Pharmacokinetics (ADME)

Mar 8
4 min read

1. Introduction

Pharmacokinetics is a fundamental concept in pharmacology that explains how the body affects a drug after administration. It describes the movement of drugs through the body from the time they are administered until they are eliminated.

Pharmacokinetics involves four main processes collectively known as ADME:

A – Absorption
D – Distribution
M – Metabolism
E – Excretion

Understanding pharmacokinetics is essential for nurses because it helps in:

Determining correct drug dosage
Predicting drug onset and duration
Preventing drug toxicity
Monitoring drug effectiveness

For example:

IV drugs act rapidly because they bypass absorption.
Drugs metabolized in the liver may cause toxicity in patients with liver disease.

2. Definition

Pharmacokinetics: The study of the absorption, distribution, metabolism, and excretion (ADME) of drugs in the body.

It describes what the body does to the drug.

3. Classification of Pharmacokinetic Processes

Pharmacokinetics consists of four main processes known as ADME.

Process	Meaning	Major Organ Involved
Absorption	Drug enters bloodstream	Intestine
Distribution	Drug spreads to tissues	Blood circulation
Metabolism	Drug chemically altered	Liver
Excretion	Drug removed from body	Kidneys

4. Mechanism of Pharmacokinetics (ADME Process)

Stepwise Drug Movement in Body

Drug administration
Drug absorption into bloodstream
Drug distribution to body tissues
Drug metabolism in liver
Drug excretion through kidneys or other routes

Flowchart students can draw:

Drug Administration
        ↓
Absorption
        ↓
Distribution
        ↓
Metabolism
        ↓
Excretion

5. Absorption (A)

Definition

Absorption is the process by which a drug moves from the site of administration into the bloodstream.

Factors Affecting Absorption

Factor	Explanation
Route of administration	IV drugs absorb fastest
Blood flow	Higher blood flow increases absorption
Drug solubility	Lipid-soluble drugs absorb faster
Surface area	Larger surface area increases absorption
Gastric pH	Affects oral drug absorption

Routes and Absorption Speed

Route	Speed of Absorption
IV	Immediate
Inhalation	Very rapid
Intramuscular	Moderate
Subcutaneous	Slow
Oral	Slowest

Bioavailability

Bioavailability refers to the fraction of administered drug reaching systemic circulation.

Example:

IV route → 100% bioavailability

Clinical Example

Paracetamol taken orally:

Tablet → stomach → intestine → absorbed into bloodstream.

6. Distribution (D)

Definition

Distribution is the movement of drugs from the bloodstream to tissues and organs.

Once absorbed, drugs are carried by the blood circulation to different body parts.

Factors Affecting Distribution

Factor	Effect
Blood flow	Organs with high blood supply receive drugs faster
Plasma protein binding	Bound drugs remain inactive
Tissue permeability	Determines drug entry into tissues
Lipid solubility	Fat-soluble drugs distribute widely

Organs Receiving Drugs First

Highly perfused organs receive drugs rapidly:

Brain
Liver
Kidneys
Heart

Blood-Brain Barrier

Some drugs cannot enter the brain due to the blood-brain barrier (BBB).

Example:

Lipid-soluble drugs cross BBB easily.

Clinical Example

Sedatives reach the brain rapidly, causing quick effects.

7. Metabolism (M)

Definition

Metabolism is the chemical transformation of drugs into metabolites in the body.

It mainly occurs in the liver.

The purpose of metabolism is to:

Convert drugs into water-soluble forms
Facilitate excretion

Two Phases of Drug Metabolism

Phase I Reactions

Also called functionalization reactions.

Includes:

Oxidation
Reduction
Hydrolysis

These reactions are mainly performed by cytochrome P450 enzymes.

Example:

Diazepam metabolism in liver.

Phase II Reactions

Also called conjugation reactions.

Drug combines with substances such as:

Glucuronic acid
Sulfate
Glycine

These reactions make drugs more water soluble.

Factors Affecting Metabolism

Factor	Effect
Liver disease	Reduced metabolism
Age	Neonates have immature liver enzymes
Genetics	Individual variation in metabolism
Drug interactions	Some drugs increase or decrease metabolism

Clinical Example

Patients with liver failure require lower drug doses.

8. Excretion (E)

Definition

Excretion is the removal of drugs and their metabolites from the body.

The kidneys are the primary organs of drug excretion.

Routes of Drug Excretion

Route	Example
Kidneys (urine)	Most drugs
Liver (bile)	Some antibiotics
Lungs	Anesthetic gases
Sweat	Trace drug elimination
Breast milk	Some medications

Renal Excretion Process

Drug elimination through kidneys occurs in three steps:

Glomerular filtration
Tubular secretion
Tubular reabsorption

Clinical Example

Patients with kidney disease may accumulate drugs and develop toxicity.

Example:

Aminoglycoside antibiotics

9. Drug Interactions in Pharmacokinetics

Drug interactions can occur at any stage of ADME.

Stage	Example
Absorption	Antacids reduce antibiotic absorption
Distribution	Competition for plasma protein binding
Metabolism	Enzyme inhibition or induction
Excretion	Diuretics affect renal drug elimination

10. Nursing Responsibilities

Pre-Administration

Assess liver and kidney function
Check drug dose and route
Review drug interactions

Monitoring During Therapy

Observe:

Therapeutic response
Adverse reactions
Signs of drug toxicity

Example:

Monitor renal function during antibiotic therapy.

Patient Education

Teach patients:

Importance of correct dosing schedule
Avoid alcohol with certain medications
Report adverse effects immediately

11. Memory Tricks for Students

ADME Mnemonic

“A Drug Moves Easily”

A → Absorption

D → Distribution

M → Metabolism

E → Excretion

Easy Trick

Think:

“Drug Journey in the Body”

Entry → Absorption
Travel → Distribution
Processing → Metabolism
Exit → Excretion

12. Clinical Pearls for Nurses

✔ IV drugs bypass absorption phase

✔ Liver is the main organ for drug metabolism

✔ Kidneys are the major route of drug excretion

✔ Elderly patients may require dose adjustment

✔ Liver or kidney disease increases drug toxicity risk

13. Exam Key Points

Pharmacokinetics studies what the body does to drugs.
It consists of ADME processes.
Absorption is the entry of drug into bloodstream.
Distribution spreads drug to body tissues.
Metabolism occurs mainly in the liver.
Excretion occurs mainly through kidneys.
Bioavailability refers to amount of drug reaching circulation.

14. Suggested Diagram

Students should draw this flowchart.

Pharmacokinetics (ADME)

Drug Administration
        ↓
Absorption
        ↓
Distribution
        ↓
Metabolism (Liver)
        ↓
Excretion (Kidneys)

15. References

Standard pharmacology textbooks:

• Lippincott Illustrated Reviews: Pharmacology

• Essentials of Medical Pharmacology – KD Tripathi

• Pharmacology and Pharmacotherapeutics – Satoskar

60-Second Revision Card