Pharmacology: Absorption of Drugs
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Questions and Answers

What is the primary driving force for passive diffusion of a drug?

  • Saturable energy-dependent process
  • Carrier proteins that facilitate the passage of large molecules
  • Hydrolysis of adenosine triphosphate (ATP)
  • Concentration gradient across a membrane (correct)

    • Which type of diffusion does not require energy?

  • Passive diffusion
  • Endocytosis
  • Facilitated diffusion (correct)
  • Active transport

    • What type of drugs can penetrate the cell membrane through aqueous channels or pores?

  • Water-soluble drugs (correct)
  • Facilitated diffusion drugs
  • Active transport drugs
  • Lipid-soluble drugs

    • Which mode of drug entry involves specific carrier proteins that span the membrane?

    <p>Active transport</p> Signup and view all the answers

    What type of diffusion is capable of moving drugs against a concentration gradient?

    <p>Active transport</p> Signup and view all the answers

    What is the difference between facilitated diffusion and active transport?

    <p>Energy requirement</p> Signup and view all the answers

    What type of drug absorption is influenced by the route of administration?

    <p>Bioavailability</p> Signup and view all the answers

    What is the result of administering drugs through routes other than intravenous?

    <p>Partial absorption</p> Signup and view all the answers

    What percentage of bioavailability is conferred by IV administration?

    <p>100%</p> Signup and view all the answers

    What occurs when a drug is rapidly metabolized in the liver or gut wall during its initial passage?

    <p>First-pass metabolism</p> Signup and view all the answers

    Why are very hydrophilic drugs poorly absorbed?

    <p>Because they are unable to cross lipid-rich cell membranes</p> Signup and view all the answers

    What type of drugs are often administered via the sublingual, transdermal, or intravenous route?

    <p>Drugs with high first-pass metabolism</p> Signup and view all the answers

    What determines the rate and extent to which a drug reaches the systemic circulation?

    <p>All of the above</p> Signup and view all the answers

    What is the area under the curve (AUC) used to measure?

    <p>Plasma concentrations of the drug versus time</p> Signup and view all the answers

    Why are drugs that are extremely lipophilic poorly absorbed?

    <p>Because they are insoluble in aqueous body fluids</p> Signup and view all the answers

    What is the reason why many drugs are either weak acids or weak bases?

    <p>To ensure solubility in aqueous solutions</p> Signup and view all the answers

    What is the primary characteristic of active transport systems?

    <p>They are selective and may be competitively inhibited</p> Signup and view all the answers

    What is the process by which a drug is engulfed by the cell membrane and transported into the cell?

    <p>Endocytosis</p> Signup and view all the answers

    Which of the following substances is transported across the gut wall by endocytosis?

    <p>Vitamin B12</p> Signup and view all the answers

    What is the primary factor that determines the effective concentration of the permeable form of a drug at its absorption site?

    <p>All of the above</p> Signup and view all the answers

    Which of the following is true about the permeation of weak acids through membranes?

    <p>The uncharged, protonated HA can permeate through membranes</p> Signup and view all the answers

    What is the primary characteristic of weak bases that influences their permeation through membranes?

    <p>The protonated form is usually charged</p> Signup and view all the answers

    Which of the following is an example of a substance that is stored in intracellular vesicles in the nerve terminal and released by exocytosis?

    <p>Norepinephrine</p> Signup and view all the answers

    What is the primary difference between endocytosis and exocytosis?

    <p>Endocytosis is used for absorption, while exocytosis is used for secretion</p> Signup and view all the answers

    What is the formula to calculate Vd?

    <p>Dose / C0</p> Signup and view all the answers

    What is the primary route of elimination for lipophilic drugs?

    <p>Hepatic metabolism</p> Signup and view all the answers

    What is the result of drug metabolism?

    <p>Production of products with increased polarity</p> Signup and view all the answers

    What is the approximate volume of distribution of heparin in a 70-kg individual?

