Digestion and Excretion The Excretory System Functions of the Excretory System

Digestion and Excretion
The Excretory System
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
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.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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.
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.
• 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.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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.
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.
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.
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).
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.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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.