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Renal & Biliary Drug Excretion: Definition & Process

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

There are many ways in which the body gets rid of drugs. In this lesson, we will focus on the kidneys and liver, and the roles that these organs play in drug excretion.

Detoxification

Have you ever been on a detox diet or seen body detoxifying products advertised? Guess what? You have a detoxification system that is way more powerful than anything you can buy - and you got it for free!

Your low-priced, hyper-efficient, detoxifying system starts with your kidneys and your liver. There are many other systems that also help you detoxify, but this lesson will concentrate on these two. Your body also excretes drugs and other toxic waste products through your saliva, sweat, and lungs and if you have it, through breast milk as well.

Sometimes your body excretes drugs intact. Sometimes your body changes the drugs at a molecular level before excreting them. We call these altered forms metabolites.

Your Kidneys

Renal means 'pertaining to the kidneys,' so renal drug excretion describes how your kidneys get rid of drugs and their waste products. One of the primary means by which kidneys remove drugs is through glomerular filtration.

A glomerulus is a glob of capillaries in the kidneys. Your kidneys filter all your blood plasma through the capillaries into the surrounding structure, known as Bowman's capsule. Your kidneys filter about 180L of plasma per day! Most of what is filtered gets reabsorbed, which is why you only urinate around 2L per day. (Otherwise, you'd have to drink a lot more water!)

You reabsorb water and most electrolytes, but you can't reabsorb most polar compounds (compounds in which some atoms are slightly negative while some are slightly positive). Since most drug metabolites are polar compounds, these molecules will be excreted unless your body has a mechanism to reabsorb them.

Glomerulus and Bowmans capsule
Glomerulus and Bowmans capsule

The pH (acidity or alkalinity) of your urine also affects how well drugs will be excreted. Alkaline drugs, such as codeine, will be more readily excreted in acidic urine; acidic drugs, such as aspirin, will be more readily excreted in alkaline urine. Sometimes, when a person has taken too much of a drug, a doctor may try to alter the pH of their urine to help them excrete the drug more readily, perhaps by giving them an antacid.

Your Liver

Hepatic means 'pertaining to the liver.' Your liver is the second-largest organ in your body; Renaissance anatomist William Harvey called it the 'noble organ.' It has many functions, but for our purposes today, let's focus on bile production.

Bile is a greenish-yellow fluid that is secreted by the liver. If you've heard of the 'four humors' of ancient medicine, bile was one of them. From the ancient Greeks through the development of modern medicine, an overabundance of bile was supposed to be responsible for a pushy, demanding personality that the ancients would describe as choleric. But we know now that that's almost as silly as shelling out good money for a miracle diet detox product. Right? Right.

The four humors
The four humors

Sometimes drugs will pass through your liver without being altered at all; sometimes your liver will change the drugs into metabolites. Either way, the drugs end up in your bile.

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