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 The Excretory System



                         Your community treats wastewater       

                       after it leaves your house to remove

large particles, contaminants, bacteria and other

material that make the water supply unsafe.


   Your excretory system works in much the same

way as the equipment that purifies the water. Your

kidneys, lungs and skin help your body to get rid of wastes your body produces by the chemical reactions that occur in life processes. The excretory system lets you excrete wastes that would interfere with body functions and eventually damage other organs if they were not removed.


Learning Objectives

By the end of this lesson, students should be able to:

  • Explain the function of the urinary system and why it is important to your health.

  • Describe how the kidneys work.

  • List the function of the parts of the urinary system.

  • Describe the excretory function of the lungs, intestines and skin.



bladder—a bag-shaped muscular organ that stores urine until it leaves the body.


kidneys—two organs that filter blood to produce the waste liquid called urine.


nephron—the filtering unit of the kidneys.


urinary system—a system of excretory organs that rids the blood of wastes, excess water, and excess salts.


urine—liquid collected by the kidneys containing water, salts and wastes.


ureters—tubes that lead from each kidney to the bladder.


urethra—a tube leading from the bladder to the outside of the body.


The Urinary System

   The urinary system is made up of two kidneys, two ureters, the urinary bladder, and the urethra.


   The organs of the urinary system filter your blood of waste products. Read below to trace the flow of waste materials from the blood through the urinary system and out of the body.



     Your blood carries waste products from

metabolic processes from your cells. Blood

circulates through the kidneys. Your kidneys

are located on the back wall of the abdomen

near waist level. They are reddish-brown,

bean-shaped, and about 12 centimeters (cm)

long, 6 cm wide, and 3 cm thick.



   Kidneys are made up of millions of units called nephrons. Nephrons act as the filters. As the blood flows through the nephrons, wastes are absorbed and concentrated. The wastes, now called urine, funnels into an area for removal from the kidneys.


   Water, sugars and salts are absorbed by capillaries and the purified blood circulates through the body. All of your blood circulates through the kidneys many times a day.


   The composition of urine can vary considerably depending on the amount of water and wastes the kidneys filter. It also changes with diet, liquid intake, and physical activity.


   The average adult produces about 1 liter (L) of urine per day but can be as much as 2.3 L depending on water intake.


   Urine can be an indicator of health conditions. For example, it is not usually normal to have glucose (sugar), proteins, ketones, or blood cells in the urine. A doctor will order urine tests for a routine exam or if she suspects a certain condition. If glucose is present it could suggest a problem with sugar metabolism. If blood cells are present it could suggest an infection.



    After urine is formed in the nephrons of each kidney it passes through tube-like structures  called ureters which lead to the urinary bladder.


    A ureter is about 25 centimeters long and has muscular walls. Peristalsis (an involuntary wave-like motion) in the ureter moves the urine along the tube to the urinary bladder. A flap-like fold of tissue acts like a valve. Urine can move into the bladder, but not back up into the ureter.

Urinary Bladder

   The urinary bladder is a hollow muscular sac that stores urine until release from the body when it forces it into the urethra.



   The urethra is a tube that carries urine from the urinary bladder to the outside of the body.

   Because the pathways of the urinary system are continuous, bacteria present in one part may travel to another. Bacteria in the urethra, for example, may travel up to the bladder (cystitis), and if not treated, can travel to the kidney. Symptoms include painful and/or frequent urination and are treated with antibiotics.


Fast Facts
All the blood in your body is filtered 400 times through the body every day.

Urine stays in the bladder for up to 5 hours before release depending on the amount of liquid consumed.

Other organs with excretory functions

   Several other organs and tissues also have excretory functions. The large intestine absorbs water and eliminates solid digestive waste products from the body. The skin eliminates other metabolic wastes in the form of perspiration, and the lungs excrete CO2 from the body.           

   These organs and tissues are usually discussed in the body systems that deal more directly with those organs: the large intestine in the digestive system and the lungs in the respiratory system and the skin in the integumentary system. 


Check for Understanding

1. Describe the function of each part of the urinary system.

2. What prevents urine from backing up into the ureters and kidneys from the urinary bladder?

3. Why are the skin, intestines and lungs excretory organs?




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image of the human excretory system from kidneys to urethra
image of the structure of the human kidney
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