Do polar molecules reach equilibrium faster in brain or muscles? why?

Do polar molecules reach equilibrium faster in brain or muscles? why?

• A. Brain, because lipophilicity of brain tissue
• B. Muscles, because permeability is the limiting factor
• C. Brain, because it's highly perfused
• D. Muscles, because brain is lipophilic

Answer: (B)

For polar molecules, how quickly they reach equilibrium between plasma and tissue is governed mainly by membrane permeability, not just blood flow. The brain has a blood–brain barrier with tight junctions and transporters that greatly restrict entry of polar compounds. Even though the brain is highly perfused, this barrier slows the rate at which polar drugs can enter brain tissue, so equilibration with plasma takes longer. Muscle tissue, on the other hand, lacks this restrictive barrier, so polar molecules cross its membranes more readily. With adequate perfusion, they diffuse into muscle tissue and approach plasma concentrations faster. So polar molecules reach equilibrium faster in muscle because permeability to them is less restricted there. The brain’s limited permeability, due to the blood–brain barrier, is the reason it equilibrates more slowly.

For polar molecules, how quickly they reach equilibrium between plasma and tissue is governed mainly by membrane permeability, not just blood flow. The brain has a blood–brain barrier with tight junctions and transporters that greatly restrict entry of polar compounds. Even though the brain is highly perfused, this barrier slows the rate at which polar drugs can enter brain tissue, so equilibration with plasma takes longer.

Muscle tissue, on the other hand, lacks this restrictive barrier, so polar molecules cross its membranes more readily. With adequate perfusion, they diffuse into muscle tissue and approach plasma concentrations faster.

So polar molecules reach equilibrium faster in muscle because permeability to them is less restricted there. The brain’s limited permeability, due to the blood–brain barrier, is the reason it equilibrates more slowly.