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Chpt. 2 Pharmacokinetics I: Drug Administration, Absorption, and Distribution
Terms in this set (43)
what is Pharmacokinetics?
what does the body do to the drug
what are the two primary routes drugs can be administered?
through the alimentary canal (enteral administration)
or through nonalimentary routes (parenteral administration)
what's the primary route drugs are given enterally?
the oral route;
easiest method of taking medications,
drugs taken orally are absorbed by which organ?
disadvantages of taking drugs orally
must have a relatively high degree of lipid solubility in order to pass through the gastrointestinal mucosa and into the bloodstream;
certain medications may irritate the stomach and cause discomfort, vomiting, or even damage to the gastric mucosa;
acidic environment and presence of digestive proteases in the stomach may also cause various compounds to be degraded and destroyed prior to absorption from the gastrointestinal tract;
amount and rate at which the drug eventually reaches the bloodstream tends to be somewhat less predictable with oral administration compared with more direct routes, such as injection
describe the first-pass effect
After absorption from the alimentary canal, the drug is transported directly into the liver via the portal vein, where a significant amount of the drug may be metabolized and destroyed prior to reaching its site of action
Drugs administered sublingually ...
placed under tongue
Buccal administration of drugs
drug placed between cheeks and gums
drugs administered sublingually and the buccal route....
absorbed through the oral mucosa into the venous system that is draining the mouth region. These veins eventually carry blood to the superior vena cava, which in turn carries blood to the heart;
offer a means of enteral administration to people who have difficulty swallowing or to patients who cannot be given drugs rectally.
drugs applied the rectal route (disadvantage)
less favorable because many drugs are absorbed poorly or incompletely, and irritation of the rectal mucosa may occur
drugs applied the rectal route (advantage)
advantage of allowing drugs to be given to a patient who is unconscious, or when vomiting prevents drugs from being taken orally
Parenteral is what?
All methods of drug administration that do not use the gastrointestinal tract;
allows the drug to be delivered to the target site more directly;
not usually subject to first-pass inactivation in the liver.
Pulmonary administration is advantageous because of the large (alveolar) surface area for diffusion of the drug into the pulmonary circulation and it is generally associated with rapid entry of the drug into the bloodstream
drug to be introduced into the bloodstream over a short period of time;
advantageous in emergency situations when it is necessary for the medication to exert an immediate effect;
used to allow the prolonged, steady infusion of a drug into the venous system;
prevents large fluctuations in the plasma concentration of the drug and allows the dosage of drug to be maintained at a specific level for as long as desired
injection of a drug directly into an artery;
difficult and dangerous;
permits a large dose of the medication to reach a given site, such as a specific organ;
used occasionally in cancer chemotherapy to administer the anticancer drug directly to the tumor site with minimal exposure of the drug to other healthy tissues.
Injecting medications directly beneath the skin is used when a local response is desired;
amount of drug that can be injected in this fashion is fairly small and that the injected drug must not irritate or inflame the subcutaneous tissues.
can be used to treat a problem located directly in the injected muscle;
botulinum toxin and other substances can be injected directly into hyperexcitable muscles to control certain types of muscle spasms or spasticity;
many drugs injected directly into a muscle cause significant amounts of local pain and prolonged soreness, tending to limit the use of this route for repeated injections
injecting the medication within a sheath, and frequently refer to injections within the spinal subarachnoid space
applied to the surface of the skin or mucous membranes;
absorbed fairly poorly through the epidermis and into the systemic circulation and are used primarily to treat problems that exist on the skin itself
applying drugs directly to the surface of the skin with the intent that they willbe absorbed through the dermal layers and into either the subcutaneous tissues or the peripheral circulation;
it must be able to penetrate the skin, and it must not be degraded to any major extent by drug-metabolizing enzymes located in the dermis;
often delivered through medicated "patches"
electric current is used to "drive" the ionized form of the medication through the skin.
uses ultrasound waves to enhance transmission of the medication through the dermis.
The extent to which the drug reaches the systemic circulation is referred to as...
