Pharmacokinetics as the Basis of Medication Actions Flashcards
Pharmacokinetics
For a medication to be useful therapeutically, it must be taken into a
patient’s body; be absorbed and distributed to cells, tissues, or a specific
organ; and alter physiological functions. Pharmacokinetics is the study of
how medications enter the body, reach their site of action, metabolize, and
exit the body. You use knowledge of pharmacokinetics when timing
medication administration, selecting the route of administration, and
evaluating a patient’s response.
Absorption
Absorption occurs when medication molecules pass into the blood from
the site of medication administration. Factors that influence absorption are
the route of administration, ability of the medication to dissolve, blood
flow to the site of administration increase absorption, larger the body surface area (BSA) e.g. most of medications are
absorbed in the small intestine rather than the stomach, and lipid
solubility of medication.
Route of Administration (ALSO AFFECTS ABSORPTION)
Each route of medication administration has a different rate of absorption.
When applying medications on the skin, absorption is slow because of the
physical makeup of the skin. Because orally administered medications
pass through the gastrointestinal (GI) tract, the overall rate of absorption is
usually slow. Medications placed on the mucous membranes and
respiratory airways are absorbed quickly because these tissues contain
many blood vessels. Intramuscular (IM) and subcutaneous medications
absorb more quickly than oral medications, with IM injections entering the
bloodstream more quickly than medications in subcutaneous injections.
Intravenous (IV) injection produces the most rapid absorption because
medications are available immediately when they enter the systemic
circulation.
Ability of a Medication To Dissolve (ALSO AFFECTS ABSORPTION)
The ability of an oral medication to dissolve depends largely on its form or
preparation. The body absorbs solutions and suspensions already in a
liquid state more readily than tablets or capsules. Acidic medications pass
through the gastric mucosa rapidly. Medications that are basic are not
absorbed before reaching the small intestine.
Lipid Solubility (ALSO AFFECTS ABSORPTION)
Because the cell membrane has a lipid layer, highly lipid-soluble
medications cross cell membranes easily and are absorbed quickly.
Another factor that often affects medication absorption is whether food is
in the stomach. Some oral medications are absorbed more easily when
administered between meals because food changes the structure of a
medication and sometimes impairs its absorption. When some
medications are administered together, they interfere with one another,
impairing the absorption of both medications.
Distribution
After a medication is absorbed, it is distributed within the body to tissues
and organs and ultimately to its specific site of action.
Circulation
Membrane permeability
Protein binding
Circulation
Once a medication enters the bloodstream, it is carried throughout the
tissues and organs. How fast it reaches a site depends on the vascularity of
the various tissues and organs. Conditions that limit blood flow or blood
perfusion inhibit the distribution of a medication. For example, patients
with heart failure have impaired circulation, which slows medication
delivery to the intended site of action. Therefore, the effectiveness of
medications in these patients is often delayed or altered.
Membrane Permeability
To be distributed to an organ, a medication must pass through all of the tissues and biological membranes of the organ. Some membranes serve as barriers to the passage of medications. For example, the blood-brain barrier allows only fatsoluble medications to pass into the brain and cerebral spinal fluid.
Therefore, central nervous system infections often require treatment with
antibiotics injected directly into the subarachnoid space in the spinal cord.
Protein Binding
The degree to which medications bind to serum proteins such as albumin
affects their distribution. Most medications partially bind to albumin,
reducing the ability of a drug to exert pharmacological activity. The
unbound or “free” medication is its active form. Older adults and patients
with liver disease or malnutrition have decreased albumin in the
bloodstream. Because more medication is unbound in these patients, they
are at risk for an increase in medication activity, toxicity, or both.
Metabolism
Medications are metabolized into a less-potent or an inactive form making it easier to excrete.
Biotransformation occurs under the influence of enzymes that detoxify, break down, and remove active chemicals. Most biotransformation occurs
within the liver. The
liver degrades many harmful chemicals before they become distributed to
the tissues. If a decrease in liver function occurs such as with aging or liver
disease, a medication is usually eliminated more slowly, resulting in its
accumulation. Patients are at risk for medication toxicity if organs that
metabolize medications are not functioning correctly. For example, a small
sedative dose of a barbiturate sometimes causes a patient with liver
disease to lapse into a coma.
Excretion
Medications exit the body through the:
Kidney
Liver
Bowel
Lungs
Exocrine glands
Chemical makeup of medication determines the organ of excretion.
Gaseous and volatile compounds such as nitrous oxide and alcohol exit through the lungs. The exocrine glands excrete lipid-soluble medications. If a medication is excreted through the mammary glands,
there is a risk that a nursing infant will ingest the chemicals. Check the
safety of any medication used in breastfeeding women.
The GI tract- Medications that
enter the hepatic circulation are broken down by the liver and excreted
into the bile. After chemicals enter the intestines through the biliary tract,
the intestines resorb them. Factors that increase peristalsis (e.g., laxatives
and enemas) accelerate medication excretion through the feces, whereas
factors that slow peristalsis (e.g., inactivity and improper diet) often
prolong the effects of a medication.
The kidneys are the main organs for medication excretion. Maintenance of an adequate fluid intake (50 mL/kg/hr) promotes proper elimination of medications for the average adult. If a patient’s renal
function declines, the kidneys cannot adequately excrete medications.
Thus, the risk for medication toxicity increases. Health care providers
usually reduce medication doses when this happens.
