Digestion and Excretion The Excretory System

Digestion and Excretion
The Excretory System
Key Concepts
• What does the excretory
system do?
• How do the parts of the
excretory system work
together?
• How does the excretory
system interact with other
body systems?
What do you think? Read the two statements below and decide
whether you agree or disagree with them. Place an A in the Before column
if you agree with the statement or a D if you disagree. After you’ve read
this lesson, reread the statements and see if you have changed your mind.
Before
Statement
After
5. Several human body systems work together
to eliminate wastes.
6. Blood contains waste products that must be
removed from the body.
3TUDY#OACH
Key Concept Check
1. Describe What does
the excretory system do?
Functions of the Excretory System
You have read about nutrients in food. These nutrients
are necessary to maintain health. You also have read about
how the digestive system processes the food that you eat.
Your body does not use all the food that you take in. The
unused food parts are waste products. These wastes are
processed by the excretory system. The excretory system
collects and eliminates wastes from the body and regulates the level of
fluid in the body.
Collection and Elimination
In your home, you probably collect waste in several
places. You might have a trash can in the kitchen and
another one in the bathroom. The furnace might have a
filter that collects dust from the air. Your body also collects
wastes. The digestive system collects waste products in the
intestines. The circulatory system collects waste products in
the blood.
When the trash cans in your home are full of waste, you
must take the trash outside. The waste in your body also
must be removed. Waste that is not removed, or eliminated,
from your body can become toxic, or poisonous, and
damage your organs.
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Reading Essentials
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Identify the Main Ideas As
you read each paragraph,
write the main idea on a
sheet of paper or in your
notebook to study later.
Regulation of Liquids
The excretory system also regulates the level of fluids in
the body. Recall that water is an essential nutrient for your
body. Some of the water in your body is lost when waste is
eliminated. The excretory system controls how much water
leaves the body through elimination. This ensures that
neither too much nor too little water is lost.
Types of Excretion
Your body excretes, or eliminates, different substances
from different body systems. The excretory system is made
of four body systems.
Use a four-door book to
summarize information
about the functions of the
body systems that make up
the excretory system.
Digestive Urinary
System System
Respiratory Integumentary
System
System
• The digestive system collects and removes undigested
solids from the foods you eat.
• The urinary system processes, transports, collects, and
removes liquid wastes from the body.
• The respiratory system removes carbon dioxide and
water vapor from the body.
• The integumentary system, which includes the skin,
secretes excess salt and water from the body through
sweat glands.
Reading Check
2. Name What body
systems make up the
excretory system?
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Organs of the Urinary System
The urinary system processes, stores, and removes liquid
wastes from the body. It helps maintain homeostasis. The
organs of the urinary system include two kidneys, two ureters,
the bladder, and the urethra. These organs work together to
process, transport, collect, and excrete liquid wastes. Most
functions of the urinary system occur in the kidneys.
The Kidneys
The bean-shaped organ that filters, or removes, wastes from blood
is the kidney. You have two kidneys, one on each side of your
body. They are near the back wall of your abdomen, above
your waist, and below your rib cage.
Reading Check
3. Describe What is the
function of the urinary
system?
Each kidney is about the size of your fist. Kidneys are
dark red in color because of the large amount of blood that
passes through them.
Kidney Functions The kidneys have several functions. This
lesson will discuss the role of the kidneys in the urinary
system. But the kidneys have other important functions.
They produce hormones that stimulate the production of
red blood cells. They also control blood pressure and help
control calcium levels in the body.
Reading Essentials
Digestion and Excretion
255
WORD ORIGIN
nephron
from Greek nephros,
means “kidney”
The Kidneys’ Role in the Urinary System The kidneys
contain blood vessels and nephrons (NEH frahnz). Nephrons
are networks of capillaries and small tubes, or tubules, where filtration
of blood occurs. Each kidney contains about one million
nephrons.
Blood contains waste products, salts, and sometimes
toxins from cells that need to be removed from the body. As
blood passes through the kidneys, they filter these products
from the blood. When blood is filtered, a fluid called urine is
produced. The kidneys filter the blood and produce urine in
two stages.
1. First Filtration Blood is constantly circulating and
filtering through the kidneys. In one day, the kidneys
filter about 180 L of blood plasma, or the liquid part
of blood. That is enough liquid to fill ninety 2-L bottles.
You have about 3 L of blood plasma in your body.
This means that your kidneys filter your entire blood
supply about 60 times each day. The first filtration
occurs in the nephrons. There, groups of capillaries
filter water, sugar, salts, and wastes out of the blood.
