Oten Biology

8/16/2019 Oten Biology

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14 Maintaining a balance

the bottom, you would expect a slight expansion of the stem as aresult of that positive pressure.

• Water continues to move through a plant whose roots have been cut

off (eg. cut flowers). If root pressure were the mechanism for water movement, you might expect the removal of the roots tosignificantly reduce water movement.

• Plants which have been chilled or poisoned to kill all living cellscontinue to conduct water. This rules out active transport as amechanism of water transport.

The figure below summarises the theory of the functioning of thetranspiration stream.

water vapour escapesthrough stomata

water around soil particles

water moves alongxylem into leaf

water evaporatesinto air spaces inspongy mesophyll

water drawn upxylem in the stem

stem

water entersxylem in root

water enters root hairs by osmosis

leaf

Movement of water in plants.

Do Exercise 4.3.


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Part 4: Transport in plants 15

Movement of minerals in plants

Most minerals (ions) are dissolved in water in the soil. They move intothe plant through root hairs and then travel through xylem tissue.

Some mineral ions enter the plant by diffusion, when they are in highconcentration in the soil but are in lower concentration inside the plantroots. However, most are moved from the soil into the root by activetransport.

Some minerals are recirculated within the plant through the phloem. For example, it has been shown that radioactively marked phosphorus istransported from old leaves that are dying to new leaves which requirethis mineral for their growth.

However, other minerals do not seem to be recycled (eg. calcium).These minerals need to be constantly taken up from the soil.

Movement of products of photosynthesis

Biologists studied the ways that radioactively marked sugars movethroughout plants. Some observations include that:

• their movement in the phloem is very rapid (as quickly as 1 metre per hour)

• the direction of movement can be reversed

• movement of materials can be in different directions in different parts of the same vascular bundle.

These observations need to be explained by any theory of movement of products of photosynthesis in the phloem (usually called translocation).

Obviously, the movement of materials in phloem is different from that inxylem. Unlike xylem cells, phloem cells are living so material stopsmoving through them when phloem cells die. This suggests that the

translocation of products of photosynthesis through phloem tissue is anactive process that demands the use of energy.

The following diagram is a summary of the current theory for howtranslocation of sugars works. This theory is called the pressure flowtheory .


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Water moves into thephloem cells due to osmosisas a result of the highersugar concentration.

Water moves out ofphloem cells by osmosisdue to more sugars in thesurrounding cells.

Sugars are ‘ pumped ’ out ofphloem cells by activetransport.

HIGHosmoticpressure

LOWosmoticpressure

site of sugarproduction in leaves

Sugars are ‘ pumped ’ into thephloem cell due to activetransport. This results in ahigher sugar concentrationinside the phloem cells.

movement of sugar and water through the phloem alongan osmotic pressure gradient

site of use ofsugars in fruit,flowers, rootand stem

Diagram representing the pressure flow theory for the translocation of sugars invascular plants.


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Comparingtransport systems

The syllabus considers transport systems of flowering plants and

mammals together in this module and in the Preliminary module called Patterns in Nature . You need to make a comparison between thesetwo systems.

Construct a table to compare the transport systems in flowering plants andmammals. Remember that the word compare means giving similarities and differences.

There is a suggested table format with some information filled in for youon the following page but you can construct your own. You can also addmore categories for comparison if you like.

When you have finished the activity (which is good revision for this partof the module), compare your table with the one in the Suggested answers . Then answer the question below.

Do flowering plants, like mammals, have a circulatory system? Explain.

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A definite answer to this question is probably not possible. However,while there are similarities, it would seem that the transport systems of these two very unrelated groups are very different from each other.Almost certainly, these transport systems have been produced throughdifferent evolutionary pathways.


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Comparison of transport systems in mammals and flowering plants

Mammals Flowering plants

Nature of vessels living multicellular tubes withflexible muscular walls

rows of dead rigid cells (xylem)or living cells with cellulosecell walls (phloem)

Nature of materialtransported

a fluid made up of water andminerals (xylem sap) orwater, minerals and sugars(phloem sap)

Source of energy formovement

heat energy from the sun forevaporation (xylem) or ATPenergy from cellular respiration(phloem)

Movement of materialsinto and out of vessels

osmosis, diffusion and activetransport

Materials transported:

• water circulated via blood andlymphatic systems

predominantly one way throughthe plant; lost by transpiration

• minerals (ions) mainly circulated via blood and

lymphatic systems

• oxygen not carried in vessels

• carbon dioxide dissolved in plasma andtransported in blood vessels

not carried in vessels

• organic compounds digestion products carried inblood and lymph vessels

• hormones carried in phloem vessels

Remember to check your answers.


