E.F. ACADEMY
OCR Biology A2
F214 Communication, Homeostasis and Energy
4.2 Excretion
JWh
8/8/2014
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4.2.1 Define the term excretion;
The process of excretion is the removal of waste from metabolic
reactions from the body.

As this is the unwanted waste products of metabolic reactions they
must have been formed within the cell.

Excretion must not be confused with egestion, which is the removal of
undigested food or chemicals which have never been part of a cellular
reaction (e.g. faeces).

The main two compounds which are produced are carbon dioxide and urea
Explain the importance of removing metabolic wastes, including carbon dioxide
and nitrogenous waste, from the body;

The need to excrete carbon dioxide is shown by looking at a condition
associated with high blood CO2 concentration.

CO2 is an acid gas and build- up of H+ decreases the pH of the body
causing Respiratory Acidosis.

There are three methods of transporting carbon dioxide around the body
for removal, outlined below:
1. As Hydrogen Carbonate ions – a process which also produces H+
within Red Blood Cells.
2. Carried around directly dissolved in the blood in solution.
3. Combining
directly
with
Haemoglobin
to
form
Carbaminohaemoglobin

Haemoglobin has a higher affinity to carbon dioxide than it does for
oxygen, so formation of carbaminohaemoglobin reduces the efficiency of
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oxygen carriage around the blood.

When one haem group has taken up a molecule of carbon dioxide it
cannot take on a molecule of oxygen.

Hydrogen ions, which are produced alongside hydrogen carbonate ions
alter the blood pH level too, which means that enzyme action is slowed.

Failure to excrete carbon dioxide from the blood appropriately will result
in respiratory acidosis.

Symptoms of respiratory acidosis includes: an increased urge to breathe,
or hyperventilation.

If this occurs for a prolonged period, there can be permanent damage to
organs, in particular the brain

Urea is produced by oxidising excess amino acids in the liver in a process
called deamination.
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
The ammonium ions produced are combined with CO2 in the Ornithene
Cycle to produce urea.

The urea which is produced there is then transported, to the kidneys,
where it will be excreted through urine.

The urine is produced in the kidneys and temporarily stored in the
bladder.
4
Describe, with the aid of diagrams and photographs, the histology and gross
structure of the liver;

Hepatocytes (liver cells) carry out many metabolic functions.

It is essential that the organ has a rich blood supply to as many cells as
possible, and the liver has many secure connections to the blood system.

The liver receives oxygenated blood from the hepatic artery and
deoxygenated blood from the digestive system through the hepatic portal
vein.

This blood supplied is rich in the products of digestion, and may contain
toxic substances that have been absorbed in the intestine.

Blood leaves the liver through the hepatic vein which rejoins the vena
cava and returns to systemic circulation.

There is also the bile duct leaving the liver, where bile is secreted.

This has both digestive and excretory functions.

The bile duct runs from the liver to the gall bladder, where bile is
stored until needed for digestion.
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
The arrangement of cells within the liver has been adapted so that each
cell has a rich blood supply from adjacent blood vessels.

The liver is divided into lobes which are further divided into small
lobules.

The vessels of the hepatic portal vein and hepatic artery, both of which
supply the liver with blood, split into smaller vessels, and run alongside
other vessels called inter-lobular vessels

The Hepatic Artery, Hepatic Portal Vein and Bile duct form a TRIAD
around each lobule.
6
Describe the formation of urea in the liver, including an outline of the ornithine
cycle;

The body needs proteins to function

Excess amino acids from proteins which we do not need cannot be
stored, because their amine groups make them toxic.

The molecule is modified under two processes so that the amine
component can be excreted alone.
Deamination

This is the removal of the amine group from an amino acid.

Deamination produces the compound ammonia, which is very toxic and so
must not be allowed to accumulate.

Another by-product of the process is keto acid, an organic compound
which directly enters the mitochondrion and is respired aerobically.
The ornithine cycle.

Ammonia is very soluble and very toxic, so it needs to be converted into
a less harmful substance.

The ammonia is combined with carbon dioxide to produce urea in the
ornithine cycle.
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8
Describe the roles of the liver in detoxification;
Detoxification of alcohol

The liver is able to detoxify many harmful substances

Alcohol and drugs can be detoxified in the liver.

Hepatocytes contain enzymes which have detoxification functions, such as
catalase, which converts hydrogen peroxide (very toxic) into water and
oxygen.

Alcohol (ethanol) is broken down in the liver using the enzyme ethanol
dehydrogenase to produce ethanal, which is broken down using ethanal
dehydrogenase into ethanoic acid.

