How Does The Body Absorb Drugs & Medication

The absorption of drugs is determined by the physicochemical properties of the drug, the formulation and mode of application. Dosage forms (eg., Tablet, capsule, solution) composed of the active substance and other ingredients are (oral z. B., buccal, sublingual, rectal, parenteral, topical, inhalation) each for certain types of application formulated. Regardless of the dosage form drugs must be solved in order to be absorbed to. Therefore, solid formulations have to disintegrate (z. B. tablets) and can dissolve.

(See also Overview pharmacokinetics.) The absorption of drugs is determined by the physicochemical properties of the drug, the formulation and mode of application. Dosage forms (eg., Tablet, capsule, solution) composed of the active substance and other ingredients are (oral z. B., buccal, sublingual, rectal, parenteral, topical, inhalation) each for certain types of application formulated. Regardless of the dosage form drugs must be solved in order to be absorbed to. Therefore, solid formulations have to disintegrate (z. B. tablets) and can dissolve. Unless i.v. given a medicine must overcome several semipermeable cell membranes before it reaches the systemic circulation. Cell membranes are biological barriers, which selectively inhibit the passage of drug molecules. The membranes consist primarily of a bimolecular lipid matrix, which determines the permeability of the membrane. Drugs can cross cell membranes by passive diffusion Facilitated passive diffusion Active transport Sometimes Pinocytosis serve various globular proteins that are embedded in the matrix, as receptors and help to transport molecules through the membrane. Passive diffusion drug diffuse (gastrointestinal fluids z. B.) to such a low concentration (eg., Blood) through a cell membrane from a region of higher concentration. The diffusion rate is directly proportional to the gradient, but is also dependent on the lipid solubility of the molecule, the molecular size, the degree of ionization and the size of the absorbing surface. Since the cell membrane is lipoid, lipid-soluble drugs diffuse the fastest. Small molecules tend to penetrate more quickly through membranes than larger ones. Most drugs are weak organic acids or bases which are present in aqueous medium in non-ionized or unionized form. The unionized form is usually fat-soluble (lipophilic) and readily diffuses across cell membranes. The ionized form has a low lipid solubility (the other hand, a high water solubility – is, for example hydrophilic.) And a high electrical resistance and can therefore difficult to penetrate cell membranes. The proportion of the present non-ionized form (and thus the ability of the drug to pass through a cell membrane) is determined by the ambient pH value and the pKa-value (pKa, the acid dissociation constant) of the drug. The pKa is that pH value at which the concentrations of the ionized and unionized forms are the same. If the pH is less than the pKa outweighs the unionized form of a weak acid, on the other hand, the ionized form of a weak base. In the gastric juice (pH 1.4), the ratio is reversed (1000: 1). Therefore, in the plasma (pH 7.4), a ratio of unionized to ionized form of 1 for a weak acid (e.g., having a pKa of 4.4.): Before the 1000th Thus, in oral administration of a weak acid of the majority of the drug in the stomach in unionized form is present, which favors the diffusion through the gastric mucosa. The majority of the drug is ionised in the stomach before. the result of a weak base having a pKa of 4.4 is reversed. Theoretically, weakly acidic drug (for. Example, acetylsalicylic acid) in an acidic medium (e.g., as the stomach) more easily absorbed than weakly basic drugs (e.g.., Quinidine). However occurs most of the absorption in the small intestine, whether the drug is acidic or basic, as in the small intestine, the surface area is larger and the membranes have a higher permeability (oral administration; Oral administration). Facilitated passive diffusion Certain molecules with low lipid solubility (eg., Glucose) penetrate membranes faster than expected. One theory for this is facilitated passive diffusion: a transport molecule (carrier) in the membrane, the substrate molecule binds reversibly outside the cell membrane, diffuses as a carrier-substrate complex rapidly through the membrane and the substrate is exposed at the inner surface. In these cases, the membrane transports only substrates with a relatively specific molecular configuration, and the availability of transport molecules limits the process. This process does not require energy, and transport against the concentration gradient is not possible. Active transport Active transport is selective, requires energy and may include a transport against the concentration gradient. Active transport appears to be limited to drugs that endogenous substances are structurally similar (z. B. ions, vitamins, sugars, amino acids). These drugs are usually absorbed at specific locations in the small intestine. In pinocytosis pinocytosis liquids or particles are “swallowed” by the cell. The cell membrane everts a, encloses the fluid or particles, then fuses again, forming a vesicle that later detaches and moves to the cell interior. For this energy must be expended. With the exception of protein drugs pinocytosis probably plays a minor role in the transport of drugs. Oral application To be absorbed, an orally administered drug must secretions, including potentially survive degrading enzymes in contact with a low pH and numerous GI. Peptide drugs (eg., Insulin) are especially susceptible to degradation and will not be administered orally. The absorption of oral drugs includes transporting a through membranes of epithelial cells in the gastrointestinal tract. Absorption is influenced