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Suggested answers

A different look at vascular tissue

epidermis

small cells in cortex

large cells in cortex

xylem

phloem


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Comparing transport systems

Mammals Flowering plants

Nature of vessels living multicellular tubes withflexible muscular walls

rows of dead rigid cells (xylem)or living cells with cellulosecell walls (phloem)

Nature of materialtransported

tissue made up of living cellssuspended in a fluid

a fluid made up of water andminerals (xylem sap) orwater, minerals and sugars(phloem sap)

Source of energy formovement muscular pumping by the heart(arteries) and body muscles(veins and lymph vessels)

heat energy from the sun forevaporation (xylem) or ATPenergy from cellular respiration(phloem)

Movement of materialsinto and out of vessels



Materials transported:

• water circulated via blood andlymphatic systems

predominantly one way throughthe plant; lost by transpiration

• minerals (ions) mainly circulated via blood andlymphatic systems

some recirculated throughphloem but many lost in deadleaves and such

• oxygen carried by red blood cells throughblood vessels


• carbon dioxide dissolved in plasma andtransported in blood vessels


• organic compounds digestion products carried inblood and lymph vessels

photosynthetic products carriedin phloem vessels

• hormones carried in plasma throughblood vessels carried in phloem vessels


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Additional resources

This information comes from the Preliminary module, Patterns in Nature .

Vascular bundles

This is the term used to describe the groups of conducting tissue ina stem. Each bundle contains three types of tissue: xylem, phloem andcambium.

XylemXylem forms long tubes up to 1 m in length. They are made up of deadcells, thickened with woody material, with cross walls that have brokendown. They are known as xylem vessels. Xylem gives support, strengthand rigidity to the stem, and transports water and mineral ions upwardsfrom the roots to the leaves. Note: Water and mineral ions travel only inone direction in the xylem – upwards.

Phloem

Phloem consists of living sieve-tube cells forming long columns.There are perforations in the cell walls so that the cytoplasm of the cellsconnects along the tubes. Associated with the sieve-tube cells arecompanion cells and other supporting tissue. Organic materialsincluding sugars, amino acids and hormones are transported by the livingsieve-tube cells of phloem tissue. This movement is called translocation.Materials move both up and down the plant through phloem tissue.The movement is too fast to be caused by diffusion only. There areseveral theories suggesting possible forces involved but the exactmechanism remains unknown.


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Cambium

Cambium cells are capable of cell division. They divide to form cellswhich become new xylem and phloem tissue. In older stems, division of cambium cells results in a continuous ring of vascular tissue.

How water travels up plant stems

Why does water move upwards in plants? Some of Australia’s tallesttrees, such as the Mountain Ash in Victoria and Tasmania, are more than100 metres tall. How does water move to this height, defying gravity?

Several processes seem to be involved in the upward movement of water.

The processes include:• adhesion

• capillarity

• root pressure

• transpiration-cohesion

• guttation.

Adhesion

Adhesion refers to the forces of attraction which exist between differenttypes of particles. Using tissues or a cloth to mop up water works

because of the force of attraction between dissimilar particles, the cloth particles and the water particles, or the tissue particles and the water particles. A piece of plastic would not be used to soak up water becausethe forces of attraction between the plastic particles and water particlesare very weak while those between cloth and water are much stronger.

In plants, cellulose acts like blotting paper or a cloth. Cell walls aremade of cellulose. They help the plant to absorb water from the soil.

Capillarity

Capillarity is the name given to the action by which the surface of aliquid (usually water) is elevated when in contact with a solid surface byattraction of molecules between the liquid and solid surfaces. The water

particles at the top of a column of water help to pull up the water particles beneath them. When the liquid is in a narrow vessel, the levelof water will rise quickly, but capillarity can also occur in structures such

as soils, causing a rise in the watertable.


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The xylem vessels in plants extend from the roots to the leaves. They areextremely minute tubes or capillaries and water rises up them partly by a

process of capillarity.

Root pressure

If the stem of a well-watered plant is cut off, water can be seen to comeout of the plant from the severed xylem vessels. If a glass tube wereattached to the cut stem with a piece of rubber tubing, water would beseen to rise up the glass tube. It can rise from several centimetres tomore than one metre. This rise is said to be caused by root pressure.Root pressure is caused by the intake of water due to osmosis. Scientistshave found that root pressure is too small to account for the rise of water upward in plants which are taller than several metres.

Transpiration-cohesion

At the moment, the best theory which attempts to explain the upwardmovement of water in plants is the transpiration-cohesion theory.

Water is continually leaving the plant via the leaves. This loss of water,in the form of water vapour from the leaves, is called transpiration. Now,as each water molecule passes out of the leaf, more water is drawn up,

because of forces of cohesion (attraction) between water particles. Thecolumns of water in plants can be likened to chains of beads. As one

bead (water molecule) is pulled out of the leaf, the whole chain of beads(water molecules) is pulled up a little. As more beads (water molecules)are pulled out of the plant, more enter the plant in the root region. So,there is this continual stream of water molecules upwards in plants.

