NURS5205 Chap.2 Pharmacokinetics and Pharmacodynamics
Pharmacokinetics and Pharmacodynamics
Pharmacokinetics explains absorption, distribution, metabolism and excretion, while pharmacodynamics explains targets, effects and response. This chapter uses that separation to explain variability between patients, the roles of clearance, volume of distribution, bioavailability and half-life, and the difference between changing exposure and changing sensitivity.
Numerical work stays tied to units and a clinical interpretation.
What this chapter covers
- 01
ADME and routes of administration
- 02
Clearance and dose rate
- 03
Volume of distribution and loading dose
- 04
Bioavailability and route conversion
- 05
Half-life and dosing frequency
- 06
Targets, agonists and antagonists
- 07
Patient variability and interactions
Worked example · free
Comparing theoretical loading doses
- +1This practice weighting is not a published University mark allocation. Use loading dose equals target concentration multiplied by apparent volume when bioavailability is one.
- +1Medicine A is 8 mg/L multiplied by 40 L, giving 320 mg.
- +1Medicine B is 8 mg/L multiplied by 70 L, giving 560 mg.
- +1Interpret the difference as a volume relationship, not a real prescribing instruction or a dosing-frequency result.
Key terms
- Absorption
- Movement of a drug from its administration site into systemic circulation.
- Clearance
- The efficiency of drug elimination, expressed as a volume per unit time.
- Potency
- The amount or concentration required to produce a specified effect, which is distinct from maximal efficacy.
- Affinity
- The tendency of a drug to bind to its target, which does not alone describe the effect after binding.
Pharmacokinetics and Pharmacodynamics FAQ
What is the quickest way to separate PK from PD?
Ask whether the statement concerns drug concentration or the body's response to that concentration. Absorption, clearance and metabolism are pharmacokinetic. Receptor sensitivity, agonism, antagonism and the magnitude of effect are pharmacodynamic.
Why can two patients respond differently to the same dose?
They may differ in absorption, distribution, renal or hepatic elimination, age, pregnancy, body composition, genetics, interactions, adherence, disease progression or target sensitivity. A strong answer names the factor and whether it alters exposure, response or both.
Which parameter belongs with which dosing decision?
Clearance helps determine dose rate, volume of distribution helps determine loading dose, bioavailability supports route adjustment, and half-life informs dosing frequency and persistence. The final sentence should match the parameter rather than attaching every term to every calculation.
How does route change the concentration-time story?
Intravenous delivery avoids an absorption phase, while oral delivery depends on formulation, gastrointestinal transit and first-pass handling. Route can therefore alter onset and bioavailability even when the medicine and nominal dose appear similar.
What is the difference between potency and efficacy?
Potency concerns the amount or concentration needed for a stated effect. Efficacy concerns the maximal effect a drug can produce in the system. A more potent medicine is not automatically capable of a greater maximum response.
How can an interaction work without changing concentration?
Two medicines can produce additive or opposing physiological effects at the pharmacodynamic layer even when neither changes the other's concentration. State the shared response, such as sedation or bleeding, and the observation that makes it clinically relevant.
Why is half-life not the same as duration of effect?
Half-life describes decline in amount or concentration, while clinical effect also depends on target binding, active metabolites and the concentration-effect relationship. The two may track together, but an answer should not assume they are identical.
What does bioavailability contribute to a route comparison?
Bioavailability describes the fraction reaching systemic circulation. It helps explain why equal nominal doses by different routes may not produce equal exposure and why route conversion requires a defined relationship rather than guesswork.
Why can renal disease change both effect and harm?
Reduced elimination can raise or prolong exposure to a renally cleared medicine. That higher exposure may increase the intended effect, adverse effects or both, depending on the concentration-response relationship and patient sensitivity.
How should a receptor antagonist be described?
State that it binds a target and reduces or blocks the action of an agonist, then identify whether the course material distinguishes reversible and irreversible behaviour. Do not equate antagonism with absence of all physiological effect.
What does a unit check add to a calculation?
It shows whether the substituted quantities can produce the requested output. If litres or time units do not cancel as expected, the relationship or conversion probably needs review before interpretation.
Exam move
Draw the dose-to-outcome chain and label where PK ends and PD begins. Recompute simple relationships with units, then write one sentence about what the result changes. Create paired examples for renal disease, older age and interactions, separating altered exposure from altered sensitivity.
For drug targets, compare receptor, ion-channel, carrier and enzyme actions, then distinguish affinity, efficacy, selectivity and potency without treating them as synonyms. Build a parameter table with symbol, unit, physiological meaning and dosing decision. Practise route comparisons and interaction cases in which exposure changes, response changes or both change.
End each calculation by checking plausibility and professional limits before describing monitoring.
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