Absorption
Drug absorption covers a drug’s progress from the
time it’s admin-istered, through its passage to the tissues, until it reaches
systemic circulation.
On a cellular level, drugs are absorbed by several
means—pri-marily through active or passive transport.
Passive transport requires no cellular energy becausediffusion
allows the drug to move from an area of high-er concentration to one of lower
concentration. Passive transport occurs when small molecules diffuse across
membranes and stops when drug concentration on both sides of the membrane is
equal.
Active transport requires cellular energy to move the drug
froman area of lower concentration to one of higher concentration. Ac-tive
transport is used to absorb electrolytes, such as sodium and potassium, as well
as some drugs such as levodopa.
Pinocytosis is a unique form of active transport that
occurs whena cell engulfs a drug particle. Pinocytosis is commonly employed to
transport fat-soluble vitamins (vitamins A, D, E, and K).
If only a few cells separate the active drug from
the systemic cir-culation, absorption will occur rapidly and the drug will
quickly reach therapeutic levels in the body. Typically, absorption occurs
within seconds or minutes when a drug is administered sublin-gually, I.V., or
by inhalation.
Absorption occurs at a slower rate when drugs are
administered by the oral, I.M., or subQ routes because the complex membrane
systems of GI mucosal layers, muscle, and skin delay drug pas-sage.
At the slowest absorption rates, drugs can take
several hours or days to reach peak concentration levels. A slow rate usually
oc-curs with rectally administered or sustained-release drugs.
Other factors can affect how quickly a drug is
absorbed. For ex-ample, most absorption of oral drugs occurs in the small
intestine. If a patient has had large sections of the small intestine
surgically removed, drug absorption decreases because of the reduced sur-face
area and the reduced time that the drug is in the intestine.
Drugs absorbed by the small intestine are
transported to the liver before being circulated to the rest of the body. The
liver may me-tabolize much of the drug before it enters the circulation. This
mechanism is referred to as the first-pass
effect. Liver metabolism may inactivate the drug; if so, the first-pass
effect lowers the amount of active drug released into the systemic circulation.
Therefore, higher drug dosages must be administered to achieve the desired
effect.
Increased blood flow to an absorption site improves
drug absorp-tion, whereas reduced blood flow decreases absorption. More rapid
absorption leads to a quicker onset of drug action.
For example, the muscle area selected for I.M.
administration can make a difference in the drug absorption rate. Blood flows
faster through the deltoid muscle (in the upper arm) than through the gluteal
muscle (in the buttocks). The gluteal muscle, however, can accommodate a larger
volume of drug than the deltoid mus-cle.
Pain and stress can decrease the amount of drug
absorbed. This may be due to a change in blood flow, reduced movement through
the GI tract, or gastric retention triggered by the autonomic ner-vous system
response to pain.
High-fat meals and solid foods slow the rate
at which contents leave the stomach and enter the intestines, delaying
intestinal ab-sorption of a drug.
Drug formulation (such as tablets, capsules,
liquids, sustained-release formulas, inactive ingredients, and coatings)
affects the drug absorption rate and the time needed to reach peak blood
concentration levels.
Combining one drug with another drug, or with food,
can cause in-teractions that increase or decrease drug absorption, depending on
the substances involved.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2023 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.