4. Evaluate Why are two
filtrations necessary?
The Ureters, Bladder, and Urethra
When garbage piles up in a trash can, the can must be
emptied. In a similar way, the urine produced by your body
cannot stay in your kidneys. Urine leaves each kidney through a
tube called the ureter (YOO ruh tur).
Key Concept Check
5. Describe How do the
ureters, bladder, and urethra
work together to excrete
urine?
Each of your kidneys has a ureter. Both ureters drain into
your bladder. The bladder is a muscular sac that holds urine until
the urine is excreted. Your bladder expands and contracts like
a balloon when it fills or empties. An adult bladder can hold
about 0.5 L of urine.
Urine leaves the bladder through a tube called the urethra
(you REE thruh). The urethra contains circular muscles called
sphincters (SFINGK turz) that control the release of urine.
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2. Second Filtration What would happen if all of the liquid
from the first filtration were excreted? Your body would
quickly dehydrate, and important nutrients would be
lost. To regain some of this water, the kidneys filter the
liquid collected in the first filtration again. The second
filtration occurs in small tubes in the nephrons. During
the second filtration, up to 99 percent of the water and
nutrients from the first filtration are separated out and
reabsorbed into the blood. The remaining liquid and
waste products form urine. On average, an adult
excretes about 1.5 L of urine per day.
Urinary Disorder
Description
Kidney disease
The nephrons are damaged and
the ability of the kidneys to filter
blood is reduced. In the beginning
stages, there might not be
symptoms.
diabetes, high blood pressure,
poisons, trauma
Urinary tract infection
Infections usually occur in the
bladder or urethra but can be in
the kidneys and ureters. Symptoms
might include burning during
urination, small and frequent
urination, and blood in urine.
bacteria in the urinary system
Kidney stones
Kidney stones are solid substances
that form in the kidneys. The most
common type is made of calcium.
Stones that pass through the
urinary system can be very painful.
calcium buildup in the kidneys
Bladder control problems
Urine is released from the bladder
involuntarily. These problems
occur in women more often than
in men.
urinary tract infections, muscle
weakness, prostate enlargement
Urinary Disorders
A urinary disorder is an illness that affects one or more
organs of the urinary system. Some urinary disorders are
described above. Several of these disorders are common.
Urinary tract infections are a leading cause of doctor visits.
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Possible Causes
The Excretory System and Homeostasis
You have already read about some of the ways that the
excretory system helps to maintain homeostasis. For example,
the excretory system filters wastes from the blood. The blood
is part of the circulatory system. A buildup of wastes in the
circulatory system would be toxic to your body.
Homeostasis is also maintained by the removal of wastes
from the digestive system. Wastes would damage your body
if the excretory system did not remove them from the
digestive system.
The excretory system also interacts with the nervous
system. The hypothalamus is an area of the brain that helps
to maintain homeostasis. One function of the hypothalamus
is to control the secretion of some hormones. One hormone
causes the tubules of the kidneys to absorb more water from
the blood. This helps the body control fluid levels. Water is
kept in the blood instead of being excreted in the urine.
Reading Essentials
Visual Check
6. Identify Which urinary
disorder results from bacteria
in the urinary system? (Circle
the correct answer.)
a. kidney disorder
b. urinary tract infection
c. kidney stones
ACADEMIC VOCABULARY
area
(noun) a part of something
that has a particular function
Key Concept Check
7. Analyze How does the
excretory system interact
with the nervous system?
Digestion and Excretion
257
Mini Glossary
bladder: a muscular sac that holds urine until the urine is
excreted
ureter (YOO ruh tur): a tube through which urine leaves
each kidney
excretory system: collects and eliminates wastes from the
body and regulates the level of fluid in the body
urethra (yoo REE thruh): a tube through which urine leaves
the bladder
kidney: a bean-shaped organ that filters, or removes, wastes
urine: a fluid that is produced when blood is filtered
from blood
nephrons (NEH frahnz): networks of capillaries and small
tubes, or tubules, where filtration of blood occurs
1. Review the terms and their definitions in the Mini Glossary. Write a sentence that
describes how the excretory system helps maintain homeostasis.
2. Complete the chart below by naming the body systems that form the excretory system.
digestive system
3. Review the main ideas that you wrote about the excretory system. Write a sentence about
the most interesting idea to you.
What do you think
Reread the statements at the beginning of the
lesson. Fill in the After column with an A if you
agree with the statement or a D if you disagree.
Did you change your mind?
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Digestion and Excretion
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LESSON
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Excretory System