Pharmacology Chapter: Drug Distribution & Elimination
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Pharmacology Chapter: Drug Distribution & Elimination

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Questions and Answers

Which renal process involves the passive diffusion of drugs back across the tubular epithelium?

  • Reabsorption (correct)
  • Renal perfusion
  • Active tubular secretion
  • Glomerular filtration

    • What is the molecular weight (MW) threshold for effective biliary excretion of drugs?

  • 20 kDa
  • 500 Da (correct)
  • 1000 Da
  • 1 kDa

    • Which process is responsible for the active carrier-mediated elimination of drugs?

  • Tubular secretion (correct)
  • Reabsorption
  • Biliary excretion
  • Glomerular filtration

    • Which type of drugs are significantly excreted via pulmonary excretion?

    <p>Volatile molecules</p> Signup and view all the answers

    How does entero-hepatic circulation affect drug action?

    <p>Creates a reservoir of recirculating drug</p> Signup and view all the answers

    What particle size is generally not filtered during glomerular filtration?

    <p>Proteins greater than 68 kDa</p> Signup and view all the answers

    Which type of excretion is not efficient for most drugs due to their low molecular weight?

    <p>Biliary excretion</p> Signup and view all the answers

    What effect does conjugation to glucuronide or sulfate have on drug molecular weight?

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

    What is the primary process involved in drug distribution?

    <p>Transfer of drug from bloodstream to tissues</p> Signup and view all the answers

    What does the apparent volume of distribution (Vd) indicate?

    <p>The theoretical volume that a drug would occupy in the body</p> Signup and view all the answers

    Which organ plays a crucial role in drug metabolism?

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

    What is one of the main routes for drug elimination from the body?

    <p>Bile secretion</p> Signup and view all the answers

    Which statement describes first-order drug elimination kinetics?

    <p>The drug elimination rate is proportional to the concentration of the drug</p> Signup and view all the answers

    What is defined as the time required for the plasma concentration of a drug to decrease by half?

    <p>Half-life (t1/2)</p> Signup and view all the answers

    How does protein binding affect drug action?

    <p>It lowers the concentration of pharmacologically active drug</p> Signup and view all the answers

    Which type of drugs can readily move across most cell membranes?

    <p>Hydrophobic drugs without a net charge</p> Signup and view all the answers

    What is the general rule for achieving steady state concentration of a drug during constant IV infusion?

    <p>3-5 half-lives after starting dosing</p> Signup and view all the answers

    Which of the following statements accurately describes the plasma half-life (T1/2) of a drug?

    <p>It determines the time required to reach steady state concentration.</p> Signup and view all the answers

    What does the term 'apparent volume of distribution (Vd)' refer to?

    <p>The hypothetical volume that would be necessary to contain the total amount of drug in the body at the same concentration as in the plasma.</p> Signup and view all the answers

    What is a primary role of the liver in drug metabolism?

    <p>To convert lipophilic drugs into more polar metabolites.</p> Signup and view all the answers

    Which statement correctly describes first-order drug elimination kinetics?

    <p>The elimination rate is proportional to the drug concentration.</p> Signup and view all the answers

    What is the primary mechanism by which drugs are secreted into sweat?

    <p>Passive diffusion</p> Signup and view all the answers

    Which of the following drugs is known to cause potential harm to breast-feeding infants by incorporating into teeth?

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

    What does 'apparent volume of distribution (Vd)' refer to in pharmacokinetics?

    <p>The theoretical volume that a drug would occupy if distributed evenly in the body</p> Signup and view all the answers

    Which statement correctly describes first-order drug elimination kinetics?

    <p>Elimination is proportional to the concentration of the drug in the plasma</p> Signup and view all the answers

    What is the primary characteristic of zero-order kinetics in drug elimination?

    <p>Elimination takes place at a fixed maximum rate irrespective of the drug concentration</p> Signup and view all the answers

    Which drug is known to be associated with 'grey baby syndrome' due to ineffective metabolism in infants?

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

    What is the primary function of the liver in drug metabolism?

    <p>To detoxify and convert lipophilic compounds into hydrophilic forms</p> Signup and view all the answers

    What is the clinical relevance of a drug's half-life (t1/2)?

    <p>It determines the time required for the plasma concentration to decrease by half</p> Signup and view all the answers

    What determines the plasma half-life (t1/2) of a drug?

    <p>Activity of metabolising enzymes or excretion mechanisms</p> Signup and view all the answers

    In first-order kinetics, how is the plasma concentration of a drug generally expected to decline?

