We all take medications from regular acetaminophen to other medications that are prescribed. While we all take these medications, it is important to understand how drugs are processed, absorbed, and distributed their metabolism, and excretion after ingestion in the mouth. With this, I will be discussing Pharmacokinetics, which is the movement of drugs into and out of the body. Let's begin with no further ado.
Needpix.com
To understand this, I will be starting with absorption which is the process by which the drug moves from administration to the bloodstream. During absorption, drugs are commonly taken from the mouth (orally), they can also be taken through rectal administration, Intradermal administration (injecting into the dermis), Topical administration (rubbing on the skin), intravenous administration, inhaled administration, buccal administration, and sublingual administration. When drugs are taken especially Per Os (PO), they are able to reach the intestinal tract after which they get absorbed into the bloodstream. In other, for the blood to get through to the bloodstream, it has to pass through certain gastrointestinal linen, through certain mechanisms depending on the drug characteristics.
Mechanism of Transport for PO Drug absorption
Passive Diffusion process is when the drug passes through the cell membrane of the gastrointestinal tract, to move into the cell membrane on the basal lateral membrane and then into the blood. Where it is moving from an area of high concentration to an area of low concentration. Drugs that can easily pass with the mechanism of diffusion are small drugs, and drugs that are hydrophobic.
[Basal Lateral Membrane is the thin membrane that forms boundaries between cells made up of lipid bilayer and protein]
Large and hydrophilic drugs require proteins to be able to move through the cell membrane in other to get into the bloodstream, in a mechanism known as facilitated diffusion. Due to its size and solubility, it requires a protein pathway to be able to move through the membrane from a region of high concentration to a region of low concentration.
Another mechanism is Active transport, where the drug is pumped from a region of low concentration to a region of high concentration, requiring energy (ATP). Large, positive charged, and hydrophilic drugs use this mechanism to transport drugs through gastrointestinal membranes and the blood vessels.
Another mechanism is Endocytosis, which takes place when large molecular drugs such as B12 are given to a patient and are very difficult to pass through the cell membranes of gastrointestinal tracts to the blood vessels. These drugs bind onto receptors triggering the invagination of the drugs (endocytosis), to become endocytosed into the vasculature. Intravenous drugs go directly into the bloodstream instead of through membranes.
Factors Affecting the Absorption of Drugs
Certain factors affect the absorption of drugs which are pH. Drugs usually exist are either weak acids or bases. Base drugs are more difficult to pass through the phospholipid bilayer membrane as it is positively charged while weak acid drugs are non-polar drugs and can be easily absorbed. While weak acid drugs are easily absorbed as they pass through an acidic environment which is the stomach and the proximal duodenum allowing the weak acids to be absorbed. Basic drugs on the other hand are absorbed in the ilium (the distal part of the small intestine). Another factor that can affect drug absorption is blood flow. In reduced blood flow, drugs that are taken orally are reduced. Contact time is another factor as drugs tend not to be absorbed when the contact time is very fast such as in cases of Diarhhea, whereas, in cases where the patient is constipated, the transit time is very slow thereby giving the drug the time to get absorbed through the cell membrane to get into the bloodstream. The surface area of the gastrointestinal tract is another factor that affects the absorption of the drug into the bloodstream. Patients with multi-drug resistance such as P-glycoprotein could prevent the drug from being absorbed.
Bioavailability
With oral or other types of routes for taking drugs asides intravenously, the bioavailability of the drugs is dependent on the size, Solubility, and other factors not making it 100%. When drugs are taken PO, the bioavailability starts to rise gradually but doesn't reach 100% then it starts to fall as it becomes metabolized. When taken IV, the bioavailability is 100%.
Conclusion
Drugs are absorbed via the drug membranes through mechanisms such as passive diffusion for lipid and water-soluble drugs which are small, facilitated diffusion, and Endocytosis which is an active transport mechanism. The absorption of these drugs is dependent on their size, and solubility of the drugs. In my subsequent post, I will be looking at the distribution, metabolism, and the excretion of drugs.
Reference
<Holland-Frei Cancer Medicine. 6th edition : - Principles of Pharmacokinetics>
< MSD Manual Professional Version :- Overview of Pharmacokinetics >
<Genomind - Introduction to Pharmacokinetics:- Four Steps in a Drug’s Journey Through the Body>
<BLUK100-Calvey :-Drug Absorption, Distribution and Elimination>
<Evidence-based approach to assess passive diffusion and carrier-mediated drug transport>
Hi, I am Tobi a writer, speaker, relationship blogger, and lover of good music. I love making friends and learning from people. Want to hear me speak on relationships and general life issues, you can find my podcast channel Here and I also have a youtube channel where you can listen and watch any episode for free, do not forget to subscribe and share with friends. I sincerely appreciate every love I get from members of the community and do well to keep them coming.