Guttation

You may have noticed, occasionally, drops of water (not dew) on certain

outdoor plants and indoor plants, such as a Monstera deliciosacommonly called Monsteria (mon-stear-e-ah) or fruit salad plant.Generally, plants lose water in the form of water vapour; that is, water inthe form of a gas. However, when humidity is high and plants are wellwatered, they may lose water as drops of liquid water. This process iscalled guttation.

Many plants have special openings through which drops of water areforced out. These special openings may be found along the edges of leaves or near the ends of their veins. It is thought that high root pressuremay cause guttation.


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Summary

Several processes appear to be involved in the upward movement of water in plants. They include:

• adhesion – forces of attraction between different particles are calledforces of adhesion. The cellulose cell walls in plants soak up water

by this process, in much the same way as a blotter soaks up water

• capillarity – the rise of water in thin tubes by forces of adhesion andcohesion. Water rises up thin tubes because of attraction betweenthe particles of the plant and water particles (adhesion) and becauseof the attraction between the water particles themselves (cohesion)

• root pressure – the upward movement of water caused by the pressure from water moving into the root as a result of osmosis

• transpiration-cohesion – the loss of water molecules from the leaves(that is, transpiration) results in the upward movement of more water molecules since these molecules are attracted to each other by forcesof cohesion

• guttation – the loss of water in the form of a liquid from openings onthe leaves.

Water enters the plant through the roots. The roots are covered by fineroot hairs which increase the surface area for absorption of water. Theroot hairs are single celled extensions of the root epidermis (surface or outer layer of the root). Water enters the root hair by diffusion since theconcentration of solutes in the soil water is lower than their concentrationinside root hair cells. Water will move from an area of high water concentration (in the soil) to an area/region of low water concentration(within the root hair cells).

root hair

water

soil particles

Water moves into the plant from the soil through the root hairs.


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One of the main functions of stems is transport of substances around the plant. Internally, stems contain tubes of conducting tissue or vascular bundles, which consist of xylem and phloem, that carry materials between the shoot and root systems. The conducting tissue can bearranged in a ring or scattered throughout the stem tissue.

Cross section of plant stem. Vascular bundles are scattered throughout the cortex.© Australian Key Centre for Microscopy. The University of Sydney.


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Exercises Part 4

Exercises 4.1 to 4.3 Name: _________________________________

Exercise 4.1: For a transverse section

Draw a transverse section of a celery stem in the space below.Remember to carefully label the diagram, including the xylem and

phloem.

Exercise 4.2: For a longitudinal section

Draw a longitudinal section of a celery stem in the space below.Carefully label the diagram, including the xylem and phloem.


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Exercise 4.3: Movement of water in plants

Outline three theories that have been used to explain the movement of water through xylem tissue.

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________________________________________________________ ________________________________________________________

What is the accepted theory today and what evidence is there for thistheory?

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BiologyHSC CourseStage 6

Maintaining a balance

Part 5: Excretion


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Part 5: Excretion 1

Contents

Introduction ................................................................................2

Water and wastes ......................................................................3

Water balance ......................................................................................3

Wastes..................................................................................................5

Excretory systems......................................................................8

A review of respiratory and excretory systems ...................................8

Dissection of a mammalian kidney....................................................13

Functioning of a mammalian kidney..................................................15

What can be done when kidneys don’t function? .............................21

Suggested answers..................................................................23

Additional resources.................................................................27

Exercises – Part 5 ....................................................................39


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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 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:

• recall the role of the respiratory and excretory systems in maintaininghumans as functioning organisms

• identify the role of water as a solvent for metabolic reactions in living cellsand explain why the concentration of water in cells must be held constant

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

activity• compare the waste products of a range of terrestrial and aquatic organisms

and explain why these differences occur

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

• 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 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.

Extracts from Biology Stage 6 Syllabus 1999 © 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/syllabus99/syllabus2000_list.html


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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 of water 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 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 toosmosis. So for example, osmotic pressure is exerted by water when itmoves into cells. Movement of water from one cell to another, or

between 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) and

where that movement is through a selectively permeable membrane.

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

of water.

selectivelypermeablemembrane

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

movement of water

Do Exercise 5.1 now.


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Part 5: Excretion 5

Wastes

Do you recall what metabolism means? The definition in the glossary

tells 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 metabolic

pathway 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 quickly

eliminated 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 is produced 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.

Reptiles, birds and insects excrete a material called uric acid . It is veryinsoluble (and the least toxic form of nitrogenous waste) and so needs

little water to get rid of it.


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The table below compares some properties of the nitrogenous wastes produced by terrestrial and aquatic organisms.

