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Biology
HSC Course
Stage 6
Maintaining a balance
Part 5: Excretion
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Contents
Introduction ............................................................................... 2
Water and wastes...................................................................... 3
Water balance ......................................................................................3
Wastes ..................................................................................................6
Excretory systems ..................................................................... 9
Dissection of a mammalian kidney......................................................9
What can be done when kidneys don’t function? .............................18
Suggested answers................................................................. 21
Exercises – Part 5 ................................................................... 23
Part 5: Excretion
1
Introduction
The kidneys play a vital part in maintaining the internal balance of
animals. In this part you will look at the structure and function of a
kidney and what happens when kidneys fail. During this part you will
need to purchase a sheep’s kidney from your local butcher.
In this part you will have the opportunity to learn to:
•
explain why the concentration of water in cells should be maintained
within a narrow range for optimal function
•
explain why the removal of wastes is essential for continued metabolic
activity
•
explain why the processes of diffusion and osmosis are inadequate in
removing dissolved nitrogenous wastes in some organisms
•
distinguish between active and passive transport and relate these to
processes occurring in the mammalian kidney
•
explain how the processes of filtration and reabsorption in the mammalian
nephron regulate body fluid composition.
In this part you will have the opportunity to:
•
perform a first-hand investigation of the structure of a mammalian
kidney by dissection, use of a model or visual resource and identify
the regions involved in the excretion of waste products
•
gather, process and analyse information from secondary sources to
compare the process of renal dialysis with the function of the kidney.
Extract from Biology Stage 6 Syllabus © Board of Studies NSW, originally
issued 1999. The most up-to-date version can be found on the Board's website
at http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_lista.html
This version October 2002.
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Water and wastes
Water is an extremely important substance as it acts as a solvent for many
substances in organisms. These dissolved substances can be involved in
chemical reactions within cells and can be transported within organisms.
Within animals, these substances are usually transported in blood whereas
in vascular plants they are carried in xylem and phloem sap.
Water balance
Many of these dissolved substances also determine the movement of
water within organisms and between cells due to osmosis. In many
animals the concentration of water and dissolved substances, especially
salts, in the fluids surrounding cells and in the blood must be maintained
within very narrow limits to prevent loss or uptake of water which could
result in damage to cells.
If water is not readily available, organisms may die as a result of not
having enough. On the other hand, excess water may need to be quickly
removed from organisms to maintain osmotic balance. To maintain their
water balance, organisms must match their water gains with their water
losses.
Some reactions involving water
Water is produced by some reactions in the bodies of organisms (for
example, cellular respiration) and required by others (for example,
photosynthesis). For revision, write an equation for each of these examples.
Cellular respiration
Photosynthesis
Check your answers.
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A reminder about osmosis
Osmosis is the movement of water from where it is in higher
concentration to where it is in lower concentration through a selectively
permeable (sometimes called semi-permeable) membrane.
Osmotic is the word which describes things (an adjective) related to
osmosis. So for example, osmotic pressure is exerted by water when it
moves into cells. Movement of water from one cell to another, or
between organisms and their external environment due to concentration
differences, can be called osmotic movement. Remember that the
adjective osmotic always describes a situation where water is moving
from where water is in higher concentration (a more dilute solution) to
where water is in lower concentration (a more concentrated solution) and
where that movement is through a selectively permeable membrane.
movement of water
A concentrated solution has
a higher concentration of
solute (dissolved substance)
but a lower concentration
of water.
A dilute solution has a lower
concentration of solute but
a higher concentration
of water.
selectively
permeable
membrane
Cells that are isotonic (same concentration inside the cell as in the
surrounding fluid) function well. In a single-celled organism this
surrounding fluid is usually water. In multicellular organisms the
surrounding fluid is usually the interstitial fluid found between cells. If
the cell is not isotonic with its surroundings then water may be lost or
gained and this may lead to death.
In summary, water needs to be maintained within a narrow range
because:
4
•
water is involved in many of the essential chemical reactions
that occur in cells such as respiration and photosynthesis
•
it acts as a solvent for many substances
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•
it is important for the transport of materials around the organism
•
cells may be damaged if there is too much or not enough water.
Do Exercise 5.1 now.
Part 5: Excretion
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Wastes
Do you recall what metabolism means? The definition in the glossary
tells you that metabolism is all of the biochemical reactions occurring in
the cells of the body.
These reactions normally occur as a series of chemical reactions, which
is called a metabolic pathway. Each step in a metabolic pathway is
governed by a specific enzyme. Cellular respiration is one metabolic
pathway with which you are familiar. Many products of metabolic
reactions are wastes, which are normally eliminated from the body.
