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The gastrointestinal tract is lined with epithelial cells. Drugs must pass through or permeate these cells to be absorbed into the bloodstream. Cell membranes may act as barriers to some drugs. They are essentially lipid bilayers which form semipermeable membranes. Pure lipid bilayers are generally permeable only to small, uncharged solutes.
Once the drug cannot pass back through the barrier the drug can be concentrated and made effective for therapeutic use. [7] However drawbacks to this exist as well. Once the drug is in the brain there is a point where it needs to be degraded to prevent overdose to the brain tissue. Also if the drug cannot pass back through the blood–brain ...
The human body uses glucuronidation to make a large variety of substances more water-soluble, and, in this way, allow for their subsequent elimination from the body through urine or feces (via bile from the liver). Hormones are glucuronidated to allow for easier transport around the body. Pharmacologists have linked drugs to glucuronic acid to ...
Therefore, the chemical characteristics of a drug will determine its distribution within an organism. For example, a liposoluble drug will tend to accumulate in body fat and water-soluble drugs will tend to accumulate in extracellular fluids. The volume of distribution (V D) of a drug is a property that quantifies the extent of its distribution ...
Cells have a more acidic pH inside the cell than outside (gastric mucosal cells being an exception). Therefore, basic drugs (like bupivacaine, pyrimethamine) are more charged inside the cell than outside. The cell membrane is permeable to non-ionized (fat-soluble) molecules; ionized (water-soluble) molecules cannot cross it easily. Once a non ...
The bacteria have thinner cell walls but the external surface is coated with an additional cell membrane, called the outer membrane. The outer membrane is a lipid layer (lipopolysaccharide chain) that blocks passage of water-soluble (hydrophilic) molecules like penicillin.
This model takes into account that not all water carrying a chemical that comes into contact with skin stays on the skin long enough to allow absorption. Only that portion of a chemical in the solution that stays in contact with the skin is available for absorption. This may be modeled using water adherence factors as postulated by Gujral 2011 ...
First-pass metabolism may occur in the liver (for propranolol, lidocaine, clomethiazole, and nitroglycerin) or in the gut (for benzylpenicillin and insulin). [4] The four primary systems that affect the first pass effect of a drug are the enzymes of the gastrointestinal lumen, [5] gastrointestinal wall enzymes, [6] [7] [8] bacterial enzymes [5] and hepatic enzymes.