Explore the system
Follow a drug through the body
Choose an entry route and trace absorption, circulation, tissue distribution, metabolism, and excretion.
Sources and model limits
Original schematic. Positions show relationships; structures and concentrations are not to scale.
- Julien's Primer of Drug Action · 15 · 1 · Follow a drug through the body · PDF 8, 10, 11, 12, 13 · 1.1, 1.3, 1.4
Course notes and existing practice
Learning objective
Distinguish systemic bioavailability from entry into the brain and identify factors that affect absorption.
Reaching systemic circulation
The lecture introduces bioavailability as the proportion of a dose reaching blood. For systemic availability, the useful distinction is the fraction reaching systemic circulation unchanged: absorption and loss before systemic entry both matter. IV administration delivers drug directly into circulation, so systemic bioavailability is 100%; it does not mean that the entire dose enters the brain.
Crossing an absorption surface
Surface area, formulation and membrane permeability affect absorption. Lipid solubility and ionization influence passive membrane passage. Local pH and a drug's pKa influence ionization, but absorption cannot be predicted by how close those two numbers are alone. Whether a drug is a weak acid or weak base, and the available surface area, also matter. The deck's pH example is a simplified teaching illustration.
- Systemic bioavailability
- The fraction of an administered dose that reaches systemic circulation unchanged.
Professor deck 04, PDF pages 5, 6. Course material is paraphrased. Qualifications and excluded clinical generalizations are documented in the Pharmacokinetics editorial record. Source review does not establish clinical validation.
Examples and practice explanations are original study aids, not professor quotations, exam requirements or medication instructions.
Julien connection: a distinction to explain on your own
For a worked route comparison, distinguish the amount absorbed from the amount reaching systemic circulation unchanged. In the Julien study example, 60 of 100 units are absorbed and another 20 are lost before systemic entry: bioavailability is 40%, not 60%.
Book connection: see the Julien Chapters 1–4 · Midterm study section in the Library for worked examples and recall practice. The cited source uses consolidated PDF page numbers.