Some products of reactions in the body are in fact poisonous (toxic) and
must be broken down to less toxic substances or be very quickly
eliminated from the body.
In most vertebrate species the liver is responsible for producing many
waste products, due to enzymatic breakdown of potentially harmful
substances taken into the body or produced by metabolism. Organs
which remove these wastes are known as excretory organs. The two
main excretory organs in vertebrate animals are the respiratory surfaces
(lungs, gills) and the kidneys.
Some examples of waste products
The respiratory surfaces excrete the carbon dioxide formed during
cellular respiration (the final metabolic process in the breakdown of fats
and carbohydrates). The kidneys get rid of other metabolic wastes
including water and nitrogenous wastes. These nitrogenous wastes are
formed from the breakdown of materials which contain nitrogen,
particularly proteins.
Ammonia is a very soluble and very poisonous nitrogenous waste. It is
produced in tadpoles and in most fish and aquatic invertebrates that have
access to plenty of water to dilute it.
Terrestrial species produce nitrogenous wastes in the form of either urea
or uric acid. Both of these wastes are less toxic than ammonia.
Urea is fairly soluble in water; for example, it is a major waste in urine.
Urea is excreted by most mammals, adult amphibians, sharks and rays.
Since it needs to be diluted in water to reduce its toxicity, urine is a
source of water loss for these species.
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Reptiles, birds and insects excrete a material called uric acid. It is very
insoluble (and the least toxic form of nitrogenous waste) and so needs
little water to get rid of it.
The table below compares some properties of the nitrogenous wastes
produced by terrestrial and aquatic organisms.
Waste products (such as carbon dioxide, water and nitrogenous wastes)
must be removed from living cells to enable them to continue to
function normally. For example, an accumulation of carbon dioxide
would lead to changes in pH inside cells. You will recall that changes
in pH can bring metabolic processes to a standstill by denaturing
enzymes.
Accumulation of other wastes would cause water to move into cells by
osmosis, altering the water balance. Finally, some wastes, such as urea,
are toxic and so must be removed.
Some metabolic products are not necessarily directly detrimental.
Indeed, some are essential to cell functioning. However, the
concentrations of these substances must also be closely regulated as
they may produce conditions that also result in cell death. For example,
too much or too little salt can result in cell damage or the
malfunctioning of organ systems.
Do Exercise 5.2 now.
You read earlier about the importance of respiratory surfaces and kidneys
for removing wastes. However, these are not the only excretory organs.
For example, in mammals, some wastes are eliminated by the skin (for
example, salts, urea and lactic acid in sweat) and a few are got rid of
through the digestive system (for example, the breakdown products of
haemoglobin are added into faeces).
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The diagram below summarises processes that produce wastes and
how these can be removed.
Cellular
respiration
CO2
respiratory
surface (gills
or lungs)
water
Other metabolic
pathways
kidney
nitrogenous
wastes
skin
(for example,
urea and
lactic acid)
other
metabolic
wastes
large
intestine
(for example,
products of
haemoglobin
breakdown)
Summary of modes of excretion.
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Excretory systems
Different organisms have different kinds of excretory mechanisms for
removing their wastes.
In unicellular organisms, waste products are quickly lost into the
environment through the cell membrane by diffusion. This can occur
because each cell has a large surface area to volume ratio.
However, in multicellular organisms, complex excretory organs (such as
kidneys) are needed to provide the necessary surface area for the
elimination of wastes..
Now complete Exercise 5.3.
Dissection of a mammalian kidney
If you dissected a kidney during the Preliminary module called Patterns
in Nature, you need only to refer back to that module. However, if you
did not do the dissection then, you really need to do it here.
You can use a model or a video if you have access to one but it is not
much trouble to buy a sheep’s kidney from the local butcher and dissect
it yourself. (A sheep’s kidney is very similar to a human kidney, except
that a sheep’s kidney is smaller.)
If you know the local butcher, you could ask for a kidney ‘in the fat’.
The kidney is embedded in fat to hold it in place in the body and this also
acts to protect it. When the kidney is removed at the abattoir or when the
fat is trimmed away, the ureter and blood vessels are usually cut off too.
If you can get a kidney that has not been trimmed, you can carefully pick
the fat away and you are more likely to see the ureter and blood vessels.
Ask the butcher to give you some idea of the weight of the sheep from
which the kidney was taken. How heavy is a kidney? Although kidneys
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are very important organs, they are quite small compared with the size of
the animal.
