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The Journey of Drugs through the Body
Mar 1, 2013

We get ill due to various reasons and in order to get better, we sometimes get some rest, sometimes be extra cautious with what we eat and other times use medicine. But how does medicine get absorbed from our intestines and get transported to the sickened area? How does it get removed from the body? What are the events that affect all these?

Some medications are effective directly over the area they are applied to. Some however are transported to distant regions via blood flow and that is where they are most effective. Medication is either taken orally or through injection. When medicine passes into the blood stream from the place of administration, it is considered to be absorbed. A good example is the transportation of medicine into the blood stream of capillary vessels between the muscle cells when injected into muscle tissue. A drug taken orally however is absorbed through the blood vessels in the gastro-intestinal system.

For orally-taken medication to be absorbed, it should be able to dissolve in gastro-intestinal fluids. First, it is broken into smaller units due to the corroding effects of stomach acid and various enzymes are secreted, then chemicals in the drug composition pass into the gastro-intestinal fluid in a molecular form. This event resembles the dissolving of a sugar cube inside a glass of hot tea. First, the sugar cube gets broken into pieces and then dissolves. A mix with a tea spoon makes this event happen a little faster. In a similar fashion gastro-intestinal movements help with the absorption of medicine. Liquid drugs like syrups dissolve in the gastro-intestinal fluid faster since they are already in smaller units; therefore they get absorbed faster.

Drugs mostly get absorbed through the small intestine. The most important task of this organ is to enable the absorption of nutrients. It is approximately 10 meters long and 4 centimeters wide. The inner lining of the small intestine has finger-like projections called villus and even smaller projections that are located on these villi are called microvillus. One of the reasons, maybe the most important reason, why the our intestines are created in this way is that as a result, the inner surface area of intestines increases multifold. Such that the inner surface area of a human beings small intestine can increase up to 200 m2 and this greatly facilitates the absorption. These projections are made of intestinal cells.

The molecules carrying the medication reach the capillary vessels by passing through these cells and then join the blood stream by crossing through capillary vessel cells. Furthermore, intestinal cell membranes host a special protein that filters unwanted substances for the cell and returns them back to intestinal lumen. Thus these unwanted substances are excreted out of the body along with other unabsorbed matter. In the same way, some drugs are held by this protein and released back into the lumen thus decreasing absorption rate for drugs experiencing this reaction.

Liver: The organ responsible for eliminating the harmful effects of medication

As soon as medication joins the bloodstream after absorption, it is first transported to the liver. This is because pulmonary veins that collect blood from the intestines are connected primarily to the liver. One of the many functions of the liver is the elimination of harmful substances entering the body. For this reason, absorbed substances are directly sent to the liver. The liver is employed with the task of chemical conversion with these substances that are transported to it. One of the wisdoms behind liver metabolism is to reduce the effects of these harmful substances via these events and to convert them into an excretal material. In the same way, drugs are metabolized in the liver, lose their efficacy and prepare for excretion.

Many drugs interfere with each other’s metabolism. If one drug’s metabolism is inhibited, blood concentration of such chemicals increase and adverse effects of drugs become more frequent. Irresponsible drug use must be avoided for this reason. Drug interactions may lead to major damage in addition to the drug’s individual adverse effects. Moreover different nutrients also affect drug metabolism. For instance, grapefruit inhibits metabolism of certain medicines, as a result blood concentration of these medicines increase and adverse effects can be observed. On the other hand, some nutrients like broccoli, cabbage, and charcoal roasted meat speed up the metabolism of certain medicines. In this case, the blood concentration of the affected drug drops and may lead to reduced benefits from intended use. Because of this reason, patients on long-term medicine treatment should not consume these types of food.

The rest of the drug molecules that escape these metabolisms is directed towards blood vessels feeding other organs. Some drug metabolisms in the liver present individual differences as metabolic levels change from person to person. Thus, a drug with the same dosage develops desired blood concentrations for some people, fails to meet this level for others or can even cause high blood concentrations enough to generate adverse effects in other individuals. That is why a medicine that has benefited a patient should never be used by somebody else without consulting a doctor.

The function of bile

Bile secretion originating from the liver and gall bladder has critical importance in the digestion and absorption of fats. Bile breaks apart fats into small pieces so that digestive enzymes can affect them. As a result of this, absorption is provided for fats and vitamins like A, D, E, K that are soluble in fats. In a similar fashion bile improves solubility and absorption of some drugs that does not dissolve in gastro-intestinal fluid. Another task of bile secretion is the removal of certain substances from the body. Waste materials in the bile that is dumped into duodenum are excreted through the digestive track. Some drugs are excreted in this way.

