally released ones into the systemic circulation. Without this, every rise in plasma catecholamines would be a central event.
4. Enzymatic degradation at the endothelium. Endothelial cells contain monoamine oxidase and dopa decarboxylase, which metabolise substances in transit and so prevent them entering the brain extracellular fluid. This is an enzymatic barrier working alongside the physical one.
5. A barrier to microorganisms entering the brain.
6. Restriction of peripheral immune access — signalling molecules, antibodies and immune cells. The brain is relatively immune-privileged as a result.
7. Allowing free access to metabolic substrates. The barrier is selective rather than simply exclusive: glucose, oxygen and essential amino acids have dedicated routes in.

Naming even three or four of these, with the reason each needs an incomplete barrier, turns a memorised list into an explanation. The area postrema is the one to name first, because it is the one with daily anaesthetic consequences.

Carbon dioxide is small, uncharged and lipid-soluble, so it crosses the blood-brain barrier freely and almost instantaneously. Hydrogen ion and bicarbonate are charged, and cross only slowly, by regulated transport. That asymmetry has four consequences, and they are worth stating as a chain:

1. A rise in arterial carbon dioxide is transmitted directly to the brain interstitium and to CSF within seconds.
2. It is hydrated to carbonic acid, a reaction catalysed by carbonic anhydrase, which dissociates to hydrogen ion and bicarbonate. Interstitial and CSF pH therefore fall.
3. The fall is large, because CSF has a very low protein concentration — about 0.2-0.4 g/L against 70 g/L in plasma — and therefore very little non-bicarbonate buffering. The same change in carbon dioxide moves CSF pH further than it moves plasma pH — the buffering point is as important as the diffusion one.
4. The central chemoreceptors on the ventral medulla read that pH change and drive ventilation. This is why arterial carbon dioxide, not arterial pH, is the dominant acute stimulus to breathing, and why an acute metabolic acidosis takes far longer to stimulate ventilation than an equivalent respiratory one.

The same asymmetry sets the time course of hyperventilation. Hypocapnia produces immediate interstitial alkalosis and cerebral vasoconstriction, but bicarbonate is then transported out of the CSF over the following six to eight hours, CSF pH returns toward normal, and the vasoconstriction is lost. Two clinical rules follow. Hyperventilation is a short-term rescue, not a treatment. And abruptly restoring a normal arterial carbon dioxide after a sustained period of hypocapnia produces a CSF acidosis, cerebral vasodilatation and a rise in intracranial pressure — so carbon dioxide should be normalised gradually.

This distinction is the reason dexamethasone works around a cerebral tumour and does nothing after a stroke or a head injury, and the reason mannitol is less effective where the barrier is already open — the osmotic gradient it creates cannot be sustained across a leaking barrier, and the mannitol itself may cross and reverse the gradient with repeated dosing.

Two general rules fall out of that table and are worth stating in any answer. A quaternary ammonium compound does not enter the brain. And the barrier is a reason a drug may be less effective centrally than its lipid solubility predicts, because of active efflux.