How can drugs be excreted from the body

For this first-order input, one-compartment model, the drug concentration at any time t is given by:. F denotes the proportion of extravascularly-administered drug that reaches the system circulation after drug administration. The maximum concentration C max after extravascular administration and the time at which C max is attained t max are calculated by:.

Sometimes, drugs may display two or more phases during the declining portion of the concentration versus time profile [ 30 ]. When the plasma drug concentration exhibits a bi-exponential decay Figure 6 , lower panel on a semi-logarithmic scale following an i.

The two-compartment model divides the body into the central compartment as per the one-compartment model, as well as a peripheral compartment that lumps together slowly-perfused tissues, such as fat and muscle. As portrayed by Figure 6 lower panel , the concentration versus time profile after a single i. The initial rapid decline in concentration is a consequence of simultaneous elimination to the external environment and distribution of the drug from the plasma to tissues lumped under the peripheral compartment.

After the distribution process is completed and equilibrium is established between drug concentrations in the central compartment and the peripheral compartment, the drug concentration in the central compartment decreases at a rate dependent on the drug elimination rate. In general, the drug elimination rate is lower than the initial rate of decline in concentration due to the simultaneous distribution and elimination of the drug from the central compartment.

Without specifically describing the micro rate constants, the drug concentration at any time t after a single i. Drug disposition refers to the combination of distribution and elimination.

Distribution is a reversible process of movement of drugs from and to the site of measurement, typically the plasma or blood. Elimination comprises metabolism and excretion, and represents the total irreversible loss of the drug from the body. Drug disposition and elimination contribute to overall efficacy or toxicity and hence, up-to-date understanding of these pharmacokinetic processes is essential to safe, professional practice around medications and drug treatment.

Compartmental modelling is a useful method to understand distribution and excretion processes [ 27 , 31 ]. Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3. Help us write another book on this subject and reach those readers.

Login to your personal dashboard for more detailed statistics on your publications. Edited by Tarek A Ahmed. Lakshmana Prabu, T. Suriyaprakash, K. Ruckmani and R. Edited by James Paxton. We are IntechOpen, the world's leading publisher of Open Access books. Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. Downloaded: Introduction Pharmacokinetics is a branch of pharmacology that examines how drug concentrations change with respect to time as a function of absorption, distribution, metabolism and excretion [ 1 ].

Another way to consider pharmacokinetic processes is to group them into two components: intake, which describes the time course of drug movement from the site of administration, e. Passive diffusion across cell membranes A major factor affecting drug distribution is the physicochemical properties of the drug [ 3 ] since these would influence the permeability of the drug to various tissues.

Plasma protein and tissue binding Another factor influencing drug distribution is the preferential binding to plasma proteins and tissues [ 7 ]. Table 1. Extent of plasma protein binding of selected drugs. Table 2. Liver metabolism In the liver, a wide array of enzymes exists to biotransform drugs, producing less active or in some cases more active metabolites [ 14 ]. Phase I metabolism Phase I reactions involve the introduction into or unveiling of a polar functional group e.

Table 3. Selected CYP enzyme-substrate drugs and their respective inducers and inhibitors. Phase II metabolism The derivative from phase I metabolism may be excreted via the urine immediately if high aqueous solubility is achieved. Drug excretion Excretion is the principal mode of termination of drug and metabolite effects. Biliary excretion While in the liver, drugs or metabolites can also be secreted into the bile in much the same manner as the kidney secretes drugs into the nephron tubular filtrate [ 22 , 23 ].

One-compartment model Although more complex pharmacokinetic models may be necessary, a one-compartment model with first-order input provides a reasonable description of the time course for many drugs given at therapeutic doses Figure 5 , for example [ 27 - 29 ]. Two-compartment model Sometimes, drugs may display two or more phases during the declining portion of the concentration versus time profile [ 30 ]. More Print chapter. How to cite and reference Link to this chapter Copy to clipboard.

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More About Us. CYP enzyme. Examples of substrate drugs. Induced by. Inhibited by. Once drugs enter the body, the process of leaving the body or elimination begins. In order for drugs to leave the body, most drugs need to be metabolised. This is a chemical process in which the liver transforms the drug into a substance called a metabolite that can be passed out of the body.

While drugs and their metabolites are mostly excreted by the kidneys into urine, drugs can also leave the body in other methods, such as breath and sweat, hence the noticeable smell of alcohol on someone who has been drinking very heavily.

Drugs can also leave the body in very small quantities via saliva and breast milk. Drug testing often detects the metabolites of specific drugs in the urine of a person, rather than the drug itself. Summary Various methods of drug administration including: Injecting Smoking Inhaling Snorting Swallowing Intravenous drug use and inhaling the fumes of a substance are the two fastest methods of achieving psycho-active drug effect.

The excretion of drugs in breast milk is significant only because the drug may affect the breastfeeding infant see Drugs That Should Not Be Taken While Breastfeeding Drugs That Should Not Be Taken While Breastfeeding When mothers who are breastfeeding have to take a drug, they wonder whether they should stop breastfeeding. The answer depends on the following: How much of the drug passes into the milk Whether Excretion in exhaled air is the main way that inhaled anesthetics are eliminated.

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Other forms of drug elimination. Administration and Kinetics of Drugs. Drug Elimination and Clearance. Leon Shargel, et al. McGraw Hill; Accessed November 14, APA Citation Chapter 6. McGraw Hill. MLA Citation "Chapter 6. Download citation file: RIS Zotero. Reference Manager. Autosuggest Results. Print Frequently Asked Questions What's the difference between clearance and the rate of drug elimination? Why is clearance a useful pharmacokinetic parameter?

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