"bioavailability"; percentage of the drug administered that reaches the bloodstream
i.e.: if 100 g of a drug is given orally, and 50 g eventually make it into the systemic circulation, the drug is said to be 50 percent bioavailable.
drugs that pass through membranes, do so how?
passive diffusion, active transport, facilitated diffusion, or endocytosis
there must be some type of difference or "gradient" on one side of the membrane compared to the other;
membrane must be permeable to the diffusing substance;
rate of the diffusion is dependent on several factors, including the magnitude of the gradient, the size of the diffusing substance, the distance over which diffusion occurs, and the temperature at which diffusion occurs;
movement occurs without expending any energy
what's the advantage of a drug being in its neutral, nonionized form?
Drugs will diffuse more readily through the lipid layer;
i.e.: when a weak acid is in an acidic environment (e.g., gastric secretions of the stomach), it tends to be in its neutral, nonionized form. The same drug will become positively charged if the pH of the solution increases and becomes more basic;
A weak acid such as aspirin will be nonionized and will therefore be absorbed fairly easily from the stomach because of its lipid solubility;
This same drug will be poorly absorbed if it reaches the basic pH of the duodenum and becomes ionized.
a drug that is a weak base will be ionized and poorly absorbed from the acidic environment of the stomach
water moves from an area where it is highly concentrated to an area of low concentration
involves using membrane proteins to transport substances across the cell membrane;
Characteristics of active transport:
Carrier specificity: some degree of specificity for certain substances, usually discriminating among different compounds according to their shape and electrical charge
Expenditure of energy: active transport implies that some energy must be used to fuel the carrier system (ATP)
Ability to transport substances against a concentration gradient: may be able to carry substances "uphill"
features of both active transport and passive diffusion;
protein carrier is present in facilitated diffusion, but no net energy is expended in transporting the substance across the cell membrane;
inability to transport substances uphill against a concentration gradient;
entry of glucose into skeletal muscle cells via facilitated diffusion is probably the best example of this type of transport in the body
several factors affecting the distribution of drugs in the body...
tissue permeability, blood flow, binding to plasma proteins, binding to subcellular components
highly lipid-soluble drug can potentially reach all of the different body compartments and enter virtually every cell it reaches;
nonlipid-soluble compound will remain primarily in the compartment or tissue to which it is administered;
certain tissues have special characteristics that limit the passage of drugs
More of the drug will reach organs that receive a great deal of blood flow— such as the brain, kidneys, and exercising skeletal muscle
Binding to plasma proteins
only the unbound or "free" drug is able to reach the target tissue and exert a pharmacologic effect;
Binding to subcellular components
drugs that are bound within specific cells are unable to leave the cell and be distributed throughout other fluid compartments
Volume of distribution
distribution of a given drug within the body;
amount of drug administered/concentration of drug in plasma;
If the Volume of distribution of the drug is far less than 42 L, then the drug is being retained in the bloodstream due to factors such as plasma protein binding;
A Volume of distribution much greater than 42 L indicates that the drug is being concentrated in the tissues.
name some storage sites for drugs
adipose, bone, muscle, organs
primary site for drug storage in the body;
Once drugs have been stored in adipose tissue, they tend to remain there for long periods of time because of the low metabolic rate and poor blood perfusion of these tissues;
storage site for several toxic agents, especially heavy metals like lead
Binding of drugs to components within the muscle may create the long-term storage of these compounds
stored within certain organs such as the liver and kidneys;
drug may enter the cell passively or by active transport and then form bonds to subcellular components
what are some adverse consequences of drug storage?
can cause local damage to the tissues in which they are stored;
Another problem with drug storage occurs when the storage site acts as a reservoir that "soaks up" the drug and prevents it from reaching the target site;
Storage sites may also be responsible for the redistribution of drugs; the drug may be reintroduced to the target site long after the original dose should have been eliminated, may explain why certain individuals experience prolonged effects of the drug or extended adverse side effects
generally designed to permit a slower and more prolonged absorption of the drug from the gastrointestinal tract and other routes of administration;
advantages such as decreasing the number of doses needed each day, preventing large fluctuations in the amount of drug appearing in the plasma, and sustaining plasma levels throughout the night
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