Risk assessment
You will be using sharp instruments so take care that you do not cut
yourself. When using animal tissue you should wear rubber gloves if
available. Make sure you are wearing suitable covered footwear and dispose
of all waste materials carefully wrapped in newspaper.
Materials required:
•
sheep’s kidney
•
small kitchen knife
•
cutting board or plate
•
knitting needle or similar
•
rubber gloves if available
•
newspaper.
What to do:
1
Observe the shape of the kidney.
2
Observe the protective outer layer of skin, called the capsule.
3
Identify the three tubes which enter the kidney. These are not easy
to see because they are all connected together with tissue and may
have been cut off the kidney you have. Also, there is a lot of fat
where they are connected to the kidney. The three tubes are:
a) the ureter, which is the large tube in the centre
b) the renal artery, which has a thick wall
c) the renal vein, which has a thinner wall.
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4
To observe the internal structure of the kidney, cut through the kidney
lengthwise, carefully cutting away from your fingers.
5
Now look inside. You will notice a funnel-shaped structure with a hole
in the centre. This hole leads into the ureter. Take an object like a
knitting needle and push it gently through the opening. Discover where
the ureter leaves the kidney.
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Continue cutting down to open up the kidney as shown in the
photograph below.
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capsule
cortex
medulla
pelvis
ureter
The internal structure of a sheep’s kidney.
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Find the following structures:
•
the brown outer layer, or cortex. This is where the waste
substances are squeezed out through the membranes of the
glomeruli into the Bowman’s capsules
•
an inner pink layer of medulla. Here, water and some salts are
reabsorbed into the blood from the tubules of the nephrons
•
a hollow whitish region. This is the pelvis of the kidney where
large collecting tubes empty urine into the funnel-shaped
beginning of the ureter.
On the following page, draw a fully labelled diagram of the dissected
kidney.
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Diagram of a dissected kidney
Complete Exercise 5.4.
A review of osmosis, diffusion and active transport
Since particles in matter are constantly moving, materials move from
where they are more concentrated to where they are less concentrated;
this is diffusion. If the diffusion of water occurs through a selectively
permeable membrane, the process is called osmosis.
However, living cells can make substances move from where they are
less concentrated to where they are more concentrated by using energy;
this is called active transport. Active transport may also involve changes
in the structure of the membranes, thus permitting materials to be moved
against the concentration gradient.
As you will see, all of these processes – diffusion, osmosis and active
transport – are very important in the functioning of the kidney.
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Try this short quiz to test your knowledge of these substance-moving
processes.
1
2
3
4
Osmosis is a special case of diffusion because it:
a
involves the movement of water only
b
involves the movement of water only and always occurs through
a membrane
c
occurs in plants only where the cell wall prevents cells from
bursting
d
occurs in plants and animals but not in microorganisms.
Diffusion occurs in:
a
liquids, gases and solids
b
liquids, gases and solutions
c
liquids and solutions only
d
liquids and gases only.
The energy necessary for osmosis and diffusion is due to the:
a
size of the particles involved
b
process of cellular respiration
c
number of particles present
d
movement of the particles involved.
Active transport occurs in:
a
solutions, liquids and gases
b
all cells
c
living cells
d
animal cells but not in plant cells.
Check your answers.
How did you go? Now that you are familiar with the structure of the
kidney and the mechanisms responsible for movement of particles in
organisms, the information below about the functioning of the kidney
should be much easier to follow.
Part 5: Excretion
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The functional units of the kidney – nephrons
The diagram below shows the structure of the kidney and its blood
supply. (Turn back to check that you correctly labelled your diagram of
a dissected kidney.)
medulla
cortex
renal vein
renal artery
ureter
The following diagram shows the position of tiny structures, called
nephrons, which make it up the kidney.
position of
nephon
There are around 1.2 million of these nephrons in each of your kidneys,
making a surface area of approximately 12 m2 in humans. The great
surface area created by so many nephrons in the kidney makes it efficient
in carrying out its two important functions. These are:
•
excretion – the elimination of harmful and unwanted products of
metabolism
•
osmoregulation – the control of body water and salt levels.
The kidneys also have some role in regulating blood pH by the secretion
of H+ ions into the nephron by active transport.
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An individual nephron is shown below, where the parts are named and
the complex blood capillary network associated with each nephron is
shown. The Bowman’s capsule and the proximal and distal tubules are
found in the cortex, which you will remember from your dissection is the
outer dark brown-coloured layer of the kidney. The loop of Henle and
the collecting tubule (or collecting duct) protrude down into the medulla,
which is the lighter-coloured part towards the centre of the kidney.
glomerulus
proximal
tubule
distal
tubule
branch of
renal artery
Bowman’s
capsule
branch of
renal vein
collecting
tubule
loop of
Henle
capillaries
A mammalian nephron.
