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Biology 30
Module 2 Body Actions and Systems:
Maintaining Life
Lesson 6
Body (Cell) Wastes
Copyright: Ministry of Education, Saskatchewan
May be reproduced for educational purposes
Biology 30
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Lesson 6
Biology 30
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Lesson 6
Lesson 6
Body (Cell) Wastes
Directions for completing the lesson:

Text References for suggested reading:
Read BSCS Biology 8th edition
Section 14.10 - Pages 359-360
Section 16.10 – Pages 418-420
OR
Nelson Biology
Pages 204-214

Study the instructional portion of the lesson

Review the vocabulary list

Do Assignment 6
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Lesson 6
Vocabulary
ADH
Bowman’s capsule
closed digestive system
deamination
dialysis
egesthan
excretion
flamecell
glomerolus
green glands
guttation
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Henle;s loop
kidney
malpighian tubules
nephridial system
nephron
open digestive system
secretion
transpiration
urea
uric acid
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Lesson 6
Body (Cell) Wastes
Introduction
Two important processes going on in the bodies of living organisms have to do with
the release of energy and with the building and repair of cell and tissue parts. In
comparing these to similar types of actions carried out by humans on a larger scale,
certain common characteristics appear. When humans "produce" electrical, heat and
other forms of energy, the process is usually accompanied by the formation of water
or chemical vapors (steam or smoke) or ash, tar and other liquid or solid byproducts
of burning fuels. In following through all phases in the construction and finishing of
a house, apartment complex or office building, one would see many left-overs. Such
items as pieces of boards, insulation, empty or partially empty glue or caulking
containers, bits of masonry, electrical wires and pipes are just some of the things
which may no longer be viewed as useful items and are discarded. Similarly, release
of energy and the synthesis of matter inside bodies result in some substances being
regarded as wastes.
In the actions occurring on a
broader scale outside of bodies,
wastes are either removed by some
physical forces in the natural
environment (wind or water) or are
periodically gathered and hauled
away to some dump site. Leaving or
not attending to such wastes in
some way could reduce the
efficiency of some operation or clog
or clutter it so badly as to stop it
completely. Conditions within the
bodies of living organisms follow
the same pattern. Wastes must be
continually removed to certain
"dump sites" which may be
permanent or, more frequently, which are temporary as the wastes are eventually
released from the bodies. A serious effect of waste accumulation can be seen during
kidney failure, due to injury or disease, which can lead to death.
Methods of disposing of wastes range from simple to complex, according to the
nature and complexity of organisms themselves. In general, the removal of wastes
becomes more critical and the methods more active and complex as body sizes and
activity levels increase.
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Lesson 6
After completing this lesson you should be able to:
•
define what "wastes" are.
•
discuss the importance of removing wastes from living
organisms.
•
indicate the types or kinds of substances which can be
wastes to particular organisms at certain times.
•
distinguish between the processes of secretion, excretion
and egestion.
•
describe the ways in which plants deal with wastes.
•
describe some of the kinds of excretory systems in some
representative animal groups:
-
•
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unicellular (protist) organisms.
colonial-type, simple multicellular organisms.
worms
arthropods
vertebrates
identify some other functions or roles of excretory organs
or systems in bodies.
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Lesson 6
The Meaning of "Waste"
Commonly in everyday life, anything
which no longer has any use and is
simply "cluttering" up an area, is
regarded as waste or garbage. A
similar meaning can be applied to
some substances in organisms
Waste: anything which has no
useful value to a body at a
particular time and/or of which
there is an excess
Some forms of matter can be useful to a body at one time and of no value or even
harmful at another time. Further mention of this will be made shortly, but oxygen
and carbon-dioxide gases fit this category in relation to green plants. Depending on
whether their dominant activities are photosynthesis or respiration at certain times,
green plants could be alternating in their manner of using each of these gases as a
nutrient or as a waste. Too much of a particular substance at a particular moment,
even though it is usually an essential nutrient, can result in attempts at its removal.
Excess amounts of vitamins or minerals taken by some people as supplements are
generally removed with the urine. Excess water in plant or animal bodies is also
treated as waste.
Plant and animal body wastes originate or accumulate from two general types of
actions:


internal metabolic processes, where energy is converted from one form to
another or where body parts are either built up or broken down, result in waste
products. In addition, the different metabolic processes require different kinds of
nutrients.
