The human stomach is capable of absorbing most acidic drugs and the very weakly basic drugs. Salicylic acid, aspirin, thiopental, secobarbital and antipyrine, which are undissociated in the acidic gastric contents, were readily absorbed.
Theoretically, weakly acidic drugs (eg, aspirin) are more readily absorbed from an acid medium (stomach) than are weakly basic drugs (eg, quinidine).
Apart from being lipid-soluble, another factors which favour drug absorption in the stomach is sufficiently high concentration gradient to drive simple passive diffusion. This means the drug has to be low-potency (i.e. given as a very large dose).
In acidic medium, lots of protons are present. Therefore, greater amount of acidic drug is unionized (shift towards left of eq 1). Thus in acidic medium acidic drug is present more in unionized form, which increases its absorption. This is why acidic drugs are better absorbed from the stomach.
The small intestine is the main site for drug absorption, as it has a high surface area of approximately 400 m2 [13]. The microvilli are lined with goblet cells whose major role is to secrete the glycoproteins that form the mucosal lining of the small intestine.
Because most absorption occurs in the small intestine, gastric emptying is often the rate-limiting step.
Active transport requires energy to facilitate the transport of drug molecules against a concentration gradient, which usually occurs at specific sites in the small intestine.
Most drugs given by mouth are designed to be absorbed from the small intestine (and to a small extent from the stomach). However, drugs are also absorbed from, and designed to be given by, alterna- tive sites in the gastrointestinal tract.
In the stomach, drugs that are weak acids (such as aspirin) will be present mainly in their non-ionic form, and weak bases will be in their ionic form. Since non-ionic species diffuse more readily through cell membranes, weak acids will have a higher absorption in the highly acidic stomach.
In contrast, the small intestines have a large surface area available for absorption owing to extensive villi and microvilli. As a result, acidic drugs are most likely to be absorbed in the acidic areas of the proximal duodenum; whereas, basic drugs will be best absorbed in more alkaline areas of the distal ileum.
Owing to its enlarged surface area, the small intestine is considered to be the major site for passive transcellular drug absorption. The anatomical surface area of the intestinal epithelium is, however, not necessarily analogous to an effective absorptive surface area (Figure 3b).
The primary site of drug absorption is usually the small intestine, and the bioavailability of the medication is influenced by the amount of drug absorbed across the intestinal epithelium. The first-pass effect is an important consideration for orally administered medications.
Once the medication arrives, it is broken down by stomach acids before it passes through the liver and then enters the bloodstream. Certain medications may stay in the bloodstream longer – it all depends on the dosage and drug family consumed.
Three substances are absorbed through the stomach water, Simple sugar like Glucose, and alcohol like ethanol.
The most common mechanism of absorption for drugs is passive diffusion. This process can be explained through the Fick law of diffusion, in which the drug molecule moves according to the concentration gradient from a higher drug concentration to a lower concentration until equilibrium is reached.
Antacids are a combination of various compounds with various salts of calcium, magnesium, and aluminum as active ingredients. The antacids act by neutralizing the acid in the stomach and by inhibiting pepsin, which is a proteolytic enzyme.
Aspirins absorption is pH sensitive at the level of the small intestine. Absorption is higher through the small intestine than the stomach for the same pH range. At pH 3.5 or 6.5, aspirin's intestinal absorption is greater than the gastric absorption of the compound. The stomach does not absorb aspirin at pH 6.5.
Proton pump inhibitors are the most potent of the medications that reduce acid production. Proton pump inhibitors promote healing of ulcers in a greater percentage of people in a shorter period of time than do histamine-2 (H2) blockers and thus are typically preferred to H2 blockers for treating ulcers.
Lipid-soluble drugs are absorbed more rapidly than non–lipid-soluble drugs. Gastric fluid has a pH of approximately 1.4. Drugs that are organic acids, such as aspirin, freely diffuse across the gastric mucosa into the circulatory system.
Upon oral administration, the drug is passing through the stomach to the small intestine where most of the drugs are absorbed. Poorly absorbed drugs, or drugs in modified release formulations will pass to the colon, where further absorption can occur.
The small intestine is the most important absorbing organ in the GI tract. About 90% of nutrient absorption takes place in the small intestine.
The small intestine is responsible for absorption of nutrients, salt, and water.
When given intravenously, a drug is delivered immediately to the bloodstream and tends to take effect more quickly than when given by any other route.
The route by which the medicines are directly introduced into the bloodstream through a vein is known as intravenous route of administration. The intravenous route is considered to be the fastest route of drug administration. The injections and the infusions are administered by this route have 100% bioavailability.