Questions and Answers
What is pharmacokinetics?
Pharmacokinetics denotes the effects of biologic systems on drugs.
What are the major routes of drug administration?
Enteral, parenteral, and topical.
What is absorption in pharmacokinetics?
Absorption is the transfer of a drug from the site of administration to the bloodstream.
What is bioavailability?
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What is half-life in pharmacology?
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What is a mechanism of absorption?
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Intravenous (IV) administration results in 100% bioavailability.
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What factors influence drug absorption?
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Which of the following factors can influence bioavailability?
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What does the volume of distribution (Vd) relate to?
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Zero-order elimination means the rate of elimination is proportional to concentration.
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Study Notes
Pharmacokinetics Overview
- Pharmacokinetics involves the effects of biological systems on drugs, including absorption, distribution, metabolism, and elimination.
- Bioavailability refers to the fraction of an administered drug dose that reaches systemic circulation.
Routes of Drug Administration
- Determined by drug properties (water/lipid solubility, ionization) and therapeutic objectives (rapid onset, long-term treatment, localized delivery).
- Major routes include enteral (oral), parenteral (IV, IM), and topical.
Absorption of Drugs
- Absorption is the transfer of a drug from administration site to bloodstream.
- Rate and extent of absorption are influenced by the environment, drug characteristics, and administration route.
- IV administration achieves 100% bioavailability; non-IV routes may have partial absorption.
Mechanisms of Drug Absorption
- Passive Diffusion: Most common; relies on concentration gradient and does not require energy or a carrier.
- Facilitated Diffusion: Involves specialized carrier proteins; does not require energy but can be saturated.
- Active Transport: Energy-dependent; capable of moving against a concentration gradient and is saturable.
- Endocytosis/Exocytosis: Engulfment of drug by cell membranes; examples include Vitamin B12 (endocytosis) and norepinephrine (exocytosis).
Factors Influencing Absorption
- Drug Ionization: Weak acids/bases affect absorption; uncharged (non-ionized) forms pass through membranes more readily.
- Blood Flow: Intestinal blood flow is higher than gastric, favoring absorption in the intestines.
- Surface Area: Intestinal surface area is vastly larger than stomach, enhancing absorption.
- Contact Time: Rapid transit through the GI tract (e.g., diarrhea) reduces absorption; food can slow gastric emptying.
- P-glycoprotein: A transporter that can limit absorption by pumping drugs back into blood or bile.
Bioavailability Considerations
- Determined by comparing plasma drug levels from different administration routes.
- Influenced by factors such as first-pass metabolism, drug solubility (lipophilic vs. hydrophilic), chemical stability, and formulation characteristics.
Drug Distribution
- Distribution refers to how a drug exits the bloodstream and enters tissues.
- Dependent on blood flow, capillary permeability, binding to proteins, lipophilicity, and volume of distribution (Vd).
Volume of Distribution (Vd)
- Relates the total amount of drug in the body to its plasma concentration.
Elimination Kinetics
- First-Order Elimination: Rate proportional to drug concentration; characteristic half-life remains constant across concentrations.
- Zero-Order Elimination: Rate is constant regardless of concentration; occurs when elimination pathways are saturated (e.g., ethanol, phenytoin).
Half-Life
- Represents the time for the drug concentration to reduce by half.
- Steady-state is typically achieved after 4-5 half-lives at a constant dosing rate.
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Description
Explore the fundamentals of pharmacokinetics in this quiz, designed to assess your understanding of drug administration routes, absorption mechanisms, and bioavailability. Delve into the critical factors affecting drug absorption and learn about the importance of half-life in pharmacology.
