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In maternal circulation, most drugs are primarily transported bound to albumin. Albumin is a major plasma protein that serves several important functions, one of which is to bind and transport various substances, including drugs, hormones, and fatty acids. The binding of drugs to albumin helps regulate the free (active) concentration of the drug in the bloodstream, which is crucial for pharmacokinetics and pharmacodynamics. When drugs are bound to albumin, they are generally in a biologically inactive form, and only the free drug is available to exert effects on tissues.

Other proteins in the blood, although present, do not play as significant a role in drug transport as albumin. For instance, hemoglobin specifically transports oxygen and carbon dioxide in red blood cells rather than being primarily involved in drug transport. Collagen is not involved in protein transport within circulation. Globulins, while involved in immune responses and various other functions, are not the dominant carrier proteins for drugs in circulation like albumin. Therefore, albumin's role as the main transport protein underscores its importance in the context of maternal pharmacology.