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Questions and Answers
What is the primary driving force for passive diffusion of a drug?
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?
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?
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?
Which mode of drug entry involves specific carrier proteins that span the membrane?
What type of diffusion is capable of moving drugs against a concentration gradient?
What type of diffusion is capable of moving drugs against a concentration gradient?
What is the difference between facilitated diffusion and active transport?
What is the difference between facilitated diffusion and active transport?
What type of drug absorption is influenced by the route of administration?
What type of drug absorption is influenced by the route of administration?
What is the result of administering drugs through routes other than intravenous?
What is the result of administering drugs through routes other than intravenous?
What percentage of bioavailability is conferred by IV administration?
What percentage of bioavailability is conferred by IV administration?
What occurs when a drug is rapidly metabolized in the liver or gut wall during its initial passage?
What occurs when a drug is rapidly metabolized in the liver or gut wall during its initial passage?
Why are very hydrophilic drugs poorly absorbed?
Why are very hydrophilic drugs poorly absorbed?
What type of drugs are often administered via the sublingual, transdermal, or intravenous route?
What type of drugs are often administered via the sublingual, transdermal, or intravenous route?
What determines the rate and extent to which a drug reaches the systemic circulation?
What determines the rate and extent to which a drug reaches the systemic circulation?
What is the area under the curve (AUC) used to measure?
What is the area under the curve (AUC) used to measure?
Why are drugs that are extremely lipophilic poorly absorbed?
Why are drugs that are extremely lipophilic poorly absorbed?
What is the reason why many drugs are either weak acids or weak bases?
What is the reason why many drugs are either weak acids or weak bases?
What is the primary characteristic of active transport systems?
What is the primary characteristic of active transport systems?
What is the process by which a drug is engulfed by the cell membrane and transported into the cell?
What is the process by which a drug is engulfed by the cell membrane and transported into the cell?
Which of the following substances is transported across the gut wall by endocytosis?
Which of the following substances is transported across the gut wall by endocytosis?
What is the primary factor that determines the effective concentration of the permeable form of a drug at its absorption site?
What is the primary factor that determines the effective concentration of the permeable form of a drug at its absorption site?
Which of the following is true about the permeation of weak acids through membranes?
Which of the following is true about the permeation of weak acids through membranes?
What is the primary characteristic of weak bases that influences their permeation through membranes?
What is the primary characteristic of weak bases that influences their permeation through membranes?
Which of the following is an example of a substance that is stored in intracellular vesicles in the nerve terminal and released by exocytosis?
Which of the following is an example of a substance that is stored in intracellular vesicles in the nerve terminal and released by exocytosis?
What is the primary difference between endocytosis and exocytosis?
What is the primary difference between endocytosis and exocytosis?
What is the formula to calculate Vd?
What is the formula to calculate Vd?
What is the primary route of elimination for lipophilic drugs?
What is the primary route of elimination for lipophilic drugs?
What is the result of drug metabolism?
What is the result of drug metabolism?
What is the approximate volume of distribution of heparin in a 70-kg individual?
What is the approximate volume of distribution of heparin in a 70-kg individual?
What is the primary role of the kidney in drug elimination?
What is the primary role of the kidney in drug elimination?
What is the characteristic of drugs that distribute into the extracellular fluid?
What is the characteristic of drugs that distribute into the extracellular fluid?
What is the rate of elimination for most drugs?
What is the rate of elimination for most drugs?
What is the approximate volume of distribution of aminoglycoside antibiotics in a 70-kg individual?
What is the approximate volume of distribution of aminoglycoside antibiotics in a 70-kg individual?
What is the characteristic of drugs that distribute into the total body water?
What is the characteristic of drugs that distribute into the total body water?
What is the purpose of phase I and phase II reactions?
What is the purpose of phase I and phase II reactions?
What is clearance (CL) a measure of?
What is clearance (CL) a measure of?
What is the approximate volume of distribution of ethanol in a 70-kg individual?
What is the approximate volume of distribution of ethanol in a 70-kg individual?
What is the plot of drug concentrations in plasma after a single injection of drug at time = 0?
What is the plot of drug concentrations in plasma after a single injection of drug at time = 0?
What is the relationship between drug clearance and Vd?
What is the relationship between drug clearance and Vd?
What is the meaning of C0 in the context of Vd?
What is the meaning of C0 in the context of Vd?
What does a larger Vd indicate?
What does a larger Vd indicate?
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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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