Drugs and the Body

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

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

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

Lock & Key

Enzymes

• Drugs can interfere with the enzymes that may be catalysts for chemical reactions
• Enzymes produce a cascade effect

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

The Processes by Which Drugs Are Handled in the Body

Pharmacokinetics

• Critical concentration
  • The amount of a drug that is needed to cause a therapeutic effect
• Loading dose
  • A higher dose than that usually used for treatment
• Dynamic equilibrium
  • The actual concentration that a drug reaches in the body

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

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

Factors Affecting Absorption

Distribution

• Protein binding
• Blood–brain barrier
• Placenta/breast milk

Biotransformation

• The liver is the single most important site for biotransformation (metabolism)
• This process breaks down medications
• It helps to prevent medications from causing adverse effects on the body

Excretion

• Removal of drugs from the body
• Kidneys play the most important role in the excretion of medication

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
• Half-life is affected by the absorption, distribution, metabolism, and excretion

Pharmacokinetics

Half-life

Calculating Half-Life

Factors Influencing Drug Effects

• Weight
• Age
• Gender
• Physiologic factors
• Pathologic factors
• Genetic factors
• Immunologic factors
• Psychological factors
• Environmental factors
• Drug tolerance
• Cumulative effect

Drug-to-Drug Interactions

• Can occur any time two or more drugs are taken together
• Can occur at:
  • Site of absorption
  • During distribution
  • During biotransformation
  • During excretion
  • At the site of action

Drug–Food Interaction

• Certain foods interact with drugs
• Drugs are best taken on an empty stomach

Drug–Laboratory Test Interaction

• Drugs may alter the results of lab testing
• Laboratory tests may be used to monitor the effects of other medications