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BiologyHSC CourseStage 6

Maintaining a balance

Part 5: Excretion

Incorporating October 2002

AMENDMENTS

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Part 5: Excretion 1

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

2 Maintaining a balance

Introduction

The kidneys play a vital part in maintaining the internal balance ofanimals. In this part you will look at the structure and function of akidney and what happens when kidneys fail. During this part you willneed 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 maintainedwithin a narrow range for optimal function

• explain why the removal of wastes is essential for continued metabolicactivity

• explain why the processes of diffusion and osmosis are inadequate inremoving dissolved nitrogenous wastes in some organisms

• distinguish between active and passive transport and relate these toprocesses occurring in the mammalian kidney

• explain how the processes of filtration and reabsorption in the mammaliannephron regulate body fluid composition.

In this part you will have the opportunity to:

• perform a first-hand investigation of the structure of a mammaliankidney by dissection, use of a model or visual resource and identifythe regions involved in the excretion of waste products

• gather, process and analyse information from secondary sources tocompare the process of renal dialysis with the function of the kidney.

Extract from Biology Stage 6 Syllabus © Board of Studies NSW, originallyissued 1999. The most up-to-date version can be found on the Board's websiteat http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_lista.html

This version October 2002.

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Part 5: Excretion 3

Water and wastes

Water is an extremely important substance as it acts as a solvent for manysubstances in organisms. These dissolved substances can be involved inchemical reactions within cells and can be transported within organisms.Within animals, these substances are usually transported in blood whereasin vascular plants they are carried in xylem and phloem sap.

Water balance

Many of these dissolved substances also determine the movement ofwater within organisms and between cells due to osmosis. In manyanimals the concentration of water and dissolved substances, especiallysalts, in the fluids surrounding cells and in the blood must be maintainedwithin very narrow limits to prevent loss or uptake of water which couldresult in damage to cells.

If water is not readily available, organisms may die as a result of nothaving enough. On the other hand, excess water may need to be quicklyremoved from organisms to maintain osmotic balance. To maintain theirwater balance, organisms must match their water gains with their waterlosses.

Some reactions involving water

Water is produced by some reactions in the bodies of organisms (forexample, 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.


A reminder about osmosis

Osmosis is the movement of water from where it is in higherconcentration to where it is in lower concentration through a selectivelypermeable (sometimes called semi-permeable) membrane.

Osmotic is the word which describes things (an adjective) related toosmosis. So for example, osmotic pressure is exerted by water when itmoves into cells. Movement of water from one cell to another, orbetween organisms and their external environment due to concentrationdifferences, can be called osmotic movement. Remember that theadjective osmotic always describes a situation where water is movingfrom where water is in higher concentration (a more dilute solution) towhere water is in lower concentration (a more concentrated solution) andwhere that movement is through a selectively permeable membrane.

A concentrated solution hasa higher concentration ofsolute (dissolved substance)but a lower concentrationof water.

selectivelypermeablemembrane

A dilute solution has a lowerconcentration of solute buta higher concentrationof water.

movement of water

Cells that are isotonic (same concentration inside the cell as in thesurrounding fluid) function well. In a single-celled organism thissurrounding fluid is usually water. In multicellular organisms thesurrounding fluid is usually the interstitial fluid found between cells. Ifthe cell is not isotonic with its surroundings then water may be lost orgained and this may lead to death.

In summary, water needs to be maintained within a narrow rangebecause:

• water is involved in many of the essential chemical reactionsthat occur in cells such as respiration and photosynthesis

• it acts as a solvent for many substances

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Part 5: Excretion 5

• 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.


Wastes

Do you recall what metabolism means? The definition in the glossarytells you that metabolism is all of the biochemical reactions occurring inthe cells of the body.

These reactions normally occur as a series of chemical reactions, whichis called a metabolic pathway. Each step in a metabolic pathway isgoverned by a specific enzyme. Cellular respiration is one metabolicpathway with which you are familiar. Many products of metabolicreactions are wastes, which are normally eliminated from the body.

Some products of reactions in the body are in fact poisonous (toxic) andmust be broken down to less toxic substances or be very quicklyeliminated from the body.

In most vertebrate species the liver is responsible for producing manywaste products, due to enzymatic breakdown of potentially harmfulsubstances taken into the body or produced by metabolism. Organswhich remove these wastes are known as excretory organs. The twomain 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 duringcellular respiration (the final metabolic process in the breakdown of fatsand carbohydrates). The kidneys get rid of other metabolic wastesincluding water and nitrogenous wastes. These nitrogenous wastes areformed from the breakdown of materials which contain nitrogen,particularly proteins.