Ingestion of nutrients can bring in excess amounts of the nutrients themselves or
of other substances combined with those nutrients but not needed or utilized –
such as the extra amounts of fibre or cellulose which we may consume with our
plant foods.
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Waste Substances
Keeping in mind that some of the following forms of matter could also be nutrients at
certain times, wastes may include:
•
gases released by the processes of photosynthesis and respiration: oxygen and
carbon-dioxide.
•
water, as a byproduct of respiration or as an excess of ingestion.
•
mineral salts, such as those of sodium, chlorine and potassium.
•
ammonia or nitrogen products, primarily from cellular respiration involving
proteins.
•
excess organic substances ingested into bodies, such as cellulose or vitamins.
•
parts of cells or tissues which are no longer functioning, such as plant bark,
animal skin cells or red blood cells.
The Importance of Removing Wastes
Allowing wastes to accumulate either inside cells or in the fluids around the edges
could be detrimental in a number of ways. At the cellular levels, such
accumulations could slow or even stop the processes of diffusion, osmosis or active
transport of nutrients or wastes into or out of the cells. Shortages of nutrients or
excesses of toxic wastes could lead to cellular deaths. A comparable example may be
that of a person who has frostbite on skin tissue, with unprotected ears being
common spots. Such a person actually suffers the first damage when intercellular
lymph solidifies. This prevents the movements of nutrients and wastes into and out
of cells and causes their deaths.
Certain substances can become toxic in higher
concentrations. Accumulations of nitrogen compounds,
such as ammonia, can be particularly harmful in animal
organisms where protein respiration is common. Toxic
wastes can cause cell and tissue deaths in smaller,
localized areas. On a larger scale, wastes can produce
muscle paralysis or convulsions, improper organ
functioning and possibly coma and death.
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Lesson 6
A term which has been mentioned in previous lessons is homeostasis. Maintaining
specific kinds of internal body conditions and maintaining them at uniform levels, is
important to the general functioning and health of organisms. Cells, tissues and
organs generally operate best within a narrow range of conditions. A rise or fall in
body temperature of only a few degrees Celsius, a rise or fall in the concentration of
certain mineral (salt) ions in lymph or blood, or a change in the pH or acidity levels of
fluids, can have significant effects on actions of body parts and the overall body.
Waste removal affects these conditions, as well as others, and must be fairly precise
in what is removed and in what quantities. Removal of too much of some
substance can be as harmful as an over-accumulation.
Excretion, Secretion and Egestion
Some terms used with the movements of substances from cells or from bodies should
perhaps be clarified before going on. References could vary but most follow the
definitions or explanations stated here.
Secretion: a material that is useful to the body in some way
Certain cells produce, accumulate and then release material to the outside of their
boundaries or membranes. If the material is considered as useful to the body in some
way, it is regarded as a secretion. Digestive enzymes released into the mouth,
stomach or small intestine are secretions.
Excretion: Matter which leaves cells and is considered to be of
no value and possibly even harmful is waste
Ammonia, from protein respiration, is an excretion. Carbon-dioxide gas from the
same process is also an excretion.
Egestion/Elimination: the final removal of a substance out of a
body
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Lesson 6
The movement of a material to the outside of a cell does not necessarily mean that it
has left a body. Frequently, both secretions and excretions enter body fluids or
certain body cavities which are still inside the body. These may include lymph, blood,
parts of the digestive tract, or a number of glands or the bladder. The final removal of
a substance out of a body is egestion or elimination. Removal of urine or feces from a
body is egestion. Many references use the term excretion for this process as well.
On occasion, it may be difficult to choose the best term to describe the removal of a
particular substance. Tears in many marine organisms could be excretions which
contain extra salt which bodies are trying to get rid of. Some organisms adapted to
land and fresh water may shed tears for the same reason. For others, tears may keep
the surfaces of the eyes moist for better functioning of eyelids and also in acting as a
mild antiseptic. In these situations, tears could be considered as secretions. Bile,
produced in the liver, contains dead red blood cells as well as bile salts used in
digestion. Since bile eventually leaves the body in the feces it could have all three
terms applied to it.
Plants
Dealing with wastes is not as critical an action with plants as it is with animals. This
does not mean that there are necessarily fewer processes taking place within plants
or that some of their metabolic rates are lower; in fact, some of these may equal those
of animals.
