Chapter 2
Drugs and the Body
AVA MARIE D. CLARO RN.USRN.RM.MAN
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• To understand what happens when a
drug is administered – know
pharmacodynamics!!!
• To understand how the drugs affect the
body or how the body acts on the
drug- pharmacokinetics!!!
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Pharmacodynamics
• The study of biochemical & physiologic effects of drug &
their mechanism of action.
• The study of the action of drugs in living tissues
• “It is what drugs do to the body & how drugs interact
with the body tissues”
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Pharmacodynamics
• Pharmacodynamics is the science of dealing
with interactions between living organisms and
foreign chemicals
• Chemical reactions occur continuously in the
body of each living system
• When other chemicals (drugs) are added to the
body, additional effects occur
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PHARMACOKINETICS
• The process of drug movement to achieve a drug action
• The way the body handles drug absorption, distribution,
metabolism & excretion.
• Involves the study of absorption, distribution &
metabolism of biotransformation & excretion of drugs.
• It is what the body does to drugs
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Drug Actions
• To replace or act as substitutes for missing
chemicals
• To increase or stimulate certain cellular activities
• To depress or slow certain cellular activities
• To interfere with the functioning of foreign cells
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Receptor Cells
• Receptor site reacts to certain chemicals
• The better the fit between receptor site and
chemical, the more pronounced the reaction
• Enzymes within the body are needed to break
down the chemicals to open up the receptor
site
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Lock & Key
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•Agonists- drugs interact directly
with receptor sites to cause the
same activity that natural
chemicals would cause at that site.
•EX: insulin reacts with specific
insulin receptor sites to change cell
membrane permeability, this
promoting the movement of
glucose into the cell.
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• Some drugs react with receptor sites to block normal
stimulation, producing no effect. Competitive
antagonist
• EX- curare occupies receptor sites for acetylcholine
necessary for muscle contraction & movement.
• CURARE-
prevents
muscle
stimulation,
causing
paralysis.
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Noncompetitive antagonists
– other drugs react with
specific receptor sites on a cell
and, by reacting there, prevent
the reaction of another
chemical with a different
receptor site on that cell.
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Enzymes
• Drugs can interfere with the enzymes that may
be catalysts for chemical reactions
• Enzymes produce a cascade effect, with one
enzyme activating another & eventually causing
a cellular reaction
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Pharmacokinetics
• Onset of drug action
• Drug half-life
• Timing of the peak effect
• Duration of drug effects
• Metabolism or biotransformation of the drug
• Site of excretion
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• ONSET OF ACTION- begins when the drugs enter the
plasma until it reaches the maximum effective
concentration.
• PEAK ACTION- when the drugs reach its highest blood or
plasma concentration
• DURATION OF ACTION- the length of the time drug has
pharmacologic effect.
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Pharmacokinetics
• Critical concentration
– The amount of a drug that is needed to cause a
therapeutic effect too much toxic/poisonous; too little will not produce the desired effects
• Loading dose
– A higher dose than that usually used for
treatment to reach the critical concentration
• Dynamic equilibrium
– The actual concentration that a drug reaches in
the body
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The Processes by Which Drugs Are
Handled in the Body
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Dynamic Equilibrium
• The actual amount of drug that reaches the body
results in a dynamic equilibrium
• Dynamic equilibrium is affected by:
– Absorption
– Distribution
– Biotransformation
– Excretion
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Absorption
• Refers to what happens to a drug from
the time it is introduced to the body
until it reaches the circulating fluids
and tissues.
• GI tract- orally/rectally
• Mucous membrane
• Through the skin, lungs or through
muscle/ SQ
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Absorption
• Administration
– Affected by route of administration
– Oral medications affected by presence of
food in the stomach
• First-pass effect
– Medications are extensively metabolized by
the liver
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FIRST PASS EFFECT
• Drugs taken orally are usually absorb from the
small intestine directly into the portal venous
system (the blood vessels that flow trough the
liver on their way back to the heart). aspirin & alcohol 2 drugs that are
known to be absorbed from the lower end of the stomach
• Portal veins deliver these absorbed molecules
into the liver, which immediately transforms most
of the chemicals delivered to it by a series of liver
enzymes.
• These enzymes break the drug into metabolites,
some of w/c are active & cause effects in the body
& some of w/c are deactivated & can be readily
excreted from the body
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Drugs can be absorbed into cells through
various processes
•Passive diffusion
•Active transport
•filtration
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Passive diffusion
• Major process through w/c drugs are absorbed into the
body
• Occurs across concentration gradient
• When there is greater concentration of drug on one side
of a cell membrane, the drug will move through the
membrane to the area of lower concentration.
