Certain physiological barriers regulate drug distribution, protecting tissues from harmful substances but also limiting drug access:
1) Blood-Brain Barrier (BBB)
Composed of tight endothelial junctions, it prevents many drugs from entering the brain.
Only highly lipophilic drugs or those using specialized transporters can cross.
2) Blood-Testes Barrier (BTB)
Protects developing sperm from harmful substances but restricts drug entry to the testes.
3) Placental Barrier
Separates maternal and fetal circulation, protecting the fetus from toxins but limiting drug transfer.
4) Epithelial Barriers
Found in the skin, respiratory tract, GIT, and urinary tract.
Regulates the movement of molecules but can hinder drug penetration.
5) Renal Barrier
Prevents loss of essential proteins while allowing waste excretion, restricting drug passage to the kidneys.
6) Hepatic Barrier
Limits harmful substances from entering circulation and plays a role in drug metabolism and excretion.
7) Pulmonary Barrier
Protects against airborne toxins while allowing gas exchange but can restrict drug absorption in the lungs.