    <p>In a logarithmic fashion</p> Signup and view all the answers

    What does a high volume of distribution (Vd) imply about a drug's pharmacokinetics?

    <p>Drug is mainly located in tissues</p> Signup and view all the answers

    What occurs when steady-state is reached during a drug infusion?

    <p>Rate of drug administered equals rate of drug excreted</p> Signup and view all the answers

    After stopping a drug, how many half-lives are generally required for the plasma concentration to fall significantly?

    <p>5 to 6 half-lives</p> Signup and view all the answers

    What factor is crucial for determining the characteristics of first-order kinetics for a particular drug?

    <p>Specific metabolic pathways involved</p> Signup and view all the answers

    During which condition does the rate of drug concentration decrease align with first-order kinetics?

    <p>In the presence of excess substrate</p> Signup and view all the answers

    If the plasma concentration of a drug follows first-order kinetics, what would be expected at time 0 hours?

    <p>100% of drug in plasma</p> Signup and view all the answers

    What drugs are associated with potential harm to breast-feeding infants due to their ability to weaken teeth?

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

    Which statement accurately reflects zero-order kinetics in drug elimination?

    <p>Elimination occurs at a fixed maximum rate regardless of drug concentration.</p> Signup and view all the answers

    How does the pH of sweat affect the diffusion of drugs into sweat?

    <p>Higher pH reduces drug diffusion.</p> Signup and view all the answers

    What is a significant clinical consequence of Chloramphenicol for infants?

    <p>Bone marrow toxicity leading to 'grey baby' syndrome.</p> Signup and view all the answers

    What does the term 'half-life (t1/2)' indicate in pharmacokinetics?

    <p>Period needed for half of the drug's active substances to be eliminated.</p> Signup and view all the answers

    What best describes the concept of 'apparent volume of distribution (Vd)'?

    <p>The theoretical volume in which a drug would be uniformly distributed.</p> Signup and view all the answers

    In pharmacokinetics, what is primarily influenced by the liver's metabolic processes?

    <p>The transformation and elimination of drugs from systemic circulation.</p> Signup and view all the answers

    Which pharmacokinetic process describes drug elimination proportional to its concentration in the body?

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

    What is the expected percentage of drug concentration at steady state after 4 half-lives?

    <p>94%</p> Signup and view all the answers

    Which statement best describes the plasma half-life (T1/2) of a drug?

    <p>It determines the time to achieve steady state during dosing.</p> Signup and view all the answers

    What is generally understood by the term 'apparent volume of distribution (Vd)'?

    <p>The theoretical volume in which a drug appears to be distributed in the body.</p> Signup and view all the answers

    How long does it typically take for a drug to reach 50% of steady state during a constant IV infusion?

    <p>1 half-life</p> Signup and view all the answers

    Which option accurately describes a primary factor affecting the plasma half-life (T1/2) of a drug?

    <p>The patient's metabolic rate.</p> Signup and view all the answers

    What defines the apparent volume of distribution (Vd) in pharmacokinetics?

    <p>It is the volume in which a drug would need to be diluted to achieve its concentration in blood.</p> Signup and view all the answers

    Which body tissues generally experience rapid drug equilibration due to high perfusion?

    <p>Liver and heart</p> Signup and view all the answers

    What role does protein binding play in drug pharmacokinetics?

    <p>It can create a drug reservoir where bound drug can be released as free drug levels decrease.</p> Signup and view all the answers

    Which characteristic describes hydrophobic drugs?

    <p>They can easily permeate most cell membranes, facilitating distribution.</p> Signup and view all the answers

    What is a significant factor influencing the elimination rate of drugs in zero-order kinetics?

    <p>The elimination process remains constant regardless of the drug concentration.</p> Signup and view all the answers

    What determines the half-life (t1/2) of a drug?

    <p>It depends on both the volume of distribution and the clearance rate of the drug.</p> Signup and view all the answers

    How does the structure of capillaries in the brain affect drug distribution?

    <p>Endothelial cell tight junctions limit drug entry, creating the 'blood-brain barrier'.</p> Signup and view all the answers

    What is the consequence of protein binding for drug activity?

    <p>It lowers the concentration of active free drug in circulation.</p> Signup and view all the answers

    What factors can lead to altered protein binding of drugs in patients?

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

    What is the effect of administering aspirin in a patient already on warfarin?

    <p>Displaces warfarin from its binding sites</p> Signup and view all the answers

    Which statement correctly defines the apparent volume of distribution (Vd)?