There are a number of characteristics unique or more common to plants make
excretion or egestion less of a problem for them.
1. Non-living areas do not form wastes
Large portions of plant parts and organs are non-living in perennial plants such as
woody shrubs and trees and to a lesser extent in annual and biannual plants. Inner
pith and the xylem cells of heartwood and sapwood, or of fibrovascular bundles, are
no longer living at maturity. Even the outer cellulose walls of living cells have this
characteristic. Such non-living areas do not form wastes themselves. In some plants,
pith or xylem cells even become areas where some solid wastes such as mineral
crystals, gums and resins are deposited permanently.
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2. Cycles of photosynthesis and cellular respiration
The occurrence of both photosynthesis and cellular respiration, even though they are
likely taking place at different rates at different times, also sets up cycles not possible
in animals. Waste materials released by one process could become nutrients for the
other. For instance, oxygen released by photosynthesis can be used in cellular
respiration. Cellular respiration, or the breakdown of organic compounds, forms
carbon-dioxide and water which can be used to synthesize new organic compounds
later on. The system is not perfect in that not all wastes can be reused. However, it
greatly reduces the total amount of waste present that the plant eventually has to
cope with.
3. End products of cellular respiration in plants and aniimals
Catabolism in plants centers largely on carbohydrates. The endproducts of starch
and sugar digestion can be used again in photosynthesis. Even if not used, these
endproducts or wastes are not usually harmful or toxic to plant cells. In animals, a
considerable amount of cellular respiration uses protein as a nutrient. When a
protein is broken down – either to provide energy or to supply building parts to form
an organism's own proteins, the amino or nitrogen containing group has to first be
broken off from the rest of the molecule. This process is called deamination. Many of
the amino groups then combine with carbon to form ammonia. This is a very toxic
waste to cells and tissues. Animal bodies therefore have a more serious type of waste
to deal with. Some plants or plant parts do break down a limited amount of protein.
New seedlings, before the ability to photosynthesize becomes developed, and plants
experiencing shortages of sunlight, will break down some proteins. However, some
plants have the ability to reuse the ammonia or nitrate waste products later on.
Other plants can temporarily or permanently convert the toxic nitrogen compounds
into nontoxic amides or alkaloids, which can be safely stored in leaves or fruits.
Nicotine, morphine, caffeine and digitalis are all such stored wastes.
4. Cell Physiology
A dominating feature of many plant
cells is a large, central vacuole,
which may take up 80 to 90% of a
cell's volume. This fluid-filled space
contains stored nutrients and foods;
but it can also store wastes for the
life of the cell and possibly the life of
the plant.
Plant Cell
Vacuole
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Plant Excretion and Egestion
Since plants do not usually have large concentrations of wastes, there is very little in
the way of development or adaptation of cells specifically for handling wastes. Some
excretions and egestions do occur using the same pathways or movement processes
that nutrients use.
Solid or fluid wastes: These wastes, which may be stored in pith or xylem cells, can
reach their final destinations along the xylem vessels, pith or vascular rays. Water is
the most common waste fluid egested or released to the outside. This loss of water to
the atmosphere is called transpiration. Most of the water departs from the vascular
bundles (veins) in leaves by passing through cell walls. Vaporizing, the water droplets
diffuse through the same stomata which allow movements of gases. Some plant
varieties have stomata-like openings, which are at the ends of veins, at the leaf edges
or surfaces. Remaining open, these modified "stomata" permit actual droplets of
water to ooze out of the ends of veins in a process called guttation. Wet soil
conditions, along with a high atmospheric humidity, produce the guttations
commonly observed some mornings on the leaves of potatoes, tomatoes, cabbages,
strawberries and lawn grasses. (Guttation may be confused with dew but, whereas
dew is a film of moisture over the whole plant, guttation is apparent as a drop or
droplets along a leaf edge.) The actual driving force behind guttation appears to be
root pressure.
Gaseous wastes: Gas wastes, which include carbon-dioxide or oxygen at different
times, takes place by diffusion through leaf stomata and stem lenticels. Lenticels
enable gases within the stem to have fairly easy access to the outside by diffusion
alone. (They also permit gases to enter when needed.) In woody trees this is further
assisted by the characteristic of living cells existing as a layer towards the outside
area of the stem while non-living cells are located towards the interior.
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Why do Trees Lose their Leaves?
