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Skin is the most commonly studied biological barrier and its structure is similar to some of others (such as the tympanic membrane). Transdermal drug delivery can be intended to deliver drugs locally, i.e. to the skin itself.

Although proteins are a significant fraction in plasma composition, they also exist beyond the bloodstream and bind with drugs in the skin, tissues, and organs. Protein binding influences the bioavailability and distribution of active compounds and is a limiting factor in the passage of drugs across biological membranes and barriers: drugs are often unable to cross membranes mainly due to the high molecular mass of the drug-protein complex, thus resulting in the accumulation of the active compounds and a significant reduction of their pharmacological activity.

Explanation:

mistakes are ok

vapour or gas assuming atmospheriv pressure