    <p>Theoretical volume reflecting drug distribution outside of plasma</p> Signup and view all the answers

    What does a high volume of distribution (Vd) signify about a drug?

    <p>Drug extensively distributes to tissues</p> Signup and view all the answers

    What pharmacokinetic concept describes how plasma concentration relates to total drug amount in the body?

    <p>Volume of distribution (Vd)</p> Signup and view all the answers

    In what situation is a loading dose necessary?

    <p>To reach steady-state faster</p> Signup and view all the answers

    How is volume of distribution (Vd) calculated?

    <p>Dose divided by plasma concentration</p> Signup and view all the answers

    What effect does age have on drug distribution volume?

    <p>Lower capacity in elderly and neonates</p> Signup and view all the answers

    What happens to the distribution of a drug with a large volume of distribution?

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

    Which of the following conditions might cause competition for protein binding sites in pharmacotherapy?

    <p>Use of two drugs that bind proteins</p> Signup and view all the answers

    What determines the plasma half-life (t1/2) of a drug?

    <p>Activity of metabolizing enzymes and clearance mechanisms</p> Signup and view all the answers

    What characterizes first-order kinetics in drug elimination?

    <p>The rate of elimination is proportional to the drug's plasma concentration</p> Signup and view all the answers

    Which of the following describes how the volume of distribution (Vd) affects plasma half-life?

    <p>A high Vd leads to prolonged half-life</p> Signup and view all the answers

    What occurs at steady-state during drug infusion?

    <p>The amount of drug administered equals the amount eliminated</p> Signup and view all the answers

    How long does it generally take for the plasma concentration to fall significantly after stopping a drug?

    <p>5-6 half-lives</p> Signup and view all the answers

    What is the significance of a drug's plasma half-life (t1/2)?

    <p>It indicates how long a drug stays effective in the body</p> Signup and view all the answers

    In pharmacokinetics, what is implied by a drug being primarily located in tissues with a high volume of distribution (Vd)?

    <p>The drug distribution leads to slower elimination from the body</p> Signup and view all the answers

    What is represented by the logarithmic decay in the context of first-order kinetics?

    <p>A decline in percentage of drug in plasma consistent with half-lives</p> Signup and view all the answers

    What is the general rule for determining the time at which a drug reaches steady state during constant intravenous infusion?

    <p>3-5 half-lives after starting dosing</p> Signup and view all the answers

    At which time point is the plasma concentration of a drug expected to reach approximately 75% of steady state?

    <p>2 hours</p> Signup and view all the answers

    Which statement accurately describes the significance of the plasma half-life (T1/2) of a drug?

    <p>It indicates the time required for plasma concentration to decrease by half.</p> Signup and view all the answers

    How does the apparent volume of distribution (Vd) impact a drug's pharmacokinetics?

    <p>It indicates the distribution of a drug throughout body tissues relative to plasma level.</p> Signup and view all the answers

    What percentage of drug concentration is typically achieved at steady state after 3 half-lives?

    <p>87%</p> Signup and view all the answers

    Which renal process is specifically responsible for eliminating drugs that are protein-bound?

    <p>Active tubular secretion</p> Signup and view all the answers

    Which factor primarily contributes to the inefficiency of biliary excretion for most drugs?

    <p>The low molecular weight of most drugs</p> Signup and view all the answers

    What is the primary mechanism through which entero-hepatic circulation influences drug levels in the body?

    <p>Prolongs drug action by recycling drugs</p> Signup and view all the answers

    What type of drugs are most effectively excreted through pulmonary excretion?

    <p>Lipid-soluble drugs</p> Signup and view all the answers

    Which mechanism is primarily responsible for the excretion of glucuronide conjugates?

    <p>Biliary excretion</p> Signup and view all the answers

    What is the molecular weight (MW) threshold for a drug to be effectively excreted via biliary excretion?

    <p>Greater than 500 Da</p> Signup and view all the answers

    What is a characteristic of drugs that are efficiently reabsorbed in the renal tubules?

    <p>They have a high lipophilicity</p> Signup and view all the answers

    Which statement best describes the role of carrier-mediated mechanisms in drug excretion?

    <p>They allow transport of drugs against their concentration gradient.</p> Signup and view all the answers

    What physiological condition can lead to a reduced protein binding capacity of drugs?

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

    Which factor significantly influences the volume of distribution (Vd) of a drug?

    <p>Protein binding affinity</p> Signup and view all the answers

    What is the consequence of drug displacement from protein binding sites?

