Pharmacology

🎯 Learning Objectives:

• Understand passive diffusion, facilitated diffusion, and active transport
• Compare different drug absorption routes
• Identify factors affecting drug absorption (e.g., pH, blood flow, surface area)

📘 1. Overview of Drug Absorption

Drug absorption is the process by which a drug moves from its site of administration into the bloodstream. The efficiency and rate of absorption affect a drug’s bioavailability, which influences how much of the drug reaches systemic circulation and becomes available for therapeutic effect.

🚪 2. Mechanisms of Drug Absorption

✅ a) Passive Diffusion

• Most common method for drug absorption
• Drug molecules move from an area of high concentration to low concentration
• No energy or carrier proteins required
• Lipid-soluble, small, and non-ionized drugs absorb easily via this route
• Example: Aspirin in the stomach

✅ b) Facilitated Diffusion

• Requires a carrier protein, but no energy (ATP)
• Movement is still from high to low concentration
• Specific to certain drug molecules
• Saturable and may be inhibited by competition
• Example: Vitamin B12 absorption with intrinsic factor

✅ c) Active Transport

• Uses energy (ATP) to move drugs against the concentration gradient (low to high)
• Requires specific carrier proteins
• Saturable and selective
• Common for nutrients and drugs that resemble body molecules
• Example: Levodopa transported across the blood-brain barrier

💉 3. Routes of Drug Absorption

RouteCharacteristicsAbsorption SpeedOral (PO)Convenient, may be affected by food, pH, first-pass metabolismModerate to slowIntravenous (IV)Directly into bloodstream, 100% bioavailabilityImmediate (no absorption needed)SublingualUnder the tongue, bypasses liver first-pass effectRapidIntramuscular (IM)Absorbed through muscle tissueModerate to fastSubcutaneous (SC)Injected into fat layerSlower than IMRectal (PR)Useful when oral route is not possibleVariableInhalationVia lungs, large surface area, avoids first-passVery rapid

🧬 4. Factors Affecting Absorption

• pH and Ionization:
• Drugs absorb best in their non-ionized form.
• Acidic drugs absorb better in acidic environments (e.g., stomach), basic drugs in basic environments (e.g., intestines).
• Blood Flow to Absorption Site:
• High blood flow increases absorption rate (e.g., IM injection into deltoid > gluteus)
• Surface Area:
• Larger surface area enhances absorption (e.g., small intestine > stomach)
• Drug Formulation:
• Liquid > tablet > enteric-coated → in terms of speed
• Presence of Food:
• May enhance or delay drug absorption depending on the drug
• First-pass Metabolism:
• Drugs taken orally may be metabolized in the liver before reaching systemic circulation, reducing bioavailability.