Each part of the nephron has an important role in the filtration of blood
and the osmoregulation of the animal.
renal artery – brings blood containing small particles, including
nitrogenous wastes (especially urea), water, salts, glucose and amino
acids to the kidney
glomerulus – blood passing through the glomerulus is under high
pressure. Substances are forced out of the blood in this knot of
capillaries into Bowman’s capsule. The process is largely governed by
the size of the pores in the membranes of the capillaries and Bowman’s
capsule, which let small molecules and ions through but prevent the
movement of larger molecules (such as large proteins) and blood cells.
Bowman’s capsule – a cup-shaped structure surrounding the glomerulus
that collects materials forced out of the blood
Part 5: Excretion
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proximal tubule, loop of Henle and distal tubule – these structures are
joined together, making a long, very thin tube. As the substances filtered
from the blood travel through this tube, useful substances are reabsorbed
back into the blood in the capillaries surrounding the tube. This involves
active transport. Most of the glucose and amino acids are reabsorbed in
this way. Water and salts are reabsorbed in these parts of the nephron.
The process of reabsorption involves both the movement of materials,
especially ions, by active transport and the movement of water by osmosis
collecting tubule (or collecting duct) – materials remaining after
reabsorption are the wastes that move into the collecting tubule. As these
wastes move through the tubule, more water is taken back into the
bloodstream from the tubule. The waste in the collecting tubule is urine,
which is passed down into the pelvis of the kidney
renal vein – capillaries that surround the proximal tubule, loop of Henle
and distal tubule join together into the renal vein. This blood vessel
carries blood that has been cleaned by the nephron back into the body’s
circulation.
So, in summary, osmoregulation and excretion by nephrons in the kidney
are accomplished by the production and elimination of urine. Urine is
produced by:
•
filtration of many substances, both wastes and useful ones, from the
blood (at the glomerulus/Bowman’s capsule)
•
reabsorption of useful substances into the blood (at the tubules and
loop of Henle).
Diffusion, osmosis and active transport in a nephron
Substances move from the blood into the Bowman’s capsule because of
the high pressure of the blood through the glomerulus. But why do
substances move from the tubules back into the blood?
Some substances can move by diffusion, because there is a lower
concentration of them in the blood and a higher concentration in the
tubule. However, once the concentration difference between the blood and
various parts of the nephron is balanced, energy must be used to move
useful substances, such as glucose and amino acids, back into the blood.
Since active transport is used, the body can determine the amount of each
substance that is reabsorbed. For example, all glucose will be reabsorbed
but only some salt. In this way, the amount of substances including salt
and water reabsorbed is precisely controlled to balance water and salt
intake and losses, so that the composition of blood and fluid surrounding
cells is maintained at a constant level. This process is controlled by the
endocrine system and will be discussed later.
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A summary of filtration and reabsorption in a nephron
The following table summarises the functioning of the kidney by
indicating the general composition of the fluid which enters Bowman’s
capsule (sometimes called the filtrate) and the fluid which eventually
drains out of the collecting tubules into the renal pelvis (the urine).
This shows that, for the most part, active transport is used to pump useful
materials back into the bloodstream, rather than specifically pumping
undesirable substances into the nephron.
Material
Bowman’s capsule
(filtrate)
Renal pelvis
(urine)
nitrogenous wastes
(mainly urea)
yes
yes
glucose
yes
no
amino acids
yes
no
salts (ions)
yes
variable amount
water
yes
variable amount
large proteins
no
no
blood cells
no
no
Turn back to the diagram of the nephron in this section and label:
•
where filtration and reabsorption occur
•
some substances that are reabsorbed from the tubules into the blood
•
the wastes that leave the collecting tubule as urine.
Check your answers.
Complete Exercise 5.5.
Part 5: Excretion
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What can be done when kidneys
don’t function?
In people who have impaired kidney function, waste products can be
removed from their blood using a process called renal dialysis. The
blood of the patient is passed through a coil separated by a membrane
from a salt (saline) solution which has the same concentration as the
blood (called a dialysing solution).
The dialysis membrane is permeable to water and to nitrogenous and
other waste products of metabolism, especially urea. For 4–5 hours
about three times a week, the blood of the patient is circulated through
the haemodialysis machine depicted in the diagram below.
artery
to dialyser
superficial
vein
dialyser
membrane
from
dialyser
bubble trap
fresh
dialysing
solution
constant
temperature
bath
used
dialysing
solution
Haemodialysis machine.