Drug intake before or after meals

It is a well known practice that medications are advised either to be taken after or before meals. When medications are taken after a meal, they cause less of the possible unwanted disturbances such as stomach sickness, aches or indigestion.

On the other hand, nutrients may reduce intake of certain drugs, therefore they need to be taken before meals. However medications taken right before a meal does not apply in this case since the food will still mix with the medication in the stomach. When taking these medicines, it should at least be an hour before meals. Generally consumption of a medicine before or after a meal does not really change its absorption level. But medications taken before meals pass the stomach into the intestines without delay and therefore get absorbed faster. This practice is important in cases where an immediate effect is desired such as pain relief. Plentiful water intake also helps with faster and improved absorption of drugs.

Some medications are packed into capsules made of gelatin-like substances. Medicines with undesirable taste and smell can be offered in this form for consumption. Moreover, if a drug is harmful to the stomach or gets degraded in stomach acid, then this drug can be prepared in capsules that are durable to stomach acid but soluble in the intestines. That is why consumption of medication without its intended capsule should be avoided. In a similar fashion, some medicinal tablets are designed to deliver its molecular contents particularly to the intestines. These types of medication must be taken as a whole unit. Otherwise it can be ineffective or may lead to harmful reactions.

Delivery of drugs to targeted regions

The molecules carrying medication that join the blood via absorption get dispersed by blood circulation throughout the body. These molecules reach various parts of the system via blood vessels, and then diffuse into organs via capillary vessels. However, their entry to the brain is difficult because this vital organ of the body has a special protection to guard itself from possible harmful effects of various substances that enter the body from the outside. Capillary vessels in the brain are different from other capillary structures in the rest of the body as they are created without an intercellular space in between vessel cells. Furthermore, these cells are bound to each other with their tight connective regions.

These capillaries are surrounded by a thicker membrane compared to other capillary vessels. This membrane is also host to various cells that wrap around the vessel. Therefore, because of these factors and other similar ones, some medications can enter the brain in very limited amounts. Drug molecules can display their targeted effects when they bind to target proteins, called receptors, in the organs. These proteins, which are very unique to each drug, exist on the cell membrane or in the cell. In addition, drug molecules also bind to other receptors that are not specific for them, and this causes adverse effects as a result.

Excretion of drugs from the body

Drugs are excreted from the body via the liver and kidneys. One function of these organs is to filter the blood from foreign substances. It was previously mentioned that absorbed substances from intestines are transported to the liver first where some amount gets metabolized and the remaining amount rejoins blood circulation that feeds other organs. Drug molecules that pass through the liver return back to it repeatedly many times because of continual blood circulation. In each of these arrivals, some amount is again metabolized. Molecules of metabolized drugs are excreted out of the body via the kidneys and through the bile at a limited level.

Only some portion of drug molecules get excreted via the kidneys without being metabolized. This ratio is higher with some medications. These types of medications are considered to be removed only by the kidneys, whereas some other drug types cannot be excreted without getting metabolized through the liver. As previously mentioned, the purpose of drug metabolism is to convert drugs into easily removable forms. If these types of drugs are not metabolized, they are rejoined to the blood circulation after filtration by the kidneys without joining the urine. It is impossible for the liver to sense these happening within the kidneys if it was not that the liver and every cell in it were employed by one authority who created them in the first place. Drug molecules concentrate in the liver and kidneys since these organs are employed with drug removal. As a result harmful effects of drugs are often experienced in these organs. Therefore unnecessary drug use should be avoided, otherwise the health of these organs deteriorate and eventually fail to carry out their basic functions.

As noted above, events that are taking place within many of our organs, stomach and elsewhere like kidneys impact on the journey of drugs in our body, therefore changing its effect. The harmonious creation of our organs that are home to many miraculous events is the major component of the entire process in which causations have their due role only as much as they are allowed by their Creator. As a test for humankind, both illness and the cure is provided by God, the All-Healer. Therefore it is the duty of a patient to see a doctor, take the medication properly and never forget that cure is only provided by the Almighty, without obsessing over causational chains.


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  • Brunton, Laurence, John Lazo, Keith Parker. 2006. The Pharmacological Basis of Therapeutics. McGraw-Hill Professional