Leaf fall is actually a form of excretion. Deciduous plants
which commonly lose or drop their leaves periodically, are
really removing or getting rid of plant matter which is no
longer needed. The actual mechanism of leaf fall is
centered on a special row of parenchyma cells found near
the base of a leaf petiole.
This separation layer, or abscission layer, has the pectins in its cells' walls dissolved
by enzymes at the end of the growing season. The most likely triggering actions
appear to be length of photoperiod (length of daylight and darkness in a certain time
period) and temperature. The cells in the layer separate from each other once the
pectins dissolve and the leaf remains attached only by its vascular bundles.
Shrinking or expansion due to moisture changes, wind or snow and rain eventually
cause final separation.
Comparison of Excretory Systems
Following is a discussion of excretory systems progressing from the systems of simple
unicellular organisms to the more complex systems of vertebrates.
It is important that you are able to understand:



The progression in development of the excretory systems
The key factors for each organisms’ system
How organisms in the arthropods and vertebrates manage soluble nitrogenous
wastes
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Unicellular (Protist) Organisms
Due their small sizes, diffusion and
osmosis are generally sufficient for
the excretion-egestion of gases and
fluids. This process is made easier
by the presence of a water, or fluid,
environment.
vacuoles
Some varieties have radiating
canals leading to a central vacuole,
as in the case of Paramecium. The
canals collect excess water and
transfer it to the vacuole. Upon reaching a certain size, the vacuole contracts, forcing
the water out. Depending on the salt or mineral concentration of the surrounding
water, the frequency of the contractions of the vacuoles could increase or decrease
(refer to lesson 3 application). The Paramecium has two contractile vacuoles, one at
each end.
What about solid particles that remain after the digestive enzymes have completed
their work? In many unicellular organisms, these could be passed out in their
vacuoles through any point along the surface membrane. Some varieties, such as the
Paramecium, have localized spots, or anal pores through which solid waste particles
can pass.
Colonial-Type or Simple, Multicellular Organisms
Excretion can be accomplished through diffusion or osmosis since most of the
individual cells border an external fluid environment. Since the wastes are deposited
directly into the external environment, both terms excretion and egestion can be
used. Organisms such as Hydra, or many of the jellyfish, have central or
gastrovascular cavities. However, since external fluids circulate regularly through
these cavities, there are no increased difficulties in removing matter.
Gastrovascular cavity of a hydra.
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Worms
The excretory systems of worms range from those with no, or few developments to
those having the beginnings of excretory systems similar to vertebrates. This range in
complexity is due to the variations in habitats and manners of living and the
resulting adapations for removing wastes. Flatworms, roundworms and the common
earthworm (Phylum Annelida) will be discussed
a) Flatworms can be divided into two main catagories:


Free living worms such as Planaria
Parasitic worms such as flukes and tapeworms
Planaria and flukes have either a pharynx or mouth which leads to a branched
gastrovascular cavity. Theses organisms actively ingest organic matter into their
cavities for digestion. Since they have a closed or incomplete digestive tube, solid
waste particles are egested through the same opening through which nutrients enter.
Segmented parasitic worms, such as tapeworms, have an even greater reduction in
their systems. In addition to lacking respiratory and circulatory systems, they lack
mouths and digestive tubes. Nutrients from their hosts’ intestines simply diffuse in,
although diffusion does have to occur through a much thicker cuticle or outer
membrane. (This thickened cuticle is necessary to protect the parasite from being
digested by its own host). The wastes of both the host and worm diffuse in and out
just as the nutrients do.
Flame cell system
A flame cell system is a specially developed excretory system existing in some
tapeworms and in the planarians and flukes.
Flame cell in a Planaria.
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A flame cell system consists of a series of collecting tubules running along both sides
of a body. Collecting tubules have many "pockets" along their length. Each is a flame
cell. Movements of cilia (or flagella) inside the cell, which to some observers resemble
a flickering "flame", tend to draw water and possible nitrogen waste into the cell and
collecting tubule. These excess wastes are then passed out through openings or
excretory pores, where the collecting tubules come to the surface of the organism's
outer membrane.
b) roundworms or nematodes can be


free living, scavengers, or
parasites
This group shows a digestive system which is complete or open, having both mouth
and anus. Solid waste particles are therefore able to move completely through the
tube for eventual egestion. Two excretory tubes, one along each side, seem to
function much like the collecting tubules in the previous group. However, there do
not appear to be any flame cells. Instead, the two tubes join together at the anterior
end and empty through a common excretory pore.
c) Phylum Annelida shows another specialized excretory system, the nephridial
system, an advance over flame cells. The nephridial system is closely associated
with a blood circulatory system, which becomes apparent in this group.
