The Excretory System
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Printed: January 23, 2015
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C HAPTER
Chapter 1. The Excretory System
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The Excretory System
Lesson Objectives
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Define excretion, and identify organs of the excretory system.
Outline the structures and functions of the urinary system.
Explain how the kidneys filter blood and produce urine.
Describe how the kidneys help maintain homeostasis.
Identify kidney diseases and how they are treated.
Lesson Vocabulary
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excretion
excretory system
kidney
kidney failure
kidney stone
nephron
ureter
urethra
urinary bladder
urinary system
urinary tract infection
urination
urine
Introduction
The boy in Figure 1.1 is working out on a hot day. He’s losing a lot of water in sweat. To maintain homeostasis,
his body can balance the water lost in sweat by excreting less water in urine. The amount of water lost in urine is
controlled by the kidneys. The kidneys are organs of excretion.
Excretion
Excretion is any process in which excess water or wastes are removed from the body. Excretion is the job of the
excretory system. Besides the kidneys, other organs of excretion include the large intestine, liver, skin and lungs.
• The large intestine eliminates food wastes that remain after digestion takes place.
• The liver removes excess amino acids and toxins from the blood.
• Sweat glands in the skin excrete excess water and salts in sweat.
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FIGURE 1.1
Water lost in sweat must be balanced in
some way for the body to maintain homeostasis.
• The lungs exhale carbon dioxide and also excess water as water vapor.
Each of the above organs of excretion is also part of another body system. For example, the large intestine and liver
are part of the digestive system, and the lungs are part of the respiratory system. The kidneys are the main organs of
excretion. They are part of the urinary system.
The Urinary System
The urinary system is shown in Figure 1.2. It includes two kidneys, two ureters, the urinary bladder, and the urethra.
The main function of the urinary system is to filter waste products and excess water from the blood and excrete them
from the body as urine. For a visual presentation on the urinary system and how it works, watch this video: http://w
ww.youtube.com/watch?v=OkyFPMXa28c .
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/137145
The Kidneys
The kidneys are a pair of bean-shaped organs at each side of the body just above the waist. You can see a diagram of
a kidney in Figure 1.3. The function of the kidneys is to filter blood and form urine. Tiny structures in the kidneys,
called nephrons, perform this function. Each kidney contains more than a million nephrons.
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Chapter 1. The Excretory System
FIGURE 1.2
The kidneys are the main organs of the urinary system.
FIGURE 1.3
Structures in the kidney
Formation of Urine
Blood with wastes enters each kidney through an artery, which branches into many capillaries. After passing through
capillaries and being filtered, the clean blood leaves the kidney through a vein.
The part of each nephron called the glomerulus is where blood in the capillaries is filtered. Excess water and wastes
are filtered out of the blood. The tubule of the nephron collects these substances. Some of the water is reabsorbed.
The remaining fluid is urine.
Excretion of Urine
From the kidneys, urine enters the ureters. These are two muscular tubes that carry urine to the urinary bladder.
Contractions of the muscles of the ureters move the urine along by peristalsis. The urinary bladder is a sac-like
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organ that stores urine. When the bladder is about half full, a sphincter relaxes to let urine flow out of the bladder
and into the urethra. The urethra is a muscular tube that carries urine out of the body through another sphincter. The
process of urine leaving the body is called urination. The second sphincter and the process of urination are normally
under conscious control.
How the Kidneys Maintain Homeostasis
The kidneys help the body maintain homeostasis in several ways. They filter all the blood in the body many times
each day and produce urine. They control the amount of water and dissolved substances in the blood by excreting
more or less of them in urine.
The kidneys also secrete hormones that help maintain homeostasis. For example, they produce a hormone that
stimulates bone marrow to produce red blood cells when more are needed. They also secrete a hormone that regulates
blood pressure and keeps it in a normal range.
Kidney Health and Disease
You need only one kidney to live a normal, healthy life. A single kidney can do all the work of filtering the blood
and maintaining homeostasis. However, at least one kidney must function properly to maintain life. Diseases that
threaten the health and functioning of the kidneys include kidney stones, infections, and diabetes. You can learn
more about kidney diseases in this video: http://www.youtube.com/watch?v=BodnYcHGtiA .
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/137146
Kidney stones are mineral crystals that form in urine inside a kidney, as shown in Figure 1.4. The stones may
be extremely painful. If a kidney stone blocks a ureter, it must be removed so urine can leave the kidney and be
excreted.
Bacterial infections of urinary organs, especially the urinary bladder, are common. They are called urinary tract
infections. Generally, they can be cured with antibiotic drugs. However, if they aren’t treated, they can lead to more
serious infections and damage to the kidneys.
Untreated diabetes may damage capillaries in the kidneys so the nephrons can no longer filter blood. This is called
kidney failure. The only cure for kidney failure is to receive a healthy transplanted kidney from a donor. Until that
happens, a patient with kidney failure can be kept alive by artificially filtering the blood through a machine. This is
called hemodialysis. You can see how it works in Figure 1.5.
Lesson Summary
• Excretion is any process in which excess water or wastes are removed from the body. Excretion is the job of
the excretory system. Organs of excretion include the large intestine, liver, skin, lungs, and kidneys.
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Chapter 1. The Excretory System
FIGURE 1.4
Kidney stones
FIGURE 1.5
Hemodialysis filters blood through a machine in patients with kidney failure.
• The urinary system filters wastes and excess water from the blood, forms urine, and excretes urine from the
body. It includes two kidneys, two ureters, the urinary bladder, and the urethra. Nephrons are tiny structures
in the kidneys that filter blood and form urine.
• Diseases of the urinary system include kidney stones and urinary tract infections. Untreated diabetes may
cause kidney failure and the need for hemodialysis or a kidney transplant.
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Lesson Review Questions
Recall
1. Define excretion.
2. What are the organs of the excretory system?
3. Describe the urinary system.
Apply Concepts
4. Why does a person with kidney failure need hemodialysis?
Think Critically
5. Explain how the kidneys filter blood and form urine.
6. Explain two ways that the kidneys maintain homeostasis.
Points to Consider
Although the process of urination is under conscious control, the other processes of the urinary system are not. You
can’t control the work of your kidneys, for example, but this doesn’t mean that they operate without any control.
1. What organ system controls the kidneys?
2. What organs make up this system?
References
1. Wilfredor. http://commons.wikimedia.org/wiki/File:Boy_Face_from_Venezuela.jpg?fastcci_from=2998339
. public domain
2. Courtesy of National Cancer Institute/SEER Training Modules. http://commons.wikimedia.org/wiki/File:I
llu_urinary_system.svg . Public Domain
3. Courtesy of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), National Institutes of Health (NIH). http://kidney.niddk.nih.gov/kudiseases/pubs/yourkidneys/ . Public Domain
4. BruceBlaus. http://commons.wikimedia.org/wiki/File:Blausen_0595_KidneyStones.png . CC BY 3.0
5. Jodi So. CK-12 Foundation . CC BY-NC 3.0
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