Drug distribution in the body is intricately regulated by various physiological barriers that control the passage of substances. These include the capillary endothelial barrier, the blood-brain, blood-cerebrospinal fluid, blood-placental, and blood-testis barriers.

The capillary endothelial barrier allows only smaller molecules below 600 Da (Daltons) to pass through. It also restricts drugs like heparin that are bound to blood components, limiting their movement within the bloodstream.

The blood-brain barrier (BBB) is a specialized, semi-permeable cell membrane that separates the circulating blood from the brain's extracellular fluid. This barrier consists of tightly packed endothelial cells, astrocytic end-feet, and a basement membrane, creating a highly selective interface. The BBB allows only certain substances, such as small lipophilic molecules and essential nutrients, to pass through while restricting the entry of many drugs, particularly water-soluble ones like dopamine. However, in the case of neurological disorders such as Parkinson's disease, levodopa, a drug that can cross the BBB, is administered. Inside the central nervous system (CNS), levodopa is converted into dopamine, mitigating the symptoms of Parkinson's disease.

The blood-cerebrospinal fluid (CSF) barrier, which separates the blood from the CSF surrounding the brain and spinal cord, permits the passage of highly lipophilic drugs. Although drugs can enter the CSF, they face challenges in maintaining high concentrations due to continuous removal by the CSF's bulk flow. Interestingly, certain drugs, like sulfamethoxazole, can achieve higher concentrations in the CSF than in the cerebral region.

The blood-placental barrier during pregnancy allows the passage of moderate to highly lipid-soluble drugs with a molecular weight below 1000 Da. Substances like barbiturates, steroids, and anticonvulsants can cross this barrier via simple diffusion, influencing fetal exposure to these drugs.

In addition to these barriers, the blood-testis barrier, formed by tight junctions between Sertoli cells in the testes, plays a vital role. It restricts the passage of drugs to spermatocytes, protecting developing sperm cells. Efflux pumps further reinforce this barrier by preventing the entry of cytotoxic drugs like doxorubicin, safeguarding the delicate process of spermatogenesis.

These physiological barriers act as complex gatekeepers, regulating the distribution of drugs within the body, ensuring targeted drug delivery, and preserving the integrity of vital organs and processes. Understanding these barriers is essential for developing effective drug therapies and minimizing potential side effects.