Segmented worms, such as the earthworm, have openings or nephrostomes which
collect fluids from the cavities in the segments. These fluids pass along a tubule,
into another segment, from there into an enlargement or bladder and then out
through an external opening or nephridiopore.
This tubule complex that collects fluids in one segment, then passes into a
bladder in the next segment and empties to the outside through the
nephridiopore, is called a nephridium. There are two of these per segment and
together they are called nephridia.
Cross-section of a
segmented worm
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The tubule and bladder are intertwined closely with blood capillaries. Waste
substances, such as salts, water and the especially harmful nitrogen byproducts,
pass from the blood and into the tubule and bladder. Useful fluids may be partially
reabsorbed into the blood. Solid wastes make their way through the open digestive
tube and out at the anus.
The remaining organisms described in this lesson have excretory systems which are
relatively similar in the manner of processing and egesting solid wastes.
Solid matter is passed through digestive tracts where nutrients are broken down and
then absorbed. Remaining solids may have additional water removed and cell or
tissue remains and excess body solids added before being egested from the body
through an anus.
Due to the similarities in solid waste removal, there will be
limited mention made of this in the remainder of the lesson.
Instead, the emphasis will be on the manners in which
soluble nitrogenous wastes are treated by various organisms.
Arthropods
Some crustaceans show the presence of green glands or antennule glands which
have enlargements or sacs extending into a blood sinus.
Nitrogen wastes and excess water are removed from the blood by a sac and
transferred along a duct or tube to a "bladder" where they collect. Egestion from the
body occurs through an excretory pore which opens near the base of an antenna.
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Arachnids and insects have tubules extending out from a portion of the intestine.
These Malpighian tubules extend into adjacent blood sinuses where they remove
salts, excess water and nitrogen wastes. The wastes join and mix with the other
waste material in the intestine, before being egested. In many insects the nitrogen
wastes are converted to insoluble uric acid crystals inside the Malpighian tubules
and intestines.
Vertebrates
Vertebrates demonstrate a number of ways of excreting and egesting waste materials.



Most solids are eliminated through an anal opening.
Some waste carbon-dioxide can leave blood capillaries and diffuse through moist
skin surfaces and into the external environments (amphibians), through gill
filaments into water (fish) or, through moist membranes into lung aveoli before
expulsion from lungs.
Lungs can also be a means of removing some of the excess water which is formed
by the breakdown of nutrients during cellular respiration.
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The Water–Salt Balance
Lungs in land animals may give off water vapor as an excess product of metabolism.
In saltwater fish and some reptiles, water may be lost through gills or other body
tissues. In these last two situations, however, water is not an excess. Organisms lose
it through osmosis, as seawater has a higher salt concentration than body tissues. To
make up for losses of water, saltwater organisms must constantly be "drinking" water
to replace that which they lose. In doing so, they are taking in extra amounts of salt.
To prevent the loss of too much water from tissues, this salt must be continually
removed. Kidneys can remove some salt from the blood by active transport to form
very concentrated urine. Much of the salt is removed by another means. Special salt
glands in the gills of fish, near the nostrils of seabirds and near the eyes of crocodiles
and some turtles, return salt to the external surface. When some of the latter
organisms are on land, losses of the very salty fluids from the corners of the eyes give
them an appearance of "weeping".
Freshwater organisms face an opposite situation in that excess water moves into the
body through the gills. This situation develops as body tissues have a higher salt
concentration than the water. Freshwater organisms must excrete and egest extra
amounts of water. This is accomplished mainly by the kidneys, which absorb water
from the blood to form urine. Body salts are replaced largely by inward diffusion
through the gills. Some organisms can make adjustments as they go from one type of
environment to another.
Why do some salmon die after spawning?
Atlantic salmon can make spawning
runs into freshwater and then return to
continue life in the sea. On the other
hand, Pacific salmon have a body
physiology which cannot make a final
adjustment. A lot of energy is used up during spawning. These fish simply do not have
enough energy left to drive blood through the kidneys and to carry out active transport
to remove excess water. Not being able to restore and maintain a proper water and salt
balance in their tissues, these fish die soon after spawning.
