Pharmacokinetics Chapter 3 Flashcards
What is pharmacokinetics?
Pharmacokinetics is thus the study of drug movement throughout the body. In practical terms, it describes how the body handles medications.
Explain the applications of pharmacokinetics to clinical practice.
Pharmacokinetics focuses on the movement of drugs throughout the body after they are administered. The four processes of pharmacokinetics are absorption, metabolism, distribution, and excretion.
Identify the four primary processes of pharmacokinetics.
- Absorption
- Distribution
- Metabolism
- Excretion
What is absorption?
Absorption is the process of moving a drug from the site of administration to the bloodstream. The absorption of a drug molecule depends on its size, lipid solubility, degree of ionization, and interactions with food or other medications.
Discuss factors affecting drug absorption.
Absorption is affected by many factors. Factors
related to drug absorption are based on the body area of the absorptive surface,
- vascularity,
- pH,
- the presence of other substances,
- GI motility,
- functional integrity of the surface, and
- diseases.
Explain the metabolism of drugs and its applications to pharmacotherapy.
also called biotransformation, is the process of chemically converting a drug to a form that is usually removed from the body more easily. Metabolism involves complex biochemical pathways and reactions that alter drugs, nutrients, vitamins, and minerals. The liver is the primary site of drug metabolism, although the kidneys and cells of the intestinal tract also have high metabolic rates.
is a process that changes a drug’s form and makes it more likely to be excreted. Changes in hepatic metabolism can significantly affect drug action.
For example: The rate and ability of the liver to metabolize medica-tions will be altered in a client with liver disease.
First-pass effect: An oral drug is metabolized to an inactive form before it reaches target cells.
1) Oral drug taken by patient
2) Drug absorbed across the intestinal mucosa
3) Drug enters portal circulation and travels to the liver
4) On first pass through the liver, drug is metabolized to less active forms
5) Drug metabolites (less active) leave the liver for distribution to tissues.
Discuss how drugs are distributed throughout the body.
Distribution represents how drugs are transported throughout the body. Distribution depends on the formation of drug-protein complexes and special barriers such as the fetal-placental and blood-brain barriers.
several key factors influence drug distribution:
1) Blood flow to the tissue: the simplest factor determining distribution is the amount of blood flow to body tissues. The heart, liver, kidneys, and brain receive the most blood supply. Skin, bone, and adipose tissue receive a lower blood flow; therefore, it is more difficult to deliver high concentrations of drugs to these areas.
2) Drug Solubility: The physical properties of a drug greatly influence how it moves throughout the body after administration. Lipid solubility is an important characteristic because this determines how quickly a drug is absorbed, mixes within the bloodstream, crosses membranes, and becomes localized in body tissues.
3) Tissue Storage: Some tissues have the ability to accumulate and store drugs after absorption. The bone marrow, teeth, eyes, and adipose tissue have an especially high affinity, or attraction, for certain medications.
4) Plasma protein binding: Not all drug molecules in the plasma will reach their target cells because many drugs bind reversibly to plasma proteins, particularly albumin, to form drug-protein complexes. Drug-protein complexes are too large to cross capillary membranes; therefore, the drug is unavailable for dis-tribution to body tissues.
Describe how plasma proteins affect drug distribution.
Not all drug molecules in the plasma will reach their target cells because many drugs bind reversibly to plasma proteins, particularly albumin, to form drug-protein complexes. Drug-protein complexes are too large to cross capillary membranes; therefore, the drug is unavailable for distribution to body tissues. Drugs bound to proteins circulate in the plasma until they are released or displaced from the drug-protein complex.
Identify major processes by which drugs are excreted.
1) Renal Excretions: Although drugs are removed from the body by numerous organs and tissues, the primary site of excretion is the kidney. In an average-sized person, approximately 180 L of blood are filtered by the kidneys each day. Free drugs, water-soluble agents, electrolytes, and small molecules are easily filtered by the glomerulus.
2) Pulmonary excretion: Other organs can serve as important sites of excretion. Drugs delivered by gas or volatile liquids (liquids that vaporize at room temperature) are especially suited for excretion by the respiratory system. The respiratory excretion rate depends on factors that affect gas exchange, including diffusion, gas solubility, and pulmonary blood flow.
3) Glandular secretion: Glandular activity is another elimination mechanism. Water-soluble drugs may be secreted into the saliva, sweat, or breast milk. The “funny taste” that clients sometimes experience when given IV drugs is due to the agent being secreted into the saliva.
4) Fecal and biliary excretion: Certain oral drugs travel through the GI tract without being absorbed and are excreted in the feces. Examples include mebendazole (Vermox), a drug used to kill intestinal worms, and barium sulphate, a radiological contrast agent.
Explain how enterohepatic recirculation might affect drug activity.
recycling of drugs and other substances by the circulation of bile through the intestine and liver
A percentage of the drug may be recirculated numerous times with the bile. Biliary reabsorption is extremely influential in pro-longing the activity of cardiac glycosides, certain antibiotics, and phenothiazines. Recirculated drugs are ultimately metabolized by the liver and excreted by the kidneys. Recirculation and elimination of drugs through biliary excretion may continue for sev-eral weeks after therapy has been discontinued.
Enterohepatic recirculation.
- Liver
- Bile transported from the liver to the small intestine by way of the gallbladder
- Gallbladder
- Stomach
- Small intestine
- Absorption
- Bloodstream
- Bile salts are absorbed and circulated back to the liver by way of the capillaries of the digestive tract and heaptic portal vein.
Explain the applications of a drug’s plasma half-life (t1/2) to pharmacotherapy.
defined as the length
of time required for a medication to decrease concentration in the plasma by one-half after administration. Some drugs have a half-life of only a few minutes, while others have a half-life of several hours or days.
Many variables can affect the duration of drug action, including the following:
* Drug concentration (amount of drug given)
* Dosage (how often a drug is given or scheduled)
* Route of drug administration (oral, parenteral, or topical)
* Drug-food interactions
* Drug-supplement interactions
* Drug-herbal interactions
* Drug-drug interactions
Explain how a drug reaches and maintains its therapeutic range in the plasma.
therapeutic range dosage range or serum concentration that achieves the desired drug effects
In this range, the drug produced its desired therapeutic action. After peaking, the drug plasma level slowly began to fall out of the therapeutic range due to excretion processes. A higher dose might have caused the drug plasma level to reach a toxic concentration, the level of drug that results in serious adverse effects.
The goal of pharmacotherapy is to reach and maintain a plasma drug level in the therapeutic range while avoiding the toxic concentration.
Differentiate between loading and maintenance dose
Loading dose: A loading dose is a higher amount of drug, often given only once or twice, that is administered to “prime” the bloodstream with a level sufficient to quickly induce a therapeutic response.
Maintenance dose: before plasma levels can drop back toward zero, intermittent maintenance doses are given to keep the plasma drug concentration in the therapeutic range. Although blood levels of the drug fluctuate with this approach, the equilibrium state can be reached almost as rapidly as with a continuous infusion.
