Drug Interactions in the body: Pharmacodynamics

“Drugs do not create new effects in the body. They inhibit or module reactions in the body.” 

Drugs must be hydrophilic enough to be able transported in the blood, but also lipophilic enough to travel through a membrane. 

Pharmacodynamics is the study of how drugs interact in the body at their action sites. 

It is responsible for

  • Absorption
  • Distribution
  • Excretion
  • Biotransformation / metabolism 

Of a drug. 

Pharmacokinetics and dosage determine the strength of the drug and the effects produced in the body.  

There are three different drug action phases in the body. These phases determine how a particular drug reaches the active site where it is needed. 

  • Pharmaceutic: dissolution 
    • Drug forms into a solution to pass biological boundaries 
    • Water soluble drugs need carriers such as enzymes to pass through cell membranes. 
    • GI tract requires drugs to be liquid to be easily absorbed by intestines
      • GI tract is lipid soluble, so any drug that is nonpolar and small can pass through. 
    • Ex: tablet disintegrates and dissolves into solutions in our body
    • Drugs can be coated with substances so as to not allow it to disintegrate before reaching the site where it is needed.  
  • Pharmacokinetic
    • Absorption
      • Passive
        • Diffusion (higher to lower conc)
        • Requires no energy
      • Active
        • Requires enzyme or particular protein
        • ATP is required 
      • Pinocytosis 
        • Drug is transported across cell membrane as it is engulfed and packaged into a vesicle to move across the cytoplasm
    • Distribution
      • Affected by flow of blood, and drug affinity to target, protein, or tissue
      • Drugs are very often bound to protein such as albumen
    • Metabolism 
    • elimination/excretion 
      • Eliminated unchanged or as metabolites. 
      • Lipid soluble drugs do not leave until they are metabolized into more polar drugs. 
      • The kidney is primarily responsible for excretion of drugs. 
  • Pharmacodynamic 
    • Effect of drug on body

Sources: https://nursekey.com/drug-action-pharmaceutic-pharmacokinetic-and-pharmacodynamic-phases/

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