Extra Study Notes on Medication Absorption and Bioavailability

Medication Absorption

Definition of Medication Absorption

• Medication absorption is defined as the movement of a drug from its site of administration into the bloodstream.

pathways for Oral Drug Absorption

• Process Overview:

  • Oral drugs enter the stomach.

  • They face two possible pathways:

  1. Dissolution and absorption: Drugs dissolve and pass through the cell membranes of epithelial cells lining the stomach.

  2. Undissolved travel: Drugs travel undissolved through the stomach to the small intestine (the most common site of absorption).

• Small Intestine Absorption: In the small intestine:

  • Drugs dissolve and pass through the intestinal wall into the bloodstream.

• Portal Venous System: After absorption, oral drugs travel through the portal venous system to the liver.

First Pass Effect

• Definition: The first pass effect refers to the metabolism of some of the drug by the liver before it reaches the general circulation.

• Impact:

  • The liver may either inactivate the drug or excrete it into bile for elimination from the body.

  • The remaining active drug exits the liver to reach general circulation and target organs.

Alternative Administration Routes

• Intravenous Injection:

  • Directly passes into the bloodstream, bypassing absorption in the gastrointestinal (GI) tract.

• Intramuscular or Subcutaneous Injection:

  • The drug enters muscle or subcutaneous tissue.

  • It passes through gaps between cells into capillary walls, reaching general circulation or target organs, also bypassing GI absorption.

Bioavailability

• Definition: Bioavailability is the net amount of a dose of a drug that is actually absorbed into the bloodstream.

• Bioavailability Comparison:

  • Bioavailability of oral drugs is less than 100% due to the first pass effect of the liver.

  • In contrast, bioavailability of IV drugs is 100% because they avoid the first pass effect.

Drug Formulation and Bioavailability

• Different drug formulations impact bioavailability because they vary in:

  • Absorption rates

  • Extent of absorption

• Examples:

  • Tablets dissolve at varying rates.

  • Terracotted drugs dissolve in the small intestine (not the stomach) due to differences in gastric emptying time among individuals.

  • Sustained Release Formulas: Contain tiny spheres that dissolve at different rates, leading to:

  • Steady drug release throughout the day

  • Variable absorption.

Factors Affecting Drug Absorption

• Several factors influence the absorption of drugs:

  • Rate of Dissolution:

  • Drugs that dissolve faster are absorbed faster.

  • Surface Area:

  • The lining of the small intestine has a larger surface area compared to the stomach, enabling faster absorption of drugs in the small intestine.

  • Blood Flow:

  • A greater concentration gradient between the drug-filled stomach and rapidly flowing, drug-free blood facilitates faster absorption.

  • Lipid Solubility:

  • Highly lipid-soluble drugs can more easily pass through the phospholipids in cell membranes compared to drugs with low lipid solubility.

  • pH Partitioning:

  • Absorption is faster when there is a significant difference between the pH at the site of administration and the pH of the plasma, which attracts more drug molecules to ionize in the plasma.

Recap on Medication Absorption Process

• The entire process of absorption from administration to reaching the bloodstream includes:

  1. Entry into the stomach.

  2. Differentiation between dissolution and undissolved travel.

  3. Passage through the small intestine and portal venous system.

  4. Interaction with the liver and eventual entry into circulation and target organs.

This comprehensive absorption process highlights the importance of various physiological factors, drug formulations, and individual variations in drug therapy outcomes and bioavailability.