Comparing nitrogenous wastes

Excretoryproduct

Solubility Toxicity Example

ammonia very soluble very toxic tadpoles, most fish,aquatic invertebrates

urea fairly soluble less toxic most mammals, adultamphibians, sharks

uric acid insoluble least toxic reptiles, birds, insects

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 (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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Part 5: Excretion 7

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

CO 2

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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Excretory systems

Different organisms have different kinds of excretory mechanisms for removing their wastes.

In unicellular organisms, waste products are quickly lost into theenvironment 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 areneeded to provide the necessary surface area for the elimination of wastes. You learnt about these in the Preliminary course. The relevantsections have been reprinted for you in the Additional resources at the

back of this part.

A review of respiratory and excretory systems

Scan the Additional resources to refresh your memory. Read any sectionsthat are unfamiliar. Then you can check your knowledge of the roles of respiratory and excretory systems by competing the tasks below.

Respiratory systems

1 Explain why a fish out of water cannot obtain enough oxygen fromthe air to survive, although there is much more oxygen in air than isever found in water.

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Part 5: Excretion 9

2 Insects have evolved a completely different system for gaseousexchange than is found in terrestrial vertebrate species.

a) Briefly describe the structure of this system.

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b) Briefly explain how it works.

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c) There are no insects anything like the size of any terrestrialvertebrate animals. With respect to their gaseous exchange(respiratory) system, suggest a reason why insects are restrictedin body size.

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3 On the following diagram of the respiratory system of a mammal,label the parts listed in the table and then complete the table byinserting a description of the function of each part. The first row inthe table has been completed for you as an example.

Fill in the missing labels below.

epiglottis

to the stomach

rib

right lungshowinglobes

diaphragm

air

nosetrachea wallmagnified

bronchusbronchiole

left lung

from pulmonaryartery to pulmonaryvein


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Part 5: Excretion 11

Structure Function

nasal cavity warms incoming air, removes dust particles and cools andcondenses moisture in outgoing air so that it is reabsorbedback into the capillaries lining the cavity

trachea

alveolus(plural is alveoli)

capillaries

cilia


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Excretory systems

4 Briefly describe the excretory system of an animal other than amammal.

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5 Complete the following table showing the functions of various partsof the mammalian urinary system.

Structure Function

kidney

ureter

bladder

urethra

Check your answers.

Now complete Exercise 5.3.


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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 also

acts 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 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 fromwhich the kidney was taken. How heavy is a kidney? Although kidneysare very important organs, they are quite small compared with the size of the animal.

Materials required:

• sheep’s kidney

• small kitchen knife

• cutting board or plate

• knitting needle or similar

• 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.


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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 the photograph below.

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 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.

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


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Diagram of a dissected kidney

Functioning of a mammalian kidney

You have revised some information about how kidneys work from the Additional resources . Before investigating the functioning of the kidneyin more detail, you need to quickly revise your understanding of the waymaterials move into and out of living cells.

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 selectively

permeable 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-moving processes.

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 from bursting

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.


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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 of a 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 of

nephon

There are around 1.2 million of these nephrons in each of your kidneys,making a surface area of approximately 12 m 2 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 of metabolism

• 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 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 high pressure. Substances are forced out of the blood in this knot of capillaries 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


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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 filteredfrom the blood travel through this tube, useful substances are reabsorbed

back 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 after reabsorption are the wastes that move into the collecting tubule. As thesewastes 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 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 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 andloop 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 dosubstances 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 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 reabsorbed

but 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 surrounding

cells 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 5.4.


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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.

______________________________________________________

______________________________________________________

______________________________________________________

______________________________________________________

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 concentrationas blood.

______________________________________________________

______________________________________________________

______________________________________________________

______________________________________________________

Check your answers.


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Suggested answers

Some reactions involving water

6O 26CO 2 C6H12 O66H2O

+ lightenergy

+ lightenergy

oxygenglucosecarbondioxide

Cellular respiration

+ oxygen carbondioxide + water + energyglucose

+ 6O 2 6CO 2 + 6H 2O + energyC6H12O6

Photosynthesis

+ +

+ +

many enzymecontrolled steps




water

or

6O 26CO 2 C6H12 O612H 2O +lightenergy+ +

many enzymecontrolled steps 6H2O+

A review of respiratory and excretory systems1 Gill filaments float in water to maintain their large surface area in

contact with the water. In air this surface area is reduced as the gillfilaments collapse together.

Water movement over the gills is continuous and in one direction – this maintains the concentration difference in gases between thewater and the gill capillaries. This movement would not occur in air.

Moisture is necessary for gaseous exchange. The gill filamentswould be directly exposed to the air and so would dry out.


