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Pharmacokenetics: Drug Absorption

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Absorption is the process by which a drug is transferred from its site of entry into the body to the circulating fluids of the body for distribution.

Absorption is the process by which a drug is transferred from its site of entry into the body to the circulating fluids of the body (i.e., blood and lymph) for distribution. The rate at which this occurs depends on the route of administration, the blood flow through the tissue where the drug is administered, and the solubility of the drug. It is therefor important to administer oral drugs with an adequate amount of fluid, usually large (8 oz) glass of water; give parenteral forms properly so that they are deposited in the correct tissue for enhanced absorption; and reconstitute and dilute drugs only with the diluent recommended by the manufacturer in the package literature so that the drug solubility is not impaired. Equally important are nursing assessments that reveal poor absorption (e.gg., if insulin is administered subcutaneously and a lump remains at the site of injection 2 to 3 hours later, absorption from that site may be impaired).

The three categories of drug administration are enteral, parenteral, and percutaneous routes. In the enteral route, the drug is administered directly into the gastrointestinal (GI) tract by oral, rectal, or nasogastric routes. The parenteral routes bypass the GI tract by using subcutaneous (subQ), intramuscular (IM), or intravenous (IV) injection. Methods for percutaneous administration include inhalation, sublingual (under the tongue), or tropical (on the skin) administration.

Regardless of the route of administration, a drug must be dissolved in body fluids before it can be absorbed into body tissues. For example, before a solid drug taken orally can be absorbed into the bloodstream for transport to the site of action, it must disintegrate and dissolve in the GI fluids and be transported across the stomach or intestinal lining into the blood. The process of converting the drug into a soluble form can be partially controlled by the pharmaceutical dosage form used (e.g., solution, suspension, capsule, and tablets with various coatings). This conversion process can also be influenced by administering the drug with or without food in the patient's stomach.

The rate of absorption when a drug is administered by a parenteral route depends on the rate of blood flow through the tissues. Circulatory insufficiency and respiratory distress may lead to hypoxia and further complicate this situation by resulting in vasoconstriction. For that reason, the nurse should not give an injection when circulation is known to be impaired. Another site on the rotation schedule should be used. Subcutaneous injections have the slowest absorption rate, especially if peripheral circulation is impaired, IM injections are more rapidly absorbed because of greater blood flow per unit weight of muscle compared with subcutaneous tissue. (Depositing the medication into the muscle belly is important. The nurse must carefully assess the individual patient for correct length of needle to ensure that this occurs). Cooling the area of injection slows the rate of absorption, whereas heat or massage hastens the rate of absorption. The drug is dispersed throughout the body most rapidly when administered by IV injection. (The nurse must be thoroughly educated regarding the responsibilities and techniques associated with administering IV medications. Once the drug enters the patient's bloodstream, it cannot be retrieved).

Absorption of tropical drugs applied to the skin can be influenced by the drug concentration, length of contact time, size of affected area, thickness of skin surface, hydration of tissue, and degree of skin disruption. Percutaneous absorption is greatly increased in newborns and young infants, who have thin, well-hydrated skin. When drugs are inhaled, their absorption can be influenced by depth of respirations, fineness of droplet particles, available surface area of mucous membranes, contact time, hydration state, blood supply to the area, and concentration of the drug itself.

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