Absorption is the transfer of a drug from the site of administration into the bloodstream. Drugs may be absorbed from the gastrointestinal tract via passive diffusion, facilitated diffusion, active transport, or endocytosis. Active transport uses energy from ATP to move drugs against their concentration gradient across cell membranes. Endocytosis involves engulfment of a drug by the cell membrane and transport into the cell within vesicles, while exocytosis is the reverse process used to secrete substances out of cells.

1. CHAPTER#01
Pharmacokinetics
Lecture #03
Absorption of Drugs
Dr.kainat Azhar
Mbbs(KMC)

2. Absorption of Drugs:
• Absorption is the transfer of a drug from the site of
administration to the bloodstream.

3. Mechanisms of absorption of
drugs from the GI tract
• Drugs may be absorbed from the GI tract by passive
diffusion, facilitated diffusion, active transport, or
endocytosis .

6. • Active transport:
• Active transport is a process in drug absorption where
energy, usually derived from ATP, is used to move a drug
against its concentration gradient across cell membranes.
• Endocytosis and exocytosis:
• . Endocytosis involves engulfment of a drug by the cell
membrane and transport into the cell by pinching off the
drug-filled vesicle.
• Exocytosis is the reverse of endocytosis. Many cells use
exocytosis to secrete substances out of the cell through a
similar process of vesicle formation. Vitamin B12 is
transported across the gut wall by endocytosis, whereas
certain neurotransmitters (for example, norepinephrine)
are stored in intracellular vesicles in the nerve terminals
rease by exocytosis.