by differences in the pH along the Gastrointestinallumen size of the surface area per Gastrointestinalvolumen circulatory presence of bile and phlegm The type of epithelial membranes However, contact with the mucous membrane is usually too short for a significant absorption. The oral mucosa has a thin epithelium and is rich in blood vessels, which promotes absorption. A drug which is pushed between the gum and cheeks (buccal administration) or placed under the tongue (sublingual administration), lingers longer, which increases the absorption. The stomach has a relatively large epithelial surface, but its thick mucus layer and the short residence time limit absorption. Since absorption takes place predominantly in the small intestine, gastric emptying is often the rate-determining step. Food intake, especially high fat meal, the gastric emptying is delayed (and the rate of absorption of drugs), which explains why taking some drugs on an empty stomach accelerates absorption. Drugs that affect gastric emptying (z. B. parasympatholytics), at the same time affect the rate of absorption of other drugs. Food intake can with poorly soluble drugs to increase the extent of absorption (eg. As griseofulvin), this for drugs which are degraded in the stomach, reducing (eg. As penicillin G), or only have little or no effect. The small intestine has the largest surface area for absorption of drugs in the gastrointestinal tract and its membranes have a higher permeability than in the stomach. For these reasons, most drugs are primarily absorbed in the small intestine, and acids are absorbed in the intestine faster than in the stomach, although they can reach easily through membranes than non-ionized drugs. The pH in the intestinal lumen is between 4 and 5 for the duodenum, but will increasingly alkaline and reaches the lower ileum about 8. The gastrointestinal microflora can reduce the absorption. A decreased blood flow (z. B. in case of shock) can reduce the concentration gradient along the intestinal mucosa and reduce the absorption by passive diffusion. The duration of the intestinal transit can also affect the absorption of drugs, especially for those drugs that are absorbed through active transport (eg. As B vitamins), which dissolve slowly (eg. As griseofulvin) and the polar are ( ie low fat solubility, eg. as many antibiotics). To ensure the long-term use as possible, doctors should prescribe oral suspensions and chewable tablets for children under 8 years. In adolescents and adults, most drugs are administered orally as tablets or capsules primarily for convenience, economy, stability, and patient acceptance. As solid drug formulations must first loosen so that resorption can take place, the dissolution rate determines the availability of the drug for absorption. If the resolution is slower than the absorption, the solution process is the rate-limiting step. A modification of the formulation (eg. As the drug formulation as a salt, crystal, or hydrate) can alter the dissolution rate and hence control the entire absorption. Parenteral administration drug i.v. are administered directly into the systemic circulation. Drugs that intramuscular (i.m.) or subcutaneous (s.c.) is injected, but must overcome one or more biological membranes to enter the systemic circulation. Are proteins with a molecular weight> 20,000 g / mol i.m. or s.c. injected, this move so slowly through the capillary membranes that absorption takes place largely through the lymphatic system. In such cases, the medicine passes slowly and, due to first-pass metabolism (metabolism of a drug before reaching the grand cycle) by proteolytic enzymes in the lymphatic vessels, often incomplete in the general circulation. The blood flow (blood flow / g tissue) greatly influences the capillary absorption of small molecules that i.m. or s.c. be injected. Therefore, the injection site can affect the rate of absorption. For salts of poorly soluble bases and acids (eg. B. parenteral forms of phenytoin) and in patients with poor peripheral blood circulation (eg. As in hypotension or shock), the absorption after i.m. or s.c. delayed injections or be uneven. Controlled release formulations controlled release formulations are being developed to reduce the frequency of application for drugs having a short elimination half-life and duration of action. These formulations also limit fluctuations in the plasma concentration of the drug and thus lead to a more consistent therapeutic effect, while the undesirable effects can be minimized. The rate of absorption can be slowed by drug particles are coated with wax or other water-insoluble materials by being embedded in a matrix which releases the drug during passage through the gastrointestinal tract, or by being complexed with ion exchange resin. These formulations, most of the absorption in the colon takes place. A crushing or otherwise destroy a controlled release tablet or capsule can often be dangerous. Transdermal controlled release systems are designed to release drugs over extended periods of time, sometimes for several days. For transdermal administration drugs must have suitable characteristics in terms of skin penetration properties and high potency because the penetration rate and the application surface is limited. Many do not intravenous parenteral formulations are designed to maintain the plasma concentration of the drug upright. The absorption of antibiotics can be prolonged if they i.m. in the form of their relatively insoluble salt be injected (eg., penicillin G, benzathine [n. d. Übers .: not available in Germany as a single agent]). For other drug suspensions or solutions are developed in non-aqueous vehicles to delay absorption (eg., Crystalline suspensions for insulin).

Health Life Media Team

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