The blood-brain barrier (BBB) is maintained by tight junctions between cerebral capillary endothelial cells. Which substance crosses the BBB most readily by simple passive diffusion?

• A Carbon dioxide ✓
• B Glucose
• C L-DOPA (levodopa)
• D Penicillin

Explanation

Small, lipid-soluble, non-ionized molecules cross the BBB most readily by simple passive diffusion. CO2 (and O2) are lipophilic gases that diffuse freely across all biological membranes including the BBB. Glucose requires GLUT1 transporters (facilitated diffusion — it cannot cross by simple diffusion due to its size and hydrophilicity). L-DOPA requires the large neutral amino acid (LAT1) transporter. Penicillin crosses the BBB poorly under normal conditions due to its ionized, hydrophilic nature and active efflux by P-glycoprotein.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.