The Journey of a Drug: An Overview of Absorption
Absorption is the first phase of pharmacokinetics, describing the movement of a drug from its site of administration into the bloodstream. The journey of a drug and its ultimate destination depend heavily on the route of administration, the drug's properties, and physiological factors within the patient. For a drug to exert its intended therapeutic effect, it must first be successfully absorbed and distributed throughout the body to reach its target site.
Oral (Enteral) Absorption: The Gastrointestinal Tract
The most common and convenient route of drug administration is oral ingestion. While some absorption can start in the stomach, the majority happens in the small intestine due to its large surface area and favorable environment. The acidic stomach (pH 1.5–3.5) favors weak acid absorption, but most drugs absorb better in the less acidic small intestine (pH 4–8). Factors like gastric emptying speed, intestinal movement, drug transporters, and metabolizing enzymes (like CYP enzymes) in the gut wall and liver influence absorption and can lead to the first-pass effect. Some drugs are absorbed in the large intestine, and rectal administration also allows absorption with partial bypass of first-pass metabolism.
Parenteral Absorption: Bypassing the Gut
Parenteral routes bypass the GI tract and first-pass metabolism. Intravenous (IV) injection delivers drugs directly into the bloodstream for immediate effect and 100% bioavailability. Intramuscular (IM) and subcutaneous (SC) injections involve injecting into muscle or fatty tissue, respectively. Absorption from these sites is slower than IV but faster than oral, with IM often quicker due to better blood supply.
Other Routes of Absorption
Transdermal delivery applies drugs to the skin for slow, sustained systemic absorption. Topical applications are primarily local, though some systemic absorption can occur. The skin's outer layer is a barrier. Inhaled drugs absorb rapidly through the lungs, bypassing first-pass metabolism. Sublingual and buccal routes under the tongue or in the cheek allow rapid absorption through the oral lining into the bloodstream, also avoiding first-pass metabolism.
Key Mechanisms of Drug Transport
Drugs cross membranes via several mechanisms. For detailed information on these mechanisms, including passive diffusion, active transport, facilitated diffusion, and pinocytosis, refer to {Link: Merck Manuals https://www.merckmanuals.com/professional/clinical-pharmacology/pharmacokinetics/drug-absorption}.
Comparison of Absorption Routes
A table summarizing the typical characteristics of various drug administration routes regarding absorption sites and speed is available in {Link: Merck Manuals https://www.merckmanuals.com/professional/clinical-pharmacology/pharmacokinetics/drug-absorption}.
Conclusion
The site of drug absorption varies widely depending on the chosen route of administration, from the tennis court-sized surface of the small intestine to the highly vascularized lung alveoli. The speed and extent of a drug's journey into the bloodstream are dictated by a complex interplay of the drug's physicochemical properties, the specific absorption mechanisms at play, and various physiological factors. A medication's design, from enteric-coated tablets to transdermal patches, is carefully engineered to leverage or bypass specific absorption sites to achieve the desired therapeutic effect. For further reading, an authoritative resource on the principles of pharmacology can be found at the {Link: National Center for Biotechnology Information https://www.ncbi.nlm.nih.gov/books/NBK595006/}.
