Factors that might influence the pharmacokinetic and pharmacodynamic processes in a patient
Pharmacodynamics is the study of how the body reacts to drugs, while pharmacokinetics focuses on how the body processes the drugs. Pharmacokinetics and pharmacodynamics of a drug is determined by four main factors; physicochemical properties, drug formulation, route of administration, and the rate of excretion.
• Physicochemical properties
Water acts as the carrier of drugs through the bloodstream. At the site of absorption, drugs must have some degree of solubility in water for them to be absorbed into the bloodstream. The cell membrane of the double lipid layer structure also requires certain drugs to be lipid-soluble for them to penetrate through the semi-permeable membranes and into the systematic circulatory system. Thus, the absorption rate of a certain drug depends on their degree of lipid solubility. Unless administered through an IV, all drugs must pass through several semi-permeable membranes before getting into the bloodstream. However, the chemical structure of a drug can be modified to increase its solubility in fats and therefore improve absorption.
• Drug formulation
The formulation of a drug is determined by its degree of dissolution or release from the dosage form. The degree of dissolution is determined by the drug’s dissolution constant and the PH of the absorption site. Different drugs have different dissolution levels, but the same drug can also have different dissolution degrees in other parts of the body because of the varying PH conditions.
• Route of administration
Drugs can be introduced into the body in many routes which include oral ingestion, injection (intravenously or IV), placed under the tongue (sublingually), breathed into the lungs, applied to the skin (cutaneous), rectal or vaginal insertion, or through the skin (transdermally). Regardless of the form of administration, a drug must be in a solution state for it to be absorbed into the circulation system. Different drugs are most effective when administered through specific routes. For instance, proteins and peptides are mostly administered through parenteral routes such as IV because of the large molecular sizes that prevent them from being absorbed through the biological barriers.
• Rate of excretion
Excretion is the processes by which the drug is broken down and expelled out of the body. This role is taken up by the kidneys. Sometimes, excreted drugs can be reabsorbed back into the bloodstream through the renal tubules in the kidneys such that the effect of the drug in the body is prolonged. Reabsorption occurs through the urine PH effect, and the alkaline or acidic nature of the drugs can interfere with the Ph of the urine affecting the tubular reabsorption of the drugs.
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