Will ammonia cross the blood-brain barrier?

Ammonia is a small, uncharged molecule that can cross biological membranes, including the blood-brain barrier. The blood-brain barrier is selectively permeable, designed to allow certain substances to enter the brain while restricting others. Due to its small size and neutral charge, ammonia can diffuse through the lipid bilayer of the cells that make up the blood-brain barrier.

When ammonia levels in the blood become elevated, it can cross into the central nervous system, potentially leading to neurological effects and conditions such as hepatic encephalopathy. While certain conditions may influence the degree to which ammonia crosses the barrier, the fundamental capability of ammonia to cross it exists, thereby affirming that it can indeed pass into the brain under normal physiological conditions.

The other options imply restrictions that do not align with the properties of ammonia as a molecule. Ammonia does not require specific conditions, such as severe liver disease or acute metabolic crises, to cross the blood-brain barrier, although those conditions might exacerbate the impact of ammonia on neuronal function.