Ammonia is a very soluble and very poisonous nitrogenous waste. It isproduced in tadpoles and in most fish and aquatic invertebrates that haveaccess to plenty of water to dilute it.

Terrestrial species produce nitrogenous wastes in the form of either ureaor 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 asource of water loss for these species.

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Part 5: Excretion 7

Reptiles, birds and insects excrete a material called uric acid. It is veryinsoluble (and the least toxic form of nitrogenous waste) and so needslittle water to get rid of it.

The table below compares some properties of the nitrogenous wastesproduced 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 tofunction normally. For example, an accumulation of carbon dioxidewould lead to changes in pH inside cells. You will recall that changesin pH can bring metabolic processes to a standstill by denaturingenzymes.

Accumulation of other wastes would cause water to move into cells byosmosis, 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, theconcentrations of these substances must also be closely regulated asthey may produce conditions that also result in cell death. For example,too much or too little salt can result in cell damage or themalfunctioning of organ systems.

Do Exercise 5.2 now.

You read earlier about the importance of respiratory surfaces and kidneysfor removing wastes. However, these are not the only excretory organs.For example, in mammals, some wastes are eliminated by the skin (forexample, salts, urea and lactic acid in sweat) and a few are got rid ofthrough the digestive system (for example, the breakdown products ofhaemoglobin are added into faeces).


The diagram below summarises processes that produce wastes andhow these can be removed.

CO2

respiratorysurface (gills

or lungs)

water

kidney

nitrogenouswastes

skin(for example,

urea andlactic acid)

othermetabolic

wastes

Cellularrespiration

Other metabolicpathways

largeintestine

(for example,products of

haemoglobinbreakdown)

Summary of modes of excretion.

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Part 5: Excretion 9

Excretory systems

Different organisms have different kinds of excretory mechanisms forremoving their wastes.

In unicellular organisms, waste products are quickly lost into theenvironment through the cell membrane by diffusion. This can occurbecause each cell has a large surface area to volume ratio.However, in multicellular organisms, complex excretory organs (such askidneys) are needed to provide the necessary surface area for theelimination of wastes..

Now complete Exercise 5.3.

Dissection of a mammalian kidney

If you dissected a kidney during the Preliminary module called Patternsin Nature, you need only to refer back to that module. However, if youdid 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 notmuch trouble to buy a sheep’s kidney from the local butcher and dissectit yourself. (A sheep’s kidney is very similar to a human kidney, exceptthat 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 alsoacts to protect it. When the kidney is removed at the abattoir or when thefat 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 pickthe 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 fromwhich the kidney was taken. How heavy is a kidney? Although kidneys


are very important organs, they are quite small compared with the size ofthe animal.

Risk assessment

You will be using sharp instruments so take care that you do not cutyourself. When using animal tissue you should wear rubber gloves ifavailable. Make sure you are wearing suitable covered footwear and disposeof 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 easyto see because they are all connected together with tissue and mayhave been cut off the kidney you have. Also, there is a lot of fatwhere 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.

4 To observe the internal structure of the kidney, cut through the kidneylengthwise, carefully cutting away from your fingers.

5 Now look inside. You will notice a funnel-shaped structure with a holein the centre. This hole leads into the ureter. Take an object like aknitting needle and push it gently through the opening. Discover wherethe ureter leaves the kidney.

6 Continue cutting down to open up the kidney as shown in thephotograph below.

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Part 5: Excretion 11

cortex

medulla

pelvis

ureter

capsule

The internal structure of a sheep’s kidney.

7 Find the following structures:

• the brown outer layer, or cortex. This is where the wastesubstances are squeezed out through the membranes of theglomeruli into the Bowman’s capsules

• an inner pink layer of medulla. Here, water and some salts arereabsorbed into the blood from the tubules of the nephrons

• a hollow whitish region. This is the pelvis of the kidney wherelarge collecting tubes empty urine into the funnel-shapedbeginning of the ureter.

8 On the following page, draw a fully labelled diagram of the dissectedkidney.


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 fromwhere they are more concentrated to where they are less concentrated;this is diffusion. If the diffusion of water occurs through a selectivelypermeable membrane, the process is called osmosis.

However, living cells can make substances move from where they areless concentrated to where they are more concentrated by using energy;this is called active transport. Active transport may also involve changesin the structure of the membranes, thus permitting materials to be movedagainst the concentration gradient.

