Lesson 30.4
The Excretory
System
Assumption College
Mathayom 4, Universal Biology 1
Miss Anna
Key Questions
O What is the principal role of the structures of
the excretory system?
O How do the kidneys clean the blood?
O How do the kidneys help maintain
homeostasis?
O Question: What might happen if a
city’s trash removal program
stopped working?
O Answer: Trash would begin to pile
up in the city.
How might this relate to what
we are learning about?
O  The excretory system collects
and removes waste much like a
city’s trash removal system
Vocabulary
O
O
O
O
O
O
O
O
O
O
Excretion
Ureter
Urinary bladder
Urethra
Nephron
Filtration
Glomerulus
Bowman’s capsule
Reabsorption
Loop of Henle
What is the principal role of
the structures of the excretory
system?
O Every living system (including humans)
produce chemical waste products and must
be eliminated in order to survive
O For example, every cell in the body produces
waste compounds, including excess salts
and carbon dioxide
O The process by which these metabolic
wastes are eliminated to maintain
homeostasis is called excretion
O Excretion is one part of the processes
that maintain homeostasis
The excretory system includes:
O Skin
O Lungs
O Liver
O Kidneys
These organs work as a system to
excrete metabolic waste from the body.
O The ureters, urinary bladder, and urethra are
also involved in excretion
O Figure 30-18 on page 883 in your book
shows the major organs of excretion
The Skin
O Excretes water, salts, and a small amount of
urea in sweat
O By releasing sweat in very small amounts,
this process eliminates wastes even when
you may not think you’re sweating!
The Skin
The Lungs
O The blood transports carbon dioxide, a
waste product of cellular respiration, from
the body cells to the lungs
O When you exhale, your lungs excrete carbon
dioxide and small amounts of water vapor
The Lungs
The Liver
O The liver plays many important roles in
excretion
O One of its principle activities is the
conversion of potentially dangerous nitrogen
wastes (a product of protein breakdown) into
less toxic urea
O Urea is then transported through the blood
to the kidneys for elimination from the body
The Liver
The Kidneys
O The kidneys are the major organs of the
excretion system
O Two fist-sized organs located on either side
of the spinal column near the lower back
O Through a complex filtering process, the
kidneys remove excess water, urea, and
metabolic waste from the blood
The Kidneys
The Kidneys (continued)
O The kidneys produce and excrete a waste
product known as urine
O Ureters transport urine from the kidneys to
the urinary bladder, where the urine is
stored until it is released through the
urethra
Excretion and the Kidneys
O What does a kidney do?  As blood
filled with waste products enters the
kidney through the renal artery, the
kidney removes urea, excess water
and minerals, and other waste
products
Excretion and the kidneys
continued
O Each kidney contains nearly a million
individual processing units called nephrons
O Nephrons are where most of the work of the
kidney takes place – impurities are filtered
out, wastes are collected, and purified blood
is returned to circulation
Excretion and the kidneys
continued
O Blood purification in the kidneys is
complex and involves two distinct
processes: filtration and reabsorption
Filtration: The first step in
blood purification
O Passing a liquid or gas through a filter to remove
waste is called filtration
O The filtration of blood mainly takes place in the
glomerulus
O A glomerulus is a small but dense network of
capillaries (very small blood vessels) encased in
the upper end of each nephron by a hollow, cup
shaped structure called Bowman’s capsule
O Please see Figure 30-19 on page 885 in your
book for a detailed picture
Zooming in on the Kidney
Reabsorption: The second step
in blood purification
O The process by which water and other
dissolved substances are taken back into
the blood is called reabsorption
O A section of the nephron tubule called the
loop of Henle is responsible for conserving
water and minimizing the volume of filtrate
O The waste material (urine) that remains in
the tubule is emptied into a collecting duct
Zooming in on a nephron
Urine Excretion
O From the collecting ducts, urine flows to the
ureter of each kidney
O The ureters carry urine to the urinary
bladder for storage until the urine leaves the
body through the urethra
Urine Excretion
Figure 30-19 Task:
O List in order the structures that
blood flows through a kidney
Figure 30-19 Answer:
O Blood first enters the kidney through the
renal artery
O Once in the kidney, blood enters a nephron
and moves through a network of capillaries
called the glomerulus where filtration occurs
O It then passes through more capillaries
where reabsorption takes place
O Finally, blood leaves the kidney through the
renal vein
The Kidneys and Homeostasis
O How do the kidneys help maintain
homeostasis?
 The kidneys respond directly to the
composition of blood. They are also influenced
by the endocrine system. Disruption of proper
kidney function can lead to serious health
problems.
Control of Kidney Function
O The activity of the kidneys is largely
controlled by the composition of the blood
O For example: if you eat salty food, the
kidneys will respond to the excess salt in
your blood by returning less salt to your
blood during reabsorption
Control of Kidney function
O Glands release hormones that also control
kidney function
O For example: if you have not consumed
enough fluids or you have sweat excessively,
your pituitary gland releases antidiuretic
hormone (ADH) into your blood
Urine Testing
O Medical professionals can learn a lot about a
person’s health from a simple urine sample
O The presence of protein or glucose can be an
indicator of diseases such as high blood
pressure or diabetes
O Certain drugs are excreted in the urine and can
be detected during a urine test
Kidney Disorders
O Kidneys are the master chemists of the
blood supply
O If anything goes wrong with the kidneys,
serious medical problems will follow
O Three of these problems are kidney stones,
kidney damage, and kidney failure
Kidney Stones
O Sometimes substances such as calcium,
magnesium, or uric acid salts in the urine
crystallize to form kidney stones
O When kidney stones block a ureter, it causes
great pain
O Sometimes kidney stones are treated using
ultrasound waves, or sometimes they are
passed naturally
Kidney Damage
O Most cases of kidney damage are related to
high blood pressure and diabetes
O Excessive blood pressure damages the
delicate filtering mechanism, and high blood
sugar levels cause the kidneys to filter more
blood than normal
Kidney Failure
O When kidneys can no longer cleanse the
blood and maintain a state of homeostasis,
a person is said to be in kidney failure
O A person in kidney failure must receive
dialysis or undergo a kidney transplant
(Figure 30-20 on page 887)
Question:
O The kidneys are essential for maintaining
homeostasis in the body. Why?
Answer:
O Kidneys remove waste from the body.
Without the kidneys, wastes can accumulate
to levels that disrupt homeostasis and can
cause damage to the body or death.
End of Lesson 30.4
O Homework: Please complete the 30.4
assessment on page 887 (all 4 questions),
and fill out the vocabulary worksheet