The blood-brain barrier (BBB) is primarily formed by the tight junctions of cerebral capillary endothelial cells. Which of the following substances crosses the intact BBB most readily?
- A Glucose (via GLUT1) ✓
- B Albumin (MW 66,000 Da)
- C Penicillin G (high ionisation at physiological pH)
- D Dopamine (a polar catecholamine)
Correct answer: A. Glucose (via GLUT1)
Explanation
The intact BBB is highly restrictive to large molecules, polar molecules, and ions. Glucose, despite being a polar molecule, crosses readily via GLUT1 (SLC2A1) carrier-mediated facilitated diffusion — the brain is entirely dependent on this for its primary energy substrate. Albumin, penicillin G (highly ionised), and dopamine (polar catecholamine) cross poorly. Lipid-soluble drugs (like anaesthetics, diazepam) and small gases (CO2, O2) cross freely by diffusion.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.