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Lesson 6
Nitrogen Wastes
Organisms consume a lot of protein. More, it is said, than what is required. Unlike
other macromolecules, proteins cannot be stored in large amounts. The reason for
this is that protein molecules are made up of amino acids that contain nitrogen in
the form called the amino group (-NH2). This amino group containing the nitrogen
must be removed from the amino acid. This process is called deamination and
occurs in the liver.
The by-product from the deamination process is ammonia (NH3). Ammonia is toxic
and is also water soluble. In smaller aquatic organisms without transport systems,
ammonia can be diffused through and completely out of the bodies. Organisms with
gill structures can eliminate ammonia by diffusion from the blood capillaries running
through the filaments.
Terrestrial organisms have the nitrogen wastes eventually reaching the liver. The liver
works to get rid of the toxic ammonia. Two molecules of ammonia combine with
another waste product, carbon dioxide, to form urea - CO(NH2)2 and/or another
substance - uric acid – C5H4N4O3. (You don’t need to know these chemical formulas; they
are given for your information only).
Urea is quite toxic (only slightly less toxic than ammonia) and remains soluble. The
substance, uric acid is almost an insoluble crystal and is not very toxic in this form.
Each species has a different way of handling these nitrogen wastes.
Those species that convert the amino groups and ammonia into uric acid are:
 Insects form uric acid in their Malpighian tubules and intestines.
 Birds and reptiles convert some of the amino groups and ammonia into uric
acid. Uric acid crystals can move through the circulatory systems of birds and
reptiles until they reach the kidneys. The crystals are filtered out in the
kidneys and sent along tube-like ureters.
 Some reptiles have a small urinary bladder located just before the cloaca, to
collect uric acid and some fluids. The cloaca is a common chamber for
receiving and storing solid wastes from the intestine and uric acid or urine
from the kidneys. At certain times of the year, a cloaca may also be a
temporary storage area for eggs and sperm from reproductive organs.
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Lesson 6
 Other reptiles, as well as birds, lack urinary bladders and the uric acid is
transferred directly to the cloaca by the ureters. Uric acid mixes with the solid
wastes from the intestine and the mixture is eventually eliminated through the
rectum. The feces are darker-colored while the uric acid is in a white, paste-like
form. Before elimination, most of what little water needed to transport the uric
acid is reabsorbed from the cloaca. Transporting nitrogenous wastes as uric
acid requires far less water than needed for urine removal by amphibians and
especially mammals. Reptiles and birds need far less water and therefore do
not need to drink as much (possibly receiving all they need from food alone)
and do not necessarily need bladders or extensively developed kidneys. This is
particularly useful to birds in helping to reduce their body weight for flying.
Those species that convert the amino groups and ammonia into urea are:
 The livers of amphibians and mammals convert their nitrogen wastes into urea.
Remember urea is quite toxic and remains soluble. Its solubility means that a
fair amount of water is needed to move it through the excretory system. Urea is
transported in the blood to the kidneys. In the kidneys it is filtered and
processed to form urine. The urine then moves through ureters to a bladder. In
amphibians, a bladder empties into a cloaca. In mammals, a bladder has its
own separate opening out of the body through a tube called the urethra. When
the bladder is full, a muscular sphincter at its juncture with the urethra
relaxes and allows urine to flow to the outside as the bladder contracts.
The Functioning Kidney
In most vertebrates, kidney functioning is usually one of a number of different ways
in which wastes are removed from bodies. However, they are the most important and
any serious impairment could lead to death. The human kidney will be used to
illustrate the nature of this organ in its structure and functioning.
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Lesson 6
Kidney Structure
Adult human kidneys exist in pairs in the dorsal area of the lower back, just above
the hip area. Each fist-sized organ has a renal artery leading in and a renal vein
coming out. Also coming out of each is a tube-like ureter leading to the bladder.