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Active transport
• Process that uses energy to actively move
a molecule across a cell membrane;
• Molecule maybe large, or it may be
moving against a concentration gradient
not for absorption but for drug excretion in the kidney
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FILTRATION
• Involves movement through pores in the cell membrane,
either down the a concentration gradient or as a result of
the pull of plasma proteins. For drug excretion
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Factors Affecting Absorption
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DISTRIBUTION
• Portion of drug that gets through the 1st pass effect is
delivered to the circulatory system for transport
throughout the body.
• FACTORS AFFECTING DISTRIBUTION:
• Drug’s lipid solubility
• Ionization
• Perfusion of reaction tissue DM pt w/ lower leg infxn, cold environment-constricted BV
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Distribution
• Protein binding
• Blood–brain barrier
• Placenta/breast milk
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PROTEIN BINDING
• Most drugs are bound to proteins in the
blood to be carried for circulation.
• Protein-drug complex are relatively large &
cannot enter the capillaries & then to
tissues to react.
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Blood brain barrier
• Protective system of
cellular activity that keeps
(foreign invaders, poisons)
away from the CNS
• Highly lipid soluble- pass
BBB
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Biotransformation
• The process by w/c drugs are changed into
new, less active chemicals
• This process breaks down medications
• liver -single most important site for
biotransformation (metabolism)
• It helps to prevent medications from causing
adverse effects on the body
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•Liver- detoxifies chemicals,
neutralizes drugs
•Hepatic microsomal
system- hepatic cells
intracellular structures that
are lined w/ enzymes that
are packed together
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Excretion
• Removal of drugs from the body
• Kidneys play the most important role in the
excretion of medication
• Skin, saliva, lungs, bile & feces
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Pharmacokinetics
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Half-Life
• Half-life is the time it takes for the amount of
drug in the body to decrease to one-half the
peak level
• Vital in determining the appropriate timing
for a drug dose or determining the duration
of a drug’s effect on the body
• Half-life is affected by the absorption,
distribution, metabolism, and excretion
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Half-life
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Calculating Half-Life
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Factors Influencing Drug Effects
• Weight • Immunologic factors
• Age • Psychological factors
• Gender • Environmental factors
• Physiologic factors • Drug tolerance
• Pathologic factors • Cumulative effect
• Genetic factors
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Weight and age
Recommended dosage of a drug is based on drug
evaluation studies & is targeted at a 150-lb person.
Heavier- larger doses
Lighter- smaller dosage
Children- immature system for drug handling
Older adults- aging process- less effective
absorption, less efficient distribution less fewer plasma protein
efficient perfusion, altered biotransformation liver changes less
effective excretion
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gender
• Men- more
vascular
muscles,
effect of drugs
so soon
• Women-
more fat cells,
drugs deposit
in fats maybe
slowly
released &
cause effects
for a
prolonged
period
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Physiological & pathological factors
• Physiological differences such as diurnal
rhythm of nervous & endocrine system, acid
base balance, hydration, & electrolyte
balance can affect the way that a drug works
on the body & the way the body handles the
drug.
• GI dse- absorption
• Vascular dse & low BP- distribution of drug
• Liver/kidney- affect metabolism & excretion
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Immunological factors
• People can
develop allergy
• Exposure to its
proteins, pt can
develop
antibodies to a
drug; w/ future
exposure- may
experience full
blown allergic
reaction
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Psychological factors
• Placebo effect- drug is more likely to be effective if the pt
thinks it work than if he believes it will not work
• TOLERANCE
• May come bec of increased metabolism of the drug,
resistance to its effects, or other pharmacokinetic
factors…drugs that are tolerated no longer cause the
same reaction, & need to be taken in increasingly larger
doses to achieve therapeutic effect.
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Drug-to-Drug Interactions
• Can occur any time two or more drugs are taken
together
• Can occur at:
– Site of absorption prevents/accelerate drug absorption tetracycline not absorb in GI tract w/ calcium
– During distribution compete for protein binding site (aspirin & methotrexate)
– During biotransformation 1 drug stimulates/blocks metabolism of the other drug
– During excretion 1 drug competes 4 excretion w/ other drug leading to accumulation & toxic effects of 1 drug
– At the site of action 1drug maybe antagonist of the other drug leading to no therapeutic effect 9antihypertensive
w/ allergy drug that also causes inc BP
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Drug–Food Interaction
• Certain foods interact with drugs
• Drugs are best taken on an empty stomach
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Drug–Laboratory Test Interaction
• Drugs may alter the results of lab testing
EEG no stimulant!!!
• Laboratory tests may be used to monitor
the effects of other medications antilipidemia inc liver
enzyme ast/alt
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Thanks chapter 2
ends!!!!!
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