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2 a) The tracheal system consists of a series of branching tubes (thetrachea and tracheoles) open from pores (spiracles) in theimpervious external covering of the insect (exoskeleton).

b) Gases enter and leave the body through openings calledspiracles. The gases diffuse through tubes, called trachea, whichsubsequently divide into finer branches (tracheoles). Gases areexchanged into fluid at the ends of the tracheoles, which arefinely divided enough to come close to most body cells.Muscular activity in movement helps to move gases in and outof the tracheal system.

c) The surface area of the tracheal system is not large enough andthe transport system not efficient enough to distribute oxygenand pick up carbon dioxide for a large animal with a smallsurface area to volume ratio.

3

epiglottis

to the stomach

rib


diaphragm

air


cilia

trachea

bronchusbronchiole

left lungdissected toshowinternal

cluster ofalveoli


nasal cavity

capillaries


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Structure Function

nasal cavity warms incoming air, removes dust particles and cools andcondenses moisture in outgoing air so that it is reabsorbedback into the capillaries lining the cavity

trachea rings of cartilage keep this tube open to permit air to pass toand from the lungs

alveolus(plural is alveoli)

thin-walled air sacs which increase the surface area forgaseous exchange

capillaries provide a large surface area for gaseous exchange of oxygenand carbon dioxide between the alveoli and the bloodstream

cilia move mucus, which contains trapped dust andmicroorganisms, out of the lungs and into the throat

4 Answers will vary. For example, you may have outlined theexcretory system of a fish, a bird or an insect.

5

Structure Function

kidney produces urine which contains metabolic waste products andregulates the amount of salt and water lost from the body

ureter transports urine from the kidney to the bladder

bladder stores urine before periodically getting rid of it from the body

urethra tube which transports urine from the bladder to the outside of

the body

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


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A summary of filtration and reabsor ption in a nephron

Here is a sample answer.

salts (such as NaCl, HCO 3 –

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 equivalent

structure 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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Additional resources

Here is a description of respiratory and excretory systems in a variety of animals, extracted from the Preliminary course module called Patterns innature Sets 5 and 6.

Fish

Most fish use gills for gaseous exchange. Gills are external structures – they hang outside the main body cavity and often have a protective cover over them. Gills have a large surface area because they are thin andhighly folded.

The structures involved in gaseous exchange in a fish.Source: Brocklehurst, KG and Ward, H. (1958.) General School Biology .The English Universities Press Ltd. London.


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Water enters a fish’s mouth and passes over its gills. When most fish arestationary they gulp water to maintain the flow over the gills. This alsoexplains why so many fish (sharks included) swim with their mouth open

– this allows water to pass into the mouth and over the gills without theneed to gulp water.

Gases are exchanged between the surrounding water and the fish on thegill surface. The gases enter the circulatory system to be transported tocells throughout the body. The main blood vessels entering the gills

branch into tiny tubes called capillaries.

The capillaries are very close to the gill surface. It is the colour of the blood in the capillaries that makes gills appear red. Capillaries, beingtiny and numerous, make the surface area to volume ratio for diffusion of gases very high in the gills.

Frogs

Frogs have two methods of gaseous exchange: gaseous exchange via thelungs and gaseous exchange via the skin. The diagram below shows thestructures involved.

The structures involved in gaseous exchange in a frog.Source: Brocklehurst, KG and Ward, H. (1958.) General School Biology .The English Universities Press Ltd. London.

Gaseous exchange via the lungs

Lungs are internal organs involved in gaseous exchange. The gaseousexchange surfaces of terrestrial organisms are usually internal to preventdesiccation (drying out). You will have noticed that the gaseous exchange


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surfaces of insects (also terrestrial) are internal too. Frogs ventilate their lungs by positive pressure breathing. This means that they force air intothe lungs. This method of breathing is very different from the negative

pressure breathing seen in mammals.

Unlike human nostrils which stay open all the time, frogs are able toopen and close their nares (nostrils). To breathe, a frog:

• closes its mouth and opens its nares

• lowers the floor of the mouth which causes air to be ‘sucked’ intothe mouth cavity

• closes the nares (nostrils)

• raises the floor of the mouth.

This forces the air from the mouth into the lungs.

Lung structure of a frog

The internal structure of a frog lung is not too dissimilar to a human lung.Air enters the lungs and then moves through a series of branching tubes.The tubes become smaller and smaller as they branch. (This is becominga familiar theme for gaseous exchange). The finest tubes are in closeassociation with capillaries (small blood vessels). Gases diffuse into andout of the blood at these sites. Waste gases diffuse out and oxygendiffuses in.

Like fish, the circulatory system delivers the gases to cells. Thecirculatory system also receives waste gases from cells and delivers themto the lungs.

Gaseous exchange via the skin

The skin of a frog is thin and is kept moist by the habitat in which thefrog lives. The skin is permeable to water (unlike human skin) and frogs

dehydrate rapidly if they are not kept in a moist environment. This iswhy you find frogs in moist locations.