Blood carries wastes
via the aorta
Filtered blood returns
to the heart via the
interior vena cava
Wastes are passed
to the kidneys
Urea is filtered from
the blood
Filtered blood leaves
the kidneys
Bladder
Ureter carries wastes
to the bladder
Urethra
The Human Excretory System
The internal structure of the organ shows three relatively distinct areas: an outer,
darker cortex; a lighter-colored medulla next; and, an inner chamber or cavity called
the pelvis.
to collecting
tube
cortex
glomerulus
medulla
Bowman’s
capsule
renal vein
renal
pelvis
renal
artery
collecting
tube
Loop of Henle
ureter
Kidney
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Nephron
104
to pelvis of
kidney
Lesson 6
The basic unit of a kidney is a nephron, of which there are about one million in each
organ. Arteries in a kidney divide into smaller branches. A small branch then leads
to, and becomes, a twisted, folded, clump of capillaries which make up a glomerulus.
A blood capillary leads out of this mass and then divides and re-divides around a
tubule, before finally combining into a vein. A collecting tubule forms a double-walled
chamber or Bowman's capsule around the glomerulus. The tubule straightens out to
form Henle's loop which extends into the medulla (note the characteristic U shape as
seen in the diagram on the previous page). The capillary masses or glomeruli and the
rest of the nephron are in the cortex, which accounts for its darker color. The loop of
Henle ends in another series of coils which joins with other tubules leading to the
pelvis.
Urine Formation
Three types of actions occur in a nephron unit:



filtration,
reabsorption, and
secretion.
Major Processes in the formation of urine in a nephron.
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Filtration
At the site of the glomerulus and Bowman's capsule, blood pressure (as a continuation
along the arterial system from the heartbeat) produces filtration. This first action
results in water and other molecules passing through capillary walls and into Bowman's
capsule. Only the larger proteins and red blood cells remain to continue their way along
the capillary. The initial fluid or urine in the first part of the collecting tube is really like
the intercellular fluid or lymph in other body areas. All human blood is completely
filtered about once every four minutes. This filtering results in anywhere from 160 to
190 litres of the total blood volume entering the tubules during the course of a day.
Resorption
Survival would not long be possible if a body eliminated or egested the fluids and other
matter at the same rates that they are filtered. Most of the initially filtered material is too
valuable to the body. Instead, the capillary network in which the collecting tubule (loop
of Henle) lies, carries out resorption (or, reabsorption) of most of the matter which
entered the tube. The length of the loop of Henle and the amount of the capillary
network surrounding it in a particular organism can be an indicator of how much
reabsorption occurs. These can also be indicators of where some organisms live. A short,
almost non-existent loop and a limited surrounding capillary network suggest that little
reabsorption of water is necessary or actually occurs. This is a common situation with
fresh water organisms, such as fish, which must remove a lot of water from their bodies.
Humans do have a fairly well-developed Henle's loop and surrounding capillary network.
In a human, approximately 80 to 85% of the water initially filtered out re-enters the
blood to maintain proper blood volume. Also reabsorbed, usually by active transport, are
glucose, amino acids, lipids, vitamins and mineral ions such as sodium, potassium and
calcium. Even some of the waste materials make their way back, although most of these
do eventually make up the final urine. Of the initial 160 to 190 L of filtrate, only 1 to 2
litres is eliminated each day.
For animals living in deserts, such as camels,
nature has provided for them to get the water
they need. The Loop of Henle and surrounding
capillaries allow them to remove more moisture
from the urine to be taken back into the body.
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Secretion
Secretion occurs in a nephron unit . In a way, this is a form of selective absorption. Cells
lining the collecting tubule have an ability to draw particular substances from the blood
and carry them across, or secrete them, into the waste fluid or urine in the tubule. The
secretions of particular ions control the acidity-alkalinity or pH levels of the blood.
Excess salts, vitamins and even foreign matter can be removed from the body by this
form of active transport.
The final composition of the urine eliminated from a body could be variable. A major
part of it, approximately 95%, is water. Urea, the slightly less toxic form of ammonia
combined with carbon, makes up about 2 to 3%. The remaining wastes consist of excess
mineral salts, blood pigments or other broken down cell parts and excess materials,
such as vitamins.
Regulation of Kidney Functioning
The last two actions of absorption and secretion, described in urine formation, are
important not only in waste elimination but also in body regulation or homeostasis.
Maintaining suitable levels of water, mineral ions, glucose and other substances in body
fluids can be just as critical to proper cell functioning as getting rid of wastes.