    <p>Increased plasma concentration of the displaced drug</p> Signup and view all the answers

    Which parameter reflects a drug's tendency to distribute beyond the plasma into tissues?

    <p>Apparent volume of distribution (Vd)</p> Signup and view all the answers

    In the example of aspirin and warfarin, what happens when aspirin is introduced during warfarin therapy?

    <p>Warfarin concentration decreases due to competitive binding</p> Signup and view all the answers

    How is the volume of distribution (Vd) calculated?

    <p>Dose divided by plasma concentration at time zero</p> Signup and view all the answers

    What term describes the theoretical volume of fluid a drug would occupy if uniformly distributed in the plasma?

    <p>Volume of distribution (Vd)</p> Signup and view all the answers

    Which demographic typically has a lower protein binding capacity for drugs?

    <p>A and C only</p> Signup and view all the answers

    Which scenario represents an enhanced risk of adverse effects during drug interactions?

    <p>Concurrent use of two highly protein-bound drugs</p> Signup and view all the answers

    What is primarily responsible for determining the plasma half-life (t1/2) of a drug?

    <p>Activity of metabolising enzymes and excretion mechanisms</p> Signup and view all the answers

    In a typical first-order elimination process, how is the plasma concentration expected to decrease over time?

    <p>It decreases at a rate proportional to its current concentration</p> Signup and view all the answers

    What occurs when steady-state concentration is achieved during drug infusion?

    <p>Elimination rate matches the administration rate</p> Signup and view all the answers

    How many half-lives are needed for a drug concentration to be considered significantly eliminated from the body?

    <p>5 to 6 half-lives</p> Signup and view all the answers

    What effect does a high volume of distribution (Vd) have on the plasma half-life of a drug?

    <p>It prolongs the half-life due to increased tissue binding</p> Signup and view all the answers

    Which of the following best describes the relationship between infusion rate and steady-state concentration?

    <p>Infusion and elimination rates must be equal at steady state</p> Signup and view all the answers

    What is indicated by achieving steady-state concentration during a constant IV infusion?

    <p>Rate in equals rate out</p> Signup and view all the answers

    Which statement accurately summarizes first-order elimination kinetics?

    <p>The elimination rate increases as drug concentration increases</p> Signup and view all the answers

    What characterizes first-order kinetics in terms of drug elimination?

    <p>The elimination is proportional to the drug concentration present.</p> Signup and view all the answers

    Which statement best describes zero-order kinetics during drug elimination?

    <p>A constant quantity of drug is eliminated regardless of its concentration.</p> Signup and view all the answers

    How does drug concentration in breast milk typically relate to plasma concentration?

    <p>It generally reflects the free concentration in blood.</p> Signup and view all the answers

    What is a clinical consequence of administering tetracyclines to breastfeeding mothers?

    <p>Incorporation into the teeth, leading to damage.</p> Signup and view all the answers

    In pharmacokinetics, what does a high apparent volume of distribution (Vd) suggest about a drug?

    <p>The drug is extensively distributed into the tissues.</p> Signup and view all the answers

    What is the primary factor affecting the time it takes for a drug to reach steady state concentration during IV infusion?

    <p>The half-life of the drug.</p> Signup and view all the answers

    Which aspect of drug elimination kinetics indicates a non-saturating process?

    <p>Elimination decreases in proportion to drug concentration.</p> Signup and view all the answers

    What condition is associated with Chloramphenicol when used in infants?

    <p>Bone marrow toxicity leading to 'grey baby' syndrome.</p> Signup and view all the answers

    Study Notes

    Drug Distribution

    • The process of a drug moving reversibly from the bloodstream into the extracelluar and/or intracellular fluids of tissues.
    • This process is dependent on blood flow.
    • Well perfused tissues (e.g., lungs, kidneys, liver, heart, brain, intestines) reach equilibrium with drugs quickly.
    • Less perfused tissues (e.g., peripheral organs, skeletal muscle, skin, connective tissue, fat) have a slower equilibration time.
    • Capillary structure is important for drug distribution.
    • Hydrophobic drugs easily cross most membranes, while hydrophilic drugs do not.
    • Protein binding (albumin and α1-acid glycoprotein) lowers the concentration of free drug.
    • Bound drugs are pharmacologically inactive.

