Excretory System

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  • 0:06 Excretory System
  • 0:45 Detoxification
  • 2:07 Filtering Blood
  • 4:27 Storage and…
  • 5:38 Lesson Summary
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Lesson Transcript
Instructor: Samuel Wright
Each year in the U.S., close to 400,000 people with kidney failure undergo dialysis treatment in order to remove waste, remove excess fluid and restore electrolyte balance. Kidneys, the workhorses of the excretory system, perform these same functions more effectively than any machine. In this lesson, we'll talk about how the excretory system removes toxic substances from the body.

Excretory System

Each year, close to 400,000 people in the U.S. undergo dialysis treatment for kidney failure. You may even know someone who has had dialysis treatment to filter their blood in order to remove waste, remove excess fluid and restore electrolyte balance. These functions of dialysis highlight the major functions of the kidneys, which are fist-sized organs that filter waste products out of the blood and help regulate blood composition and pressure. In this lesson, we'll go through the excretory system and talk about how the liver, kidneys and bladder work together to remove waste products and toxic substances from the body.


All animals have toxins in their body that they must find a way to eliminate. Some toxins are ingested in food and others are byproducts of metabolism. Some toxins, like carbon dioxide, can be tolerated in small amounts and can be easily disposed of as they are, but some toxins are so dangerous that the body must change them in some way to make them less toxic, or find a way to quickly remove them. Detoxifying and modifying dangerous substances so that they can be quickly and easily removed from the body is one of many jobs of the human liver.

Illustration of the human body with the liver highlighted in green
Image of the Liver

Let's look at an example of a toxic substance that the liver modifies to make less toxic.

Ammonia is a major byproduct of human metabolism, but it's extremely toxic, so the liver converts it to a less toxic form called urea, which can be created from two ammonia molecules and one carbon dioxide molecule. Here is a depiction of the chemical reaction: two ammonia molecules, these NH3s here combine with one CO2 molecule to form one urea molecule here, and one molecule of water. Of course, then we still have to get rid of the urea, but it's far less toxic than ammonia so we can concentrate it and store it for several hours without it harming us.

Filtering Blood

So once the liver has converted the ammonia into much less toxic urea, then what? Well, first the urea is allowed to leave the liver through the bloodstream. The circulating urea will eventually reach the kidneys where it will be filtered out of the bloodstream and concentrated in the urine. Here is how it works. In the outer layer of the kidneys are structures called glomeruli, which are ball-like structures of the kidney composed of very porous capillaries.

Illustration of glomeruli
Illustration of Glomeruli

You can think of the capillaries in the glomeruli as being like garden hoses with hundreds of holes all over them that let water spray out when the pressure is on. The blood pressure in these capillaries forces large amounts of water and small molecules including urea out through the pores, but blood cells and larger molecules that are too big to fit through the pores stay in the bloodstream. Surrounding the glomeruli are the ends of the renal tubules, which are long, looping tubes in the kidney that collect filtrate from the blood and concentrate it into urine. The water, nutrients, salts, urea and other waste products in the filtrate are collected by the surrounding renal tubule and begin a twisted journey.

The filtrate travels through the tubule that crosses back and forth between the outer and inner layers of the kidney. As it does, nutrients, water, and sodium chloride are reabsorbed from the renal tubules and returned to the blood. As water is reabsorbed, the urea and other waste products become very concentrated in the tubules and become urine. The whole structure from the glomerulus to the end of the renal tubule is called a nephron and is the basic functional unit of the kidney. By filtering out a large volume of water and small molecules and then selectively allowing most of the water and nutrients to be reabsorbed, the nephrons allow humans to get rid of urea and other wastes in high concentrations and conserve water when needed. Sodium chloride and other salts can also be reabsorbed back into the bloodstream as needed, but that function of the kidney is a key component for homeostasis which is another topic for another day.

Storage and Elimination of Urine

Illustration of human body showing the kidneys, ureters and bladder
Image of the Kidneys

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