4.1
The process of pharmacokinetics
The process of pharmacokinetics is described in the following schema.
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12. The process of pharmacokinetics
Fig. 12. The process of pharmacokinetics
4.1.1
Transportation
There are two routes in which drugs can get into the body:
Enteral – The drug enters the body through the GI tract and is taken by mouth, through the rectum, under the tongue or held by the cheeks.
Parenteral – The drug enters the body through other than GI tract. It can be injected into the veins, muscles or under the skin. It can also be inhaled through the lungs, or absorbed trans-dermally through the skin via ointment or a patch.
4.1.2
Absorption
Absorption is movement of the drug from its site of administration into the blood stream. Several factors can affect drug absorption.
  • Concentration
  • Area of absorbing surface
  • Solubility
  • Ph influence
  • Route of administration
Bioavailability is the percentage of the drug that reaches to systemic circulation after administration. It depends on the route of administration as well as on the ability of the drug to cross membranes and reach its target. Drugs absorbed through the GI tract must pass through the liver before reaching the systemic circulation. However, liver can inactivate the drug, making less of the drug available to reach the target organ.
Absorption at cellular level occurs through:
  1. Passive transport - diffusion and filtration
  1. Active transport
  1. Facilitated diffusion
  1. Pinocytosis
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Video 3. Membrane transport
4.1.3
Distribution
Once a drug has accessed the blood stream, it is distributed to other tissues that initially included no drug. The concentration gradient is in the direction of plasma to tissue. There exist several factors affecting drug distribution:
  • Tissue permeability – ability of the drug to pass through the membranes.
  • Extent of plasma protein binding – drug can bind to a protein that will keep the drug inactive. Only an unbound drug can attach to the receptors.
  • Blood Ph
  • Difference in the blood flow – highly perfused tissues and organs are reached first.
  • Binding to subcellular components
Movement of the drug continues until an equilibrium is reached between unbound drug in plasma and tissue fluids.
4.1.4
Drug storage
The primary site of lipid soluble drugs is in adipose tissue. They can remain there for quite a long time because of its low metabolic rate and poor blood supply. Bones serve as the storage for toxic agents like heavy metals. Binding can cause also muscles to store drugs. Liver and kidneys are the primary organs of drug storage due to their filtering function.
4.1.5
Metabolism
Metabolism of drugs refers to chemical alteration of the drug in the body. Primary sites of drug metabolism is liver followed by kidneys, intestines and lungs. Biotransformation of a drug, chemical changes that occur in the drug after administration, can lead to its inactivation, forming active metabolite from an active drug or activation of an inactive drug. Metabolite is the altered version of the chemical compound and prodrug requires activation of drug so as it can act within the body.
4.1.6
Excretion
Excretion means the removal of the drug and its metabolite from the body. They are excreted in:
  • urine
  • saliva
  • sweat
  • faeces
  • exhaled air
Organs excreting drugs are thus kidneys, sweat glands, lungs, skin, salivary glands and GI tract. The rate of drug removal from the body is connected and proportional to its concentration in the plasma.
Half time refers to time necessary to decrease the blood levels of a drug by one-half. A one-time drug dosage will be eliminated by five half times.