JNTU B.Pharmacy IV BIOPHARMACEUTICS AND PHARMACOKINETICS Previous Question Paper (March-2021 - R17)

Convective transport, also known as pore transport or bulk flow, occurs when drug molecules move through aqueous-filled channels or pores in the biological membrane. This process is driven by the hydrostatic pressure gradient or osmotic pressure difference across the membrane. It is primarily responsible for the absorption of small, water-soluble, polar molecules (typically with a molecular weight less than 100 Da) that cannot pass through the lipid bilayer. The rate of transport is determined by the pore size and the solvent flow.

Vesicular transport is an active process involving the engulfment of substances by the cell membrane to form vesicles. It includes two primary mechanisms: pinocytosis (cell drinking), which involves the uptake of fluid or small particles, and phagocytosis (cell eating), which involves the engulfment of large particles. This mechanism is significant for the absorption of high-molecular-weight substances, such as proteins, peptides, and oil-in-water emulsions, which are otherwise unable to cross membranes via passive diffusion.

Intestinal transit time is a critical physiological factor that determines the duration a drug remains in contact with the absorptive surface of the gastrointestinal tract. Since the small intestine is the primary site for drug absorption, the transit time significantly influences the extent of bioavailability.

• Residence Time: A longer transit time in the small intestine generally allows for more complete dissolution and absorption of drugs, particularly those with low solubility or slow dissolution rates.
• Gastric Emptying: The rate at which the stomach empties its contents into the small intestine is the rate-limiting step for many drugs. Delayed gastric emptying can reduce the rate of absorption (Cmax and Tmax) for drugs absorbed primarily in the small intestine.
• Motility Disorders: Conditions such as diarrhea decrease intestinal transit time, leading to reduced absorption due to insufficient contact time. Conversely, conditions like constipation or the use of anticholinergic drugs may increase transit time, potentially enhancing the absorption of slowly dissolving drugs.
• Site-Specific Absorption: For drugs absorbed via carrier-mediated transport systems located only in specific segments of the small intestine (e.g., the duodenum or jejunum), rapid transit may move the drug past the absorption window before it can be fully absorbed, thereby reducing the total fraction absorbed.