Gases from the atmosphere can dissolve into the moisture on the skin.From there the gases can diffuse into capillaries beneath the skin. Theskin does not exchange sufficient gases for all of a frog’s needs.However, the gaseous exchange is important and allows the frog toremain submerged for longer than if it had to depend upon lungs alone.

While submerged, gaseous exchange occurs on the frog’s skin betweengases dissolved into the surrounding water and the frog.


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Mammals

Mammalian lungs are internal. This helps to reduce the loss of water and

heat through these structures since they have a high surface area tovolume ratio.

To get air into the lungs, a mammal lowers the air pressure in the lungs.When the air pressure in the lungs is lower than the surroundingatmosphere, air enters via the nose.

To remove air from the lungs, mammals increase the pressure of the air in the lungs. When air pressure in the lungs is higher than thesurrounding atmosphere, air moves out of the lungs.

Air enters the body through the nostrils. The nasal cavity warms the air and filters it to remove dust. The air then moves into the throat region or pharynx. It enters the largest air tube – the trachea – through an openingcalled the glottis. The epiglottis is a flap of tissue that closes over theglottis and stops food going down the wrong way when you swallow.

epiglottis

to the stomach

rib


diaphragm

air


cilia

trachea

bronchusbronchiole

left lungdissected toshowinternal

cluster ofalveoli


nasal cavity

capillaries

Structures involved in the exchange of gases in humans.


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The upper part of the trachea is the larynx, or voice box, which containsyour vocal cords.

The trachea branches into two bronchi (singular is bronchus). Each bronchus branches into smaller air passages called bronchioles and theseend in very thin-walled alveoli (singular is alveolus). Blood capillariesare wrapped closely around alveoli. The following diagram shows howgases are exchanged between air in alveoli and blood in capillaries.

blood from body(low in oxygen, high in carbon dioxide)

blood capillary

blood cell

oxygen

carbon dioxide

wall of alveolus

air inhaledair exhaled

blood to rest of body(high in oxygen,low in carbon dioxide)

Movement of material at an alveolus.

It has been estimated that the total surface area of the alveoli of an adultmale is about one third the area of a tennis court. A large surface areaobviously allows for a greater quantity of gases to be exchanged. Thethinness of the walls of the alveoli allows for rapid diffusion of oxygeninto the blood and carbon dioxide out of the blood. The moisture in thealveoli walls allows the gases to dissolve.


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Composition of inhaled and exhaled air is shown in the table below.

Gases Inhaled air Exhaled air

oxygen 21% 16%

carbon dioxide 0.04% 4%

nitrogen about 80% about 80%

water vapour varies according to thehumidity of the air

more than in inhaled air

Air passages

Rings of cartilage keep the trachea and bronchi open and prevent themclosing when the air pressure inside the body falls. The lining, or epithelial, cells of the air passages have numerous cilia. These minutehair-like projections sweep to and fro.

Mucus is secreted by special gland cells, also present in the lining cells.Dust particles and bacteria in the air are trapped by the mucus film. Themovements of cilia sweep them away in the mucus to the larynx then the

mucus is swallowed or coughed up.

Nasal hairs and mucus also trap dust and foreign particles.

Around the lungs is a membrane, the pleural membrane, which coversthe outside of the lungs and the inside of the chest cavity. It contains afluid which lubricates the surface so that there is no friction between thetissues during breathing movements.

The mechanism of breathing

In mammals, breathing refers to the movements of the chest that result inair entering and leaving the lungs.

The movement of air in and out of your chest is brought about bychanges in the pressure of the air in the chest cavity. This pressure varies

because the volume of the chest cavity varies. The chest cavity is airtightand enclosed by ribs with intercostal muscle between them. At the baseof the chest cavity is the diaphragm. The diaphragm is the muscular sheet which separates the chest (also called thorax) and abdomen.


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At rest, the diaphragm is curved upwards. The intercostal muscles relaxat the same time and the ribs move downwards and inwards. Thesecollapsing movements reduce the size of the chest cavity, increase the

pressure of the air in the lungs and thus force it out.

During inhalation, the diaphragm contracts and flattens, being moretaunt, or tight. At the same time, the intercostal muscles contract andmove the ribcage up and outwards. This increases the volume of thechest cavity and reduces the pressure of the air in it. Thus, air moves intothe lungs.

You can check these movements by placing your fingers over your ribcage as you inhale and exhale.

Excretion in mammals

There are a number of wastes produced in the body as a result of themany chemical changes occurring. Some metabolic wastes are:

• water

• carbon dioxide

• urea.

The removal of wastes from metabolic processes in cells is calledexcretion.

Metabolic wastes

Water is continually formed in cells as a result of respiration.The generalised equation for respiration is:

glucose + oxygen Æ water + carbon dioxide + energy

Water

Water is excreted from the body by way of the kidneys, lungs and skin.

