Factors Affecting Drug Distribution: Physiological Barriers

Drug distribution is governed by several physiological barriers.

The simple capillary endothelial barrier limits the passage of drugs larger than 600 Daltons, including those bound to blood components like heparin.

The blood-brain barrier, or BBB, restricts water-soluble drugs like dopamine. So, Parkinson's disease is treated with levodopa, which can enter the CNS, where it converts into dopamine.

The blood-cerebrospinal fluid barrier primarily permits highly lipid-soluble drugs. Drugs that enter the CSF may struggle to achieve high concentrations due to continuous removal by the CSF's bulk flow. Sometimes, a drug's CSF concentration may surpass its cerebral concentration, as with sulfamethoxazole.

The blood-placental barrier allows moderate to highly lipid-soluble drugs with a molecular weight below 1000 Daltons to cross by simple diffusion. 

Lastly, the blood-testis barrier, formed by the tight junctions between Sertoli cells, restricts drug passage to spermatocytes. Efflux pumps further prevent the entry of cytotoxic drugs like doxorubicin.