The hypothalamus area of the brain is normally sensitive to the state or condition of the
blood in the circulatory system. Detecting variations, it can stimulate or inhibit the
pituitary gland in its production of a certain antidiuretic hormone, or ADH. This
hormone, sometimes called vasopressin, increases the permeability of the cell walls in
the collecting tubules. This causes kidneys to reabsorb or retain more water in the
circulatory system (resulting in a lower volume and more concentrated urine). "Watery"
blood would result in less vasopressin being produced and less reabsorption into the
circulatory system. Tubule walls would be more impermeable to water. This results in a
higher volume of less concentrated urine.
Irregularities In Kidney Functioning
The homeostatic ability of kidneys could be upset in a number of ways. Defective
functionings of glands can affect the levels of a number of hormones which control
absorptions within the organs. Too little or no antidiuretic hormone or vasopressin can
bring on a form of diabetes. Failure by the kidneys to reabsorb much of the water and
glucose causes victims to eliminate large amounts of watery urine. Drinking of fresh
water offers no lasting relief to constant thirst.
Kidney stones may present difficulties for some people. Mineral salts or ions can begin
forming solid deposits around clumps of bacterial cells, bits of degenerated body tissues
or small blood clots. Once a stone is started, the process continues. Difficulties occur if
these stones should block kidney tubules leading into the pelvis or even lodge and block
the ureter itself. Chemicals or sound vibrations have been used in attempts to reduce
the size or to shatter the stones. Actual removal by some forms of operations are used
when initial attempts are unsuccessful.
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Injuries, poisonings or diseases may lead to severe or complete kidney failure. If one
kidney is affected, the other may enlarge somewhat and be able to do the work of both.
Failure of both kidneys may require the use of an artificial kidney machine or a dialysis
unit to purify the blood on a regular basis. An individual's blood circulates through the
unit so that waste molecules can pass from the blood through a membrane and into a
fluid medium. This fluid medium is carefully controlled so that it has the proper
concentrations of glucose, mineral and other molecules to prevent the loss of necessary
molecules from the blood. An alternative to dialysis treatments, which require long
periods of time, is a kidney transplant. However, immune rejection is a problem when
tissue types do not match up closely.
The Skin
Some of the same materials removed by the kidneys from the blood, are also eliminated
by sweat glands in the skin. Water, salts and urea are absorbed from nearby capillaries
by coiled sweat glands in the dermis. These wastes are transported through a duct to
the skin surface and then eliminated through a pore. An accumulation of wastes on the
skin surface could result in corresponding increases of certain kinds of bacteria which
use them as nutrients. The decomposing actions of these bacteria on the "nutrients"
lead to body odor.
Sweat glands carry out another important function in some organisms. Evaporation of
any liquid requires energy to change it to vapor form. Sweat, evaporating from a skin
surface, derives heat energy from the skin itself. This produces a cooling effect on the
body. The highest concentration of sweat glands in humans occurs on the hands,
followed by the feet, forehead and then other areas of the body.
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Summary
Removal of metabolic wastes or substances which may be in excess amounts in cells or
bodies is just one of a number of important actions which are part of homeostasis. In
smaller, less complex organisms, removal of wastes from cells may occur by diffusion or
osmosis to a nearby external environment. The same processes occur in plants,
although in larger, multicellular varieties, wastes are usually left in vacuoles or moved
only short distances to dead cells where they are deposited.
Animals begin to show a variety of excretory systems adapted to body complexities and
life styles. These include flame cells, nephridia, Malpighian tubules and nephrons (in
kidneys). As complexities in bodies increase, organisms may rely on more than one
organ or method of removing wastes. In mammals, the lungs, sweat glands, kidneys and
the liver all have some part in the general process, in addition to the removal of solid
wastes from the digestive tract or large intestine.
To regard excretory systems only as adaptations in removing wastes from cells or bodies
would be omitting some of their other important roles. Some "wastes" can be regarded as
substances or byproducts having no value, or even being harmful, to cells or tissues. In
other instances, certain substances could be wastes at one particular time and nutrients
at another. Excretory systems become especially important in their abilities not only to
eliminate, but at other times to retain certain matter. This becomes even more complex
if one realizes that some of the adjustments occur within very precise or narrow limits.
Excretory systems make these adjustments as they maintain certain water-salt levels in
tissue fluids, mineral ion concentrations and even help to maintain body temperatures
at certain levels such as our own sweat glands do. Any abnormalities or disruptions to
excretory systems, which in turn lead to abnormalities in concentration levels or
conditions within bodies, could have just as serious consequences as defects in any
other systems.
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