Dialysis can also be carried out within the body by a process known as
peritoneal dialysis. In this instance, a saline solution is passed into the
body cavity (peritoneum) of the patient by a catheter (fine tube). Wastes
diffuse from the body fluids and pass through the membrane that lines
the peritoneum into the saline solution, which is then drained out by
another catheter. This process avoids the necessity to circulate the blood
from the patient’s body, with the possible risk of blood clotting and
infection.
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Comparing renal dialysis with normal kidney function
Refer to the diagram of the haemodialysis machine and use your knowledge
from throughout this module to deduce answers to the following.
1
Explain the reason for the constant temperature bath in the machine.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
The membrane in the haemodialysis machine is equivalent to which
part of the nephron of the kidney?
A
the membrane of the tubule
B
the membrane of Bowman’s capsule
C
the capillaries surrounding the nephron
D
the walls of the collecting tubules
State a reason for your selected answer.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
3
Explain why the dialysing solution has the same salt concentration
as blood.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
Part 5: Excretion
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Suggested answers
Some reactions involving water
Cellular respiration
glucose
+ oxygen
C6H12O6 +
6O2
many enzyme
controlled steps
many enzyme
controlled steps
carbon + water + energy
dioxide
6CO2 + 6H2O + energy
Photosynthesis
carbon
dioxide
+
water + light
energy
many enzyme
controlled steps
glucose + oxygen
many enzyme
controlled steps
6H2O + light
C6H12O6 + 6O2
energy
or to show that the source of the oxygen gas is water and not carbon dioxide
6CO2
+
6CO2
+ 12H2O + light
energy
many enzyme
controlled steps
C6H12O6 +
6O2
+ 6H2O
A review of osmosis, diffusion and active transport
1
B
2
B
3
D
4
C
Part 5: Excretion
The energy of movement of the particles is responsible for the
movement. This energy (kinetic energy) increases with temperature.
It is not supplied by the cells themselves through respiration.
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A summary of filtration and reabsorption in a nephron
Here is a sample answer.
salts (such as NaCl, HCO3– and K+)
nutrients and water
many substances
from blood
salts, nutrients
and water
H+ (to balance pH)
FILTRATION
salts,nutrients
and water
urine (water, urea and salts)
REABSORPTION
Comparing renal dialysis with normal kidney function
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1
If the constant temperature bath were not used to keep the solution at
body temperature, the blood would lose heat to the solution in the
core and the patient could become hypothermic (have a body
temperature below normal).
2
B is correct. The membrane of Bowman’s capsule is the equivalent
structure in the nephron, where filtration occurs. Remember that
reabsorption occurs in the other parts of the nephron.
3
If the solution had a higher salt concentration than blood, the patient
would lose water into the solution by osmosis. If it were less
concentrated, water would pass into the patient’s blood by osmosis
through the membrane.
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Exercises Part 5
Exercises 5.1 to 5.5
Name: _________________________________
Exercise 5.1: Water balance
What is the solvent for metabolic reactions in living cells? _________
Why is it important that the concentration of this solvent remains
constant in living cells? (What might happen if it did not?)
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 5.2: Some examples of waste products
a)
Metabolic processes constantly produce wastes such as carbon
dioxide, nitrogenous wastes and water. Why is it essential for
continued metabolic activity that these wastes are removed from
cells?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 5: Excretion
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Exercise 5.3: A review of respiratory and excretory
systems
Simple organisms, such as unicellular animals, are able to rely on
diffusion and osmosis to remove wastes such as nitrogenous wastes and
water. However, multicellular organisms, such as mammals, require
complex organs and body systems for excretion.
Explain why the processes of diffusion and osmosis are inadequate in
removing dissolved nitrogenous wastes from multicellular organisms.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 5.4: Dissection of a mammalian kidney
Outline the safe working practices that you used during the dissection of
the mammalian kidney.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 5.5: A summary of filtration and
reabsorption in a nephron
a)
Why do substances move out of the blood into Bowman’s capsule at
the glomerulus?
______________________________________________________
______________________________________________________
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b) Why do substances move out of the tubules and loop of Henle into
the blood in the surrounding capillaries? (Discuss osmosis and
active transport in your answer.)
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
c)
List five substances that are filtered from the blood at the nephron.
Then circle the ones that are reabsorbed.
_____________________________________________________
_____________________________________________________
_____________________________________________________
d) Explain how filtering then reabsorbing some substances enables the
kidney to control the composition of body fluids, such as blood.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 5: Excretion
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