JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Pharmacy IV Year BIOPHARMACEUTICS AND PHARMACOKINETICS Previous Question Paper (January/February - 2023 - R17)

Pharmacodynamics is the study of the biochemical and physiological effects of drugs on the body and the mechanisms of their action, including the correlation between drug concentration and the magnitude of the observed effect.

The absorption of drugs from the gastrointestinal (GI) tract is governed by several anatomical and physiological barriers that a molecule must traverse to reach the systemic circulation:

• Gastrointestinal Mucus Layer: A viscous gel layer covering the epithelial surface that acts as a physical diffusion barrier, particularly for large or hydrophobic molecules.
• Unstirred Water Layer (UWL): A stagnant layer of water adjacent to the epithelial membrane that creates a significant resistance to the diffusion of highly lipophilic drugs.
• Gastrointestinal Epithelial Membrane: The primary biological barrier consisting of a lipid bilayer. It restricts the passage of ionized, polar, or high molecular weight compounds, requiring them to utilize specific transport mechanisms or passive diffusion.
• Tight Junctions: The intercellular spaces between epithelial cells that limit paracellular transport, forcing most drugs to follow the transcellular route.
• Capillary Endothelium: Once through the epithelium, the drug must cross the capillary wall to enter the blood circulation, which generally offers less resistance than the epithelial membrane but still acts as a final barrier.