As you will see, all of these processes – diffusion, osmosis and activetransport – are very important in the functioning of the kidney.

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Try this short quiz to test your knowledge of these substance-movingprocesses.

1 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 througha membrane

c occurs in plants only where the cell wall prevents cells frombursting

d occurs in plants and animals but not in microorganisms.

2 Diffusion occurs in:

a liquids, gases and solids

b liquids, gases and solutions

c liquids and solutions only

d liquids and gases only.

3 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.

4 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 thekidney and the mechanisms responsible for movement of particles inorganisms, the information below about the functioning of the kidneyshould be much easier to follow.


The functional units of the kidney – nephrons

The diagram below shows the structure of the kidney and its bloodsupply. (Turn back to check that you correctly labelled your diagram ofa dissected kidney.)

medulla

cortex

ureter

renal vein

renal artery

The following diagram shows the position of tiny structures, callednephrons, which make it up the kidney.

position ofnephon

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 greatsurface area created by so many nephrons in the kidney makes it efficientin carrying out its two important functions. These are:

• excretion – the elimination of harmful and unwanted products ofmetabolism

• osmoregulation – the control of body water and salt levels.

The kidneys also have some role in regulating blood pH by the secretionof H+ ions into the nephron by active transport.

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An individual nephron is shown below, where the parts are named andthe complex blood capillary network associated with each nephron isshown. The Bowman’s capsule and the proximal and distal tubules arefound in the cortex, which you will remember from your dissection is theouter dark brown-coloured layer of the kidney. The loop of Henle andthe collecting tubule (or collecting duct) protrude down into the medulla,which is the lighter-coloured part towards the centre of the kidney.

branch ofrenal artery

branch ofrenal vein

proximaltubule

distaltubule

glomerulus

collectingtubule

Bowman’scapsule

capillariesloop ofHenle

A mammalian nephron.

Each part of the nephron has an important role in the filtration of bloodand the osmoregulation of the animal.

renal artery – brings blood containing small particles, includingnitrogenous wastes (especially urea), water, salts, glucose and aminoacids to the kidney

glomerulus – blood passing through the glomerulus is under highpressure. Substances are forced out of the blood in this knot ofcapillaries into Bowman’s capsule. The process is largely governed bythe size of the pores in the membranes of the capillaries and Bowman’scapsule, which let small molecules and ions through but prevent themovement of larger molecules (such as large proteins) and blood cells.

Bowman’s capsule – a cup-shaped structure surrounding the glomerulusthat collects materials forced out of the blood


proximal tubule, loop of Henle and distal tubule – these structures arejoined together, making a long, very thin tube. As the substances filteredfrom the blood travel through this tube, useful substances are reabsorbedback into the blood in the capillaries surrounding the tube. This involvesactive transport. Most of the glucose and amino acids are reabsorbed inthis 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 afterreabsorption are the wastes that move into the collecting tubule. As thesewastes move through the tubule, more water is taken back into thebloodstream 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 Henleand distal tubule join together into the renal vein. This blood vesselcarries blood that has been cleaned by the nephron back into the body’scirculation.

So, in summary, osmoregulation and excretion by nephrons in the kidneyare accomplished by the production and elimination of urine. Urine isproduced by:

• filtration of many substances, both wastes and useful ones, from theblood (at the glomerulus/Bowman’s capsule)

• reabsorption of useful substances into the blood (at the tubules andloop of Henle).

Diffusion, osmosis and active transport in a nephron

Substances move from the blood into the Bowman’s capsule because ofthe high pressure of the blood through the glomerulus. But why dosubstances move from the tubules back into the blood?

Some substances can move by diffusion, because there is a lowerconcentration of them in the blood and a higher concentration in thetubule. However, once the concentration difference between the blood andvarious parts of the nephron is balanced, energy must be used to moveuseful substances, such as glucose and amino acids, back into the blood.

Since active transport is used, the body can determine the amount of eachsubstance that is reabsorbed. For example, all glucose will be reabsorbedbut only some salt. In this way, the amount of substances including saltand water reabsorbed is precisely controlled to balance water and saltintake and losses, so that the composition of blood and fluid surroundingcells is maintained at a constant level. This process is controlled by theendocrine 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 byindicating the general composition of the fluid which enters Bowman’scapsule (sometimes called the filtrate) and the fluid which eventuallydrains out of the collecting tubules into the renal pelvis (the urine).This shows that, for the most part, active transport is used to pump usefulmaterials back into the bloodstream, rather than specifically pumpingundesirable 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.


