Permeability-limited movement at both the capillary and cell membrane would occur in which tissues?

Permeability-limited movement at both the capillary and cell membrane would occur in which tissues?

• A. Skeletal muscle and adipose
• B. Lungs and liver
• C. CNS (BBB) and placenta
• D. Heart and kidney

Answer: (C)

Movement into a tissue can be limited by barriers both at the capillary wall and at the cell membranes. When both barriers are highly restrictive, a drug has to cross two tight barriers, making permeability the main limiting factor rather than blood flow. The CNS is the prime example because brain capillaries have tight junctions that form the blood-brain barrier, and once past the capillary, the lipid membranes of neural cells and the surrounding glial environment present another selective barrier. The placenta shares this dual restriction: placental capillaries are lined by selective barriers (syncytiotrophoblast and fetal endothelium) that limit transfer, often requiring specific transport mechanisms or favorable properties for passage. Other tissues tend to be governed more by blood flow or have less restrictive barriers, so permeability through two barriers isn’t the primary limiting factor there.

Movement into a tissue can be limited by barriers both at the capillary wall and at the cell membranes. When both barriers are highly restrictive, a drug has to cross two tight barriers, making permeability the main limiting factor rather than blood flow. The CNS is the prime example because brain capillaries have tight junctions that form the blood-brain barrier, and once past the capillary, the lipid membranes of neural cells and the surrounding glial environment present another selective barrier. The placenta shares this dual restriction: placental capillaries are lined by selective barriers (syncytiotrophoblast and fetal endothelium) that limit transfer, often requiring specific transport mechanisms or favorable properties for passage.

Other tissues tend to be governed more by blood flow or have less restrictive barriers, so permeability through two barriers isn’t the primary limiting factor there.