    <p>4 L</p> Signup and view all the answers

    What is the primary role of the kidney in drug elimination?

    <p>Elimination of polar drugs into the urine</p> Signup and view all the answers

    What is the characteristic of drugs that distribute into the extracellular fluid?

    <p>Low molecular weight and hydrophilicity</p> Signup and view all the answers

    What is the rate of elimination for most drugs?

    <p>First-order kinetics</p> Signup and view all the answers

    What is the approximate volume of distribution of aminoglycoside antibiotics in a 70-kg individual?

    <p>14 L</p> Signup and view all the answers

    What is the characteristic of drugs that distribute into the total body water?

    <p>Low molecular weight and lipophilicity</p> Signup and view all the answers

    What is the purpose of phase I and phase II reactions?

    <p>To increase the polarity of drugs</p> Signup and view all the answers

    What is clearance (CL) a measure of?

    <p>The amount of drug cleared from the body per unit of time</p> Signup and view all the answers

    What is the approximate volume of distribution of ethanol in a 70-kg individual?

    <p>42 L</p> Signup and view all the answers

    What is the plot of drug concentrations in plasma after a single injection of drug at time = 0?

    <p>A log scale with an exponential decrease</p> Signup and view all the answers

    What is the relationship between drug clearance and Vd?

    <p>Drug clearance is a first-order process that allows calculation of Vd</p> Signup and view all the answers

    What is the meaning of C0 in the context of Vd?

    <p>The concentration of drug that would have been achieved if the distribution phase had occurred instantly</p> Signup and view all the answers

    What does a larger Vd indicate?

    <p>Greater distribution into tissues</p> Signup and view all the answers

    Study Notes

    Mechanisms of Absorption

    • Absorption of drugs from the GI tract depends on chemical properties and can occur through:
      • Passive diffusion: driven by concentration gradient, no carrier, not saturable, low structural specificity
      • Facilitated diffusion: uses carrier proteins, no energy required, saturable, inhibited by competing compounds
      • Active transport: uses carrier proteins, energy-dependent, saturable, can move against concentration gradient
      • Endocytosis and exocytosis: for large drugs, involves engulfment and transport across cell membrane

    Factors Influencing Absorption

    • pH affects drug absorption: weak acids and bases are more permeable in their uncharged form
    • Ionization constant (pKa) affects the ratio of charged and uncharged forms
    • Bioavailability: affected by first-pass metabolism, solubility, and chemical characteristics of the drug
    • First-pass metabolism: occurs in liver and gut wall, reduces amount of unchanged drug entering systemic circulation
    • Solubility: very hydrophilic or lipophilic drugs are poorly absorbed due to inability to cross cell membranes

    Distribution of Drugs

    • Volume of distribution (Vd): the apparent volume of fluid that the drug would have to be dissolved in to produce the observed plasma concentration
    • Types of distribution:
      • Plasma volume: for drugs with low Vd, approximately 4 L in a 70-kg individual
      • Extracellular fluid: for hydrophilic drugs, approximately 14 L in a 70-kg individual
      • Total body water: for lipophilic drugs, approximately 42 L in a 70-kg individual

    Determination of Vd

    • Calculation of Vd: uses the dose and extrapolated plasma concentration at time zero
    • Formula: Vd = Dose / C0

    Drug Clearance

    • Elimination of drugs occurs through:
      • Hepatic metabolism
      • Biliary elimination
      • Urinary elimination
    • Most drugs are eliminated according to first-order kinetics
    • Clearance (CL) estimates the amount of drug cleared from the body per unit of time
    • Metabolism leads to production of polar products, allowing for elimination
    • Lipid-soluble drugs are metabolized in the liver to increase polarity before elimination

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    Description

    Learn about the factors influencing drug absorption, including environment, chemical characteristics, and route of administration. Understand the mechanisms of absorption from the GI tract.

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