What can be done when kidneysdon’t function?

In people who have impaired kidney function, waste products can beremoved from their blood using a process called renal dialysis. Theblood of the patient is passed through a coil separated by a membranefrom a salt (saline) solution which has the same concentration as theblood (called a dialysing solution).

The dialysis membrane is permeable to water and to nitrogenous andother waste products of metabolism, especially urea. For 4–5 hoursabout three times a week, the blood of the patient is circulated throughthe haemodialysis machine depicted in the diagram below.

to dialyser

freshdialysingsolution

useddialysingsolution

constanttemperature

bath

dialysermembrane

bubble trap

superficialvein

artery

fromdialyser

Haemodialysis machine.

Dialysis can also be carried out within the body by a process known asperitoneal dialysis. In this instance, a saline solution is passed into thebody cavity (peritoneum) of the patient by a catheter (fine tube). Wastesdiffuse from the body fluids and pass through the membrane that linesthe peritoneum into the saline solution, which is then drained out byanother catheter. This process avoids the necessity to circulate the bloodfrom the patient’s body, with the possible risk of blood clotting andinfection.

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Comparing renal dialysis with normal kidney function

Refer to the diagram of the haemodialysis machine and use your knowledgefrom throughout this module to deduce answers to the following.

1 Explain the reason for the constant temperature bath in the machine.

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2 The membrane in the haemodialysis machine is equivalent to whichpart 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.

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3 Explain why the dialysing solution has the same salt concentrationas blood.

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Check your answers.

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Suggested answers

Some reactions involving water

6O26CO2 C6H12O66H2O

+ lightenergy

+ lightenergy

oxygenglucosecarbondioxide

Cellular respiration

+ oxygen carbondioxide + water + energyglucose

+ 6O2 6CO2 + 6H2O + energyC6H12O6

Photosynthesis

+ +

+ +

many enzymecontrolled steps




water

or

6O26CO2 C6H12O612H2O + lightenergy

+ +many enzymecontrolled steps 6H2O+

to show that the source of the oxygen gas is water and not carbon dioxide

A review of osmosis, diffusion and active transport1 B

2 B

3 D The energy of movement of the particles is responsible for themovement. This energy (kinetic energy) increases with temperature.It is not supplied by the cells themselves through respiration.

4 C


A summary of filtration and reabsorption in a nephron

Here is a sample answer.

salts (such as NaCl, HCO3– and K+)

nutrients and water

salts, nutrientsand water

salts,nutrientsand water

many substancesfrom blood

urine (water, urea and salts)

FILTRATION

REABSORPTION

H+ (to balance pH)

Comparing renal dialysis with normal kidney function1 If the constant temperature bath were not used to keep the solution at

body temperature, the blood would lose heat to the solution in thecore and the patient could become hypothermic (have a bodytemperature below normal).

2 B is correct. The membrane of Bowman’s capsule is the equivalentstructure in the nephron, where filtration occurs. Remember thatreabsorption occurs in the other parts of the nephron.

3 If the solution had a higher salt concentration than blood, the patientwould lose water into the solution by osmosis. If it were lessconcentrated, water would pass into the patient’s blood by osmosisthrough 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 remainsconstant in living cells? (What might happen if it did not?)

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Exercise 5.2: Some examples of waste productsa) Metabolic processes constantly produce wastes such as carbon

dioxide, nitrogenous wastes and water. Why is it essential forcontinued metabolic activity that these wastes are removed fromcells?

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Exercise 5.3: A review of respiratory and excretorysystems

Simple organisms, such as unicellular animals, are able to rely ondiffusion and osmosis to remove wastes such as nitrogenous wastes andwater. However, multicellular organisms, such as mammals, requirecomplex organs and body systems for excretion.

Explain why the processes of diffusion and osmosis are inadequate inremoving dissolved nitrogenous wastes from multicellular organisms.

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Exercise 5.4: Dissection of a mammalian kidney

Outline the safe working practices that you used during the dissection ofthe mammalian kidney.

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Exercise 5.5: A summary of filtration andreabsorption in a nephrona) 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 intothe blood in the surrounding capillaries? (Discuss osmosis andactive transport in your answer.)

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c) List five substances that are filtered from the blood at the nephron.Then circle the ones that are reabsorbed.

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d) Explain how filtering then reabsorbing some substances enables thekidney to control the composition of body fluids, such as blood.

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