1.6 Excretion
Excretion
Excretion is the final stage of a medication interaction within the body. The body has absorbed, distributed, and metabolized the medication molecules – now what does it do with the leftovers? Remaining parent drugs and metabolites in the bloodstream are often filtered by the kidney, where a portion undergoes reabsorption back into the bloodstream, and the remainder is excreted in the urine. The liver also excretes byproducts and waste into the bile. Another potential route of excretion is the lungs. For example, drugs like alcohol and the anesthetic gases are often eliminated by the lungs.[1]
Routes of Excretion
Kidney
The most common route of excretion is through the kidneys. As the kidneys filter blood, the majority of drug byproducts and waste are excreted in the urine. The rate of excretion can be estimated by taking into consideration several client factors, including age, weight, biological sex, and kidney function. There are known sex differences in the three main renal functions of glomerular filtration, tubular secretion and tubular reabsorption. Renal clearance is generally higher in men than in women.[2]
Kidney function is measured by lab values such as serum creatinine, glomerular filtration rate (GFR), and creatinine clearance. If a client’s kidney function is decreased, then their ability to excrete medication is affected, and drug dosages must be altered for safe administration.
Renal disorders, such as chronic kidney disease, can reduce kidney function and hinder drug excretion. As kidney function decreases with age, drug excretion becomes less efficient, and dosing adjustments may be needed. Other medical conditions that impact blood flow to the kidneys can also affect drug elimination. For example, heart failure can affect systemic blood flow to the kidney, resulting in decreased filtration and elimination of drugs.
Liver
As the liver filters blood, some drugs and their metabolites are actively transported by hepatocytes (liver cells) to bile. Bile moves through the bile ducts to the gallbladder and then on to the small intestine. During this process, some drugs may be partially absorbed by the intestine back into the bloodstream. Other drugs are biotransformed (metabolized) by intestinal bacteria and reabsorbed. Unabsorbed drugs and byproducts/metabolites are excreted in the feces.
If a client has decreased liver function, their ability to excrete medication is affected, and drug dosages must be adjusted. Lab studies used to evaluate liver function are called liver function tests and include measurement of alanine transaminase (ALT) and aspartate aminotransferase (AST) enzymes that the body releases in response to damage to or disease of the liver.
Conditions that cause decreased blood flow to the liver can also affect the metabolism and excretion of drugs. For example, conditions such as shock, hypovolemia, or hypotension cause decreased liver perfusion and may require adjustment of dosages of medicati