Learning objective
Trace a drug from administration to systemic circulation and distinguish absorption from first-pass metabolism.
Separate movement from effect
Pharmacokinetics describes the time course of absorption, distribution, metabolism and excretion. Pharmacodynamics describes what a drug does at its targets. Giving the same amount by two routes need not produce the same concentration-time curve or response. Dose is the amount administered; concentration is an amount per volume in a sampled compartment.
Oral entry and first pass
A swallowed drug must be released from its formulation and cross the gastrointestinal lining. Absorbed drug commonly enters portal blood and reaches the liver before the general circulation. Metabolism in the gut wall and liver can reduce the amount of unchanged drug reaching systemic circulation. Absorption and first-pass loss are different steps. Bioavailability here means the fraction of an administered dose reaching systemic circulation unchanged; it does not mean the fraction bound to brain receptors.
Compare routes by the barrier crossed
Intravenous delivery enters blood directly and has no absorption step. Intramuscular and subcutaneous delivery still require movement from tissue into blood; formulation and perfusion affect the rate. Inhaled drug crosses lung tissue, while mucosal and transdermal delivery cross their respective surfaces. A patch can provide slow input; an injection is not necessarily rapid when a depot formulation is used. Rectal delivery does not guarantee complete first-pass avoidance.
| Route | Key step before systemic exposure |
|---|---|
| Oral | Dissolution and gut absorption, with possible gut/liver first-pass loss. |
| Intravenous | Direct blood entry; distribution and elimination still occur. |
| Intramuscular / subcutaneous | Absorption from tissue; depot release can prolong input. |
| Inhaled / mucosal / transdermal | Crossing lung, mucosal or skin barriers; rate depends on properties and formulation. |
Check your understanding
Supplemental knowledge check
Original PolyDiscipline practice based on the explanation and cited sources. Unscored; answers are temporary to this page visit and do not award mastery or XP.
Supplemental knowledge check
Original PolyDiscipline practice based on the explanation and cited sources. Unscored; answers are temporary to this page visit and do not award mastery or XP.
Explain it without notes
Trace oral and intravenous entry until both reach systemic blood.
Compare your explanation 1
Oral: formulation → gut lining → portal circulation → liver → systemic circulation. IV: direct systemic blood entry. Both then undergo distribution and elimination.
Why can two formulations of the same drug differ in onset?
Compare your explanation 2
Release and absorption rates can differ even when receptor action is unchanged. A concentration-time change is a pharmacokinetic explanation.
Revisit the connected course lessons and visuals
Julien’s Primer of Drug Action, supplied consolidated PDF pages 8, 9, 10, 11, 12, 13 (Chapter 1). Locators refer to PDF captures, not printed pages. Original paraphrase and practice; source conformance reviewed, not independently clinically certified.