You can see that exhaled air has a lot of water vapour. If you blow onto acold mirror surface, a film of water droplets forms. This is the exhaledwater vapour which has changed from a gas to a liquid.

This effect (called condensation) is also seen on windows inside cars asthey mist up when the windows are up and the glass is cold. This mist is

mostly exhaled water vapour which has condensed to form a liquid.Demisters are used to warm up the cold glass and prevent water vapour


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from condensing as a liquid. These observations can be used as evidencethat water is excreted from the lungs as water vapour (a gas). A personmay lose up to 400 mL of water from the lungs in a 24 hour period!

Water vapour and carbon dioxide pass through the capillary walls wherethey are in close contact with the alveoli of the lungs. The alveoli areextremely thin walled and have a large surface area of about 70 m 2.

About one third of our waste carbon dioxide is carried by the red bloodcells; the rest is carried in blood plasma. A certain amount of carbondioxide is necessary. It not only helps to keep the breathing processesgoing but also is important in maintaining the right blood acidity.

Too much carbon dioxide, however, is harmful because it can interferewith chemical changes and also prevent oxygen from reaching the cells.

Since there is a large moist surface area in the lungs, a certain amount of water is lost by evaporation into the air spaces. Heat is also lost from the

body by means of the air breathed out of the lungs. This helps to maintaina constant body temperature.

Most of the urine which is excreted from the kidneys is water. Urine alsocontains urea and excess salts.

Urea

Urea is a nitrogenous waste which has the formula CO(NH 2)2. It isformed from the breakdown of amino acids in the liver. This chemicalchange is referred to as deamination. Urea, which is soluble in water, iscarried away from the liver in the blood and is eventually excreted by thekidneys.

The urinary system

The human urinary system is shown on the next page. It includes thekidneys, ureters, urinary bladder and urethra. Its function is to removeurine from the body.

Urine is the fluid which contains water, urea, some salts and other substances. These substances are metabolic wastes which could beharmful, if not removed regularly from the body.

The formation of urine in the kidneys is a remarkable process. Each kidneyin a human contains about one million filtering units called nephrons.


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pelvis

ureter(a long tube)

bladder(a storage sac)

muscle

urethra (tube leading outof the body)

Human urinary system.

The nephron

The nephron filters blood. Each nephron consists of three major parts:• Bowman’s capsule

• glomerulus

• loop of Henle.

Blood, under pressure from the contraction of the heart, passes into thetwisted capillary known as the glomerulus. Most substances in thecapillary are literally squeezed out through membranes, due to the

pressure, into Bowman’s capsule.

The substances removed from the blood in this part of the nephroninclude urea, water, various salts and glucose. Substances which are notremoved include the blood cells and plasma proteins. These substancesare larger and do not usually pass through the blood capillary wall.

As the substances removed from the blood pass along the loop of Henle,glucose and some of the salts and water are reabsorbed into the blood.There are various regulatory processes which determine how much water and salt are reabsorbed. The water and urea which remain in the tubule

pass eventually into the ureter. From there they pass to the muscular

urinary bladder where urine is stored before being finally excretedthrough the urethra.


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The renal artery carries blood into the kidneys, while the filtered bloodleaves the kidneys via the renal vein.

The kidneys have two important functions.

• Firstly, they get rid of metabolic wastes, in particular, urea which ismoderately toxic.

• Secondly, the kidneys, by their filtration and reabsorption, keep theamount of water and concentration of salts (ions such as potassiumand calcium) fairly constant in the blood.

This maintenance of fairly constant conditions in the body is referred toas homeostasis . The kidneys play an important role in achieving this.

Summary• Metabolism is the name given to describe the sum total of chemical

changes in the body.

• Chemical changes include respiration, digestion and deamination.

• Wastes produced include water, carbon dioxide and urea.

• The removal of metabolic wastes is called excretion. (Defaecation is theremoval of substances from the digestive tract, which are not formed bymetabolic processes but rather are the unused remains of food.)

• Waste water is excreted from the lungs, skin and kidneys.• Waste carbon dioxide is excreted by the lungs.

• Urea is a soluble nitrogenous chemical formed from the deaminationof amino acids in the liver. It is excreted by the kidneys.

• The urinary system includes the kidneys, ureters, urinary bladder andurethra.

• The filtering units in the kidneys are the nephrons. Each nephronconsists of a Bowman’s capsule, glomerulus and loop of Henle.

• Wastes are passed through membranes out of the twisted capillary,or glomerulus, by pressure. The wastes include urea, water andsalts, collectively called urine. (Blood cells and plasma proteins aretoo large to pass through the same membranes.)

• The renal artery carries blood to the kidneys while the renal veincarries the filtered blood away from the kidneys.