    Drug Elimination

    • The body eliminates drugs primarily through the kidneys (renal excretion), liver (biliary excretion), lungs (pulmonary excretion), sweat (skin), and milk (mammary).
    • Renal Excretion: involves glomerular filtration, active tubular secretion, and reabsorption.
      • Glomerular filtration: Molecules less than 20 kDa are filtered. Protein-bound drugs are not filtered (albumin 68 kDa).
      • Tubular secretion: Active carrier-mediated elimination of both acidic and basic compounds. Can transport against electrochemical gradient and when drug is protein bound.
      • Reabsorption: Passive diffusion back across tubular epithelium. Lipid soluble drugs are excreted slowly.
    • Biliary excretion: Lipid-soluble drug uptake, metabolism, and excretion into bile. Reabsorption in the small intestine can occur, resulting in enterohepatic circulation.
    • Pulmonary excretion: Volatile molecules (e.g., anaesthetics, ethanol) are excreted via the lungs and breath.
    • Skin: Drugs are secreted in sweat by passive diffusion.
    • Milk: Drug concentration in milk generally reflects free concentration in blood.

    Elimination Kinetics

    • First-order kinetics:
      • The rate of elimination is proportional to the amount of drug present.
      • Characterized by a constant fraction of drug eliminated per unit of time.
      • Example: glomerular filtration.
    • Zero-order kinetics:
      • Elimination occurs at a fixed maximum rate, independent of drug concentration.
      • Example: protein-mediated reactions (ethanol metabolism).

    Half-life

    • Half-life (t1/2) is the time it takes for the plasma concentration of a drug to decrease by 50%.
    • It is a characteristic of the drug and the elimination process.
    • It is independent of dose.
    • It determines the time it takes to reach steady-state and the time required for elimination.
    • A long half-life means the drug is cleared more slowly from the body.
    • Steady-state is reached when the rate of drug administration equals the rate of drug elimination.
    • To achieve steady-state, it takes approximately 3-5 half-lives.

    Apparent Volume of Distribution (Vd)

    • Represents the theoretical volume of fluid in which a drug would distribute to produce the observed plasma concentration.
    • Not a real volume, it reflects the extent of drug distribution into tissues.
    • A high Vd signifies that the drug is extensively distributed in the tissues, meaning only a small portion resides within the plasma.
    • This can affect the half-life of the drug, leading to slower elimination.

    Liver in Drug Metabolism

    • Plays a significant role in drug metabolism.
    • The liver contains enzymes that can transform drugs into more polar, water-soluble metabolites.
    • Makes drugs easier to excrete from the body, mainly via the kidneys or bile.
    • Many drugs undergo metabolism in the liver, which alters their pharmacological activity and duration of action.

    Drug Distribution

    • Drug distribution: The reversible movement of a drug from the bloodstream into extracellular fluid and/or cells of the tissue (intracellular fluid).
    • Blood Flow: Well-perfused tissues (lungs, kidneys, liver, heart, brain, intestines) equilibrate rapidly with drugs; less perfused tissues (peripheral organs, skeletal muscle, skin, connective tissue, fat) equilibrate more slowly.
    • Capillary Permeability: The 'blood-brain barrier' restricts drug entry due to tight junctions between endothelial cells, reduced pores, and a layer of astrocytes. Hydrophobic drugs easily cross membranes.
    • Protein Binding: Reversible association of drugs with plasma proteins like albumin (acidic and neutral drugs) and α1-acid glycoprotein (basic drugs). Protein binding lowers the concentration of free drug, which is pharmacologically active.
    • Altered Protein Binding: Reduced protein binding can occur in hypoalbuminemia (liver disease), uraemia (kidney damage), and aging. Displacement from binding sites can occur due to competition, such as when aspirin displaces warfarin.

    Volume of Distribution (Vd)

    • Vd Definition: A theoretical volume that reflects the drug's distribution into tissues relative to plasma. It is NOT a real volume but a ratio.
    • Significance: Vd determines the relationship between plasma concentration and total drug in the body, as well as the dose required to achieve a specific plasma concentration.
    • Calculation: Vd = (Total amount of drug in body) / (Plasma concentration)

    Drug Elimination

    • First-order Kinetics: Elimination rate is proportional to the drug concentration (a constant fraction of the drug is eliminated per unit time).
    • Zero-order Kinetics: Elimination occurs at a constant rate, regardless of drug concentration.
    • Plasma Half-life (t1/2): Time for plasma concentration to decrease by 50%. Independent of the dose and characteristic for a specific drug.
    • Steady-state: Equilibrium reached when the rate of drug administration equals the rate of drug elimination. Aim to maintain steady-state concentration within the therapeutic range.