• The kidneys:

a) remove metabolic wastes such as urea

b) help regulate the amount of water and salts in the blood.

• The maintenance of constant conditions in the body is calledhomeostasis.


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Removal of wastes in fish

The diagram below illustrates some adaptations that fish have to survive

in salt and fresh water. Excess salt is removed by special cells located onthe gills.

water and salts

salts isotonic urine

Saltwater fish(hypotonic relative to medium)

water salts water

salts dilute urine

Freshwater fish(hypertonic relative to medium)

Salt and water exchange in fish.

Removal of wastes in insects

salivary gland

cropmidgut

Malpighian tubules

hindgut (intestine)

rectum

Organs of a grasshopper.

The grasshopper is an example of an insect. The excretory organs of

insects are called the Malpighian tubules located between the midgutand the hindgut. They are bathed in blood directly from the opencirculatory system. Fluid is absorbed from the blood into the tubules,nitrogenous material is precipitated as uric acid and much of the water and salt is reabsorbed. The concentrated material is then passed into thehindgut and then into the rectum. Any water that remains in the materialis now reabsorbed and the urine and faeces leave the rectum as very drymaterial.

The insect rectum is very efficient at reabsorbing water. It is very similar to that of the cloaca of birds and some vertebrates, where the water is

almost completely removed from the waste, leaving uric acid to beeliminated as a nearly dry powder or hard mass.


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Exercises Part 5

Exercises 5.1 to 5.4 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?)

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

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 for continued metabolic activity that these wastes are removed from

cells? _____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________


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b) Three forms of nitrogenous wastes are ammonia, urea and uric acid.Construct a table to compare the toxicity; the relative amount of water needed for dilution and examples of groups of animals thatexcrete these forms of wastes.

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: A summary of filtration andreabsorption in a nephrona) Why do substances move out of the blood into Bowman’s capsule at

the glomerulus? _____________________________________________________

_____________________________________________________

_____________________________________________________

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.)

_____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________

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 thekidney to control the composition of body fluids, such as blood.

_____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________


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BiologyHSC CourseStage 6

Maintaining a balance

Part 6: Maintaining water balance


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Part 6: Maintaining water balance 1

Contents

Introduction ............................................................................... 2

Hormonal control of body fluid composition............................... 4

Antidiuretic hormone ............................................................................4

Aldosterone...........................................................................................6

Osmoregulation......................................................................... 8

In other animals ....................................................................................8

Adaptations for demanding environments ........................................13

Adaptations for changing environments............................................18

Suggested answers................................................................. 21

Additional resources................................................................ 22

Exercises – Part 6 ................................................................... 29


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Introduction

So far in this module, you have been studying the internal mechanisms thatare responsible for maintaining the balance in organisms. In this part youwill move out into the external environment of organisms, to look at someadaptations they have evolved to sustain life in different ecosystems.

In this part you will have the opportunity to learn to:

• identify the role of the kidney in the excretory system of fish andmammals

• outline the role of the hormones, aldosterone and ADH(vasopressin), in the regulation of water and salt levels in blood

• define enantiostasis as the maintenance of metabolic and physiological functions in response to variations in the environmentand discuss its importance to estuarine organisms in maintainingappropriate salt concentrations

• describe adaptations of a range of terrestrial Australian plants thatassist in minimising water loss.

In this part you will have the opportunity to:

• gather, process, analyse and present information to outline thegeneral use of hormone replacement therapy in people who cannotsecrete aldosterone and use available evidence to discuss the

importance of this therapy• perform a first-hand investigation to gather information about

structures in plants that assist in the conservation of water

• gather, process and analyse information from secondary sources tocompare and explain the differences in urine concentration of terrestrial mammals, marine fish and freshwater fish

• process and analyse information from secondary sources and useavailable evidence to explain the relationship between theconservation of water and the production and excretion of concentrated nitrogenous wastes in a range of Australian insects andterrestrial mammals


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Part 6: Maintaining water balance 3

• process and analyse information from secondary sources and useavailable evidence to discuss processes used by different plants for salt regulation in saline environments

• process and analyse information from secondary sources and useavailable evidence to assess the impact of urban development on thehealth of aquatic organisms inhabiting these waterways.

Extracts from Biology Stage 6 Syllabus 1999 © Board of Studies NSW,originally issued 1999. The most up-to-date version can be found on theBoard’s website at:http://www.boardofstudies.nsw.edu.au/syllabus99/syllabus2000_list.html


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Hormonal control ofbody fluid composition

In Part 5, you learnt about the functioning of the kidneys to control

excretion of wastes and osmosregulation. In Part 6, you will learn abouthow osmoregulation occurs due to coordination by the endocrine system(hormonal system).

Two hormones are involved in the regulation of the levels of salt andwater in the body. These are:

• antidiuretic h

Oten Biology

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