    Drug Elimination Routes

    • Kidney: Drugs eliminated via glomerular filtration, tubular secretion, or reabsorption.
    • Liver: Primary site for drug metabolism.
    • Lungs: Important for volatile gas elimination (e.g., anesthesia).
    • Skin: Drugs secreted into sweat via passive diffusion (depending on plasma/sweat partition).
    • Mammary Gland: Milk concentration reflects free drug concentration in blood.
      • Clinical relevance: Tetracyclines can cause tooth problems; chloramphenicol can cause bone marrow toxicity and 'grey baby syndrome' in babies.

    Half-life (t1/2) and Steady State

    • Half-life Determining Factors: Activity of metabolizing enzymes and excretion mechanisms (clearance), and drug distribution between blood and tissues (high Vd prolongs t1/2).
    • Time to Steady State: Generally takes 3-5 half-lives after starting constant IV infusion or regular oral dosing.

    Drug distribution - altered protein binding

    • Hypoalbuminaemia (low albumin) can occur in liver disease
    • Excess urea in blood from kidney damage (uraemia) can affect drug binding sites
    • Lower protein-binding capacity in foetuses, neonates, and elderly patients

    Competition example

    • Highly protein-bound drugs (aspirin) can displace other drugs, like warfarin (anticoagulant), from protein binding sites
    • Warfarin is ~98% protein bound, so only a small fraction (0.1 mg) is free to work from a 5 mg dose.
    • Aspirin competes for binding sites and can displace warfarin, increasing its free concentration.

    Apparent volume of distribution (Vd)

    • Represents the theoretical volume a drug would occupy if it was evenly distributed in the body at the same concentration as in the plasma.
    • Not a real physical volume but indicates the ratio of drug in extravascular space versus plasma space.
    • Higher Vd indicates a greater tendency of the drug to move out of the plasma into other tissues.

    Calculation of volume of distribution (Vd)

    • Formula for Vd: Total amount of drug in the body / Plasma concentration.
    • Can also be calculated by dividing the dose by the plasma concentration at time zero.

    Renal Drug Excretion

    • Three main processes:
      • Glomerular filtration: Filters molecules smaller than 20 kDa, protein-bound drugs are not filtered.
      • Tubular secretion: Active carrier-mediated elimination, transporting drugs against electrochemical gradients.
      • Reabsorption: Passive diffusion of lipid-soluble drugs back across the tubular epithelium, leading to slower excretion.

    Biliary Excretion

    • Hepatocytes take up lipid-soluble drugs, metabolize them, and excrete them into bile.
    • Drugs can be reabsorbed in the small intestine, leading to inefficient excretion.
    • Efficient biliary excretion requires a high molecular weight (MW > 500 Da).
    • Drug conjugation with glucuronide or sulphate can increase MW for efficient excretion.

    Enterohepatic Circulation

    • Drug conjugates are hydrolysed by bacteria in the lower intestine, releasing active drug.
    • Free drug is reabsorbed, creating a reservoir of recirculating drugs, which prolongs their action.

    Pulmonary Excretion

    • Volatile molecules are excreted via the lungs, e.g., anaesthetics, ethanol.

    Other Excretion Routes

    • Sweat: Drugs secreted into sweat by passive diffusion, depends on plasma/sweat partition.
    • Milk: Drug concentration in milk reflects free concentration in blood.

    Rates of Reaction (Elimination)

    • First-Order Kinetics: rate of elimination proportional to the amount of drug present. Non-saturating kinetics.
    • Zero-Order Kinetics: elimination occurs at a fixed maximum rate, independent of drug concentration. Saturation kinetics.

    Plasma half-life (t1/2)

    • Time for plasma concentration to fall by 50%.
    • Determined by:
      • Activity of metabolising enzymes or excretion mechanisms
      • Distribution of drug between blood and tissues
    • Clinical relevance:
      • Time it takes for a drug to reach a steady-state.
      • Time it takes for a drug to be eliminated from the body.

    Steady-State

    • When the rate of drug administration equals the rate of drug elimination.
    • Goal of drug therapy is to maintain a steady-state concentration within the therapeutic range.

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    Pharmacokinetics: Drug Distribution, Metabolism, And Elimination PDF

    Description

    Explore the key concepts of drug distribution and elimination in this quiz. Understand how blood flow and tissue perfusion influence drug movement and the processes involved in drug excretion from the body. Test your knowledge on the factors affecting these critical pharmacological processes.

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