This end product combines with coenzyme-A to produce acetyl coenzymeA which can be directly respired

The hydrogen atoms released in this reaction can also combine with a
molecule of NAD to produce reduced NAD.
9
Describe, with the aid of diagrams and photographs, the histology and gross
structure of the kidney;
10
The human body has two kidneys.
The urinary system consists of four major parts:
1. the two kidneys
2. the ureters which transport urine from the kidney to the bladder
3. the bladder temporarily stores urine
4. the urethra to take urine to the outside world

There are approximately one million tubules within a kidney called
nephrons.

A nephron exists through each section of the kidney: the outer cortex,
the medulla in the middle, and the innermost pelvis
The nephron A single nephron consists of the:
a. Bowman’s Capsule in which there is a network of capillaries called the
Glomerulus - ultrafiltration
b. Proximal (first) Convoluted Tubule which runs from the capsule to the
loop of Henle – absorption of useful material.
c. Loop of Henle, creates the conditions for water reabsorption.
d. Distal Convoluted Tubule – ion exchange.
e. Collecting Duct which connects to many nephrons and ultimately ending
up in the pelvis region – water absorption.
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Describe, with the aid of diagrams and photographs, the detailed structure of a
nephron and its associated blood vessels;
Bowman’s Capsule – as blood enters the Bowman’s Capsule, under high
pressure, it passes through tree layers as it moves from the blood vessel in
the Glomerulus to the Nephron proper.
Endothelial cells

The endothelial cells lining the glomerular capillaries are thin and flat
with a large nucleus.

The cells contain pores, which allow plasma components to cross the
vessel wall, but not blood cells, platelets or large proteins.
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Basement membrane

The basement membrane is a continuous layer of connective tissue and
glycoproteins.

It is a non-cellular structure that prevents any large molecules from
being filtered.

It is the only real barrier to molecules being forced out of the blood
Epithelial lining

The epithelial lining of Bowman’s capsule consists of a single layer of
cells; podocytes, which rest on the basement membrane.

The podocytes have large extensions which extend out from the cell body
and are embedded in the basement membrane surrounding a capillary.

This leads to the formation of slit pores, which control the movement
of substances through the final layer of the filter.

The podocytes have a well-developed Golgi apparatus, used to produce
and maintain the glomerular basement membrane.
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Describe and explain the production of urine, with reference to the processes of
ultrafiltration and selective reabsorption;
Ultrafiltration

The kidney is concerned with filtering out waste products from the
blood plasma, which are to be excreted in the urine.

It is important that useful substances are kept.

All molecules from the blood plasma below a certain size are passed
through the filtration membrane, and those materials which need to be
kept are selectively reabsorbed later on.

The afferent arteriole which supplies the glomerulus has a much larger
width than the efferent arteriole leading away from the glomerulus.

This means that there is an increased pressure inside the glomerular
capillaries than in most capillaries.

This forces fluid from the blood through from the blood capillaries into
the renal capsule.

This process is called ultrafiltration.

The pressure forces molecules within the blood with a relative molecular
mass (RMM) of below 69,000 through the glomerular capillaries and
into the Bowman’s capsule.

This pressure is called filtration pressure.

Molecules of water, glucose, urea, amino acids and ions are all removed
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from the blood plasma and filtered into the nephron.

Proteins are too
big to fit through
the gaps in the basement
membrane, and so remain in the blood. Similarly, all blood cells do not
enter the glomerular filtrate
Selective reabsorption

The filtrate which results in the Bowman’s capsule flows along the
nephron, and it is at the proximal convoluted tubule that the majority
of substances are reabsorbed.

This process is known as selective reabsorption.

All of the glucose and amino acids are reabsorbed back into the blood.

80% of the Sodium ions (and Cl-), as well as 80% of the water are
reaborbed.
Selective reabsorption occurs in a several staged process:
1. On the cell surface membrane closest to the blood capillary, there are
sodium-potassium pumps which actively transport sodium ions out of the
cell.
2. As sodium ions are pumped out of the cell, the concentration of sodium
ions inside the cell drops
3. This allows the transport of sodium ions back into the cell from the
surface membrane in contact with the proximal convoluted tubule – the
ions flow in through co-transporter proteins which couple the movement
of sodium ions into the cell with the movement of glucose and amino
acids (this is facilitated diffusion)
4. As sodium moves into the cell, taking glucose and amino acids in with it,
the concentration of glucose and amino acids within the cell lining the
convoluted tubule increases, and so these substances are pumped out of
the cell and into the blood capillary via active transport, using carrier
proteins.
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5. The movement of ions, glucose and amino acids from the convoluted
tubule into the cells leads to an increased water potential (Ψ) inside the
convoluted tubule, and so water moves, by osmosis, into the cells lining
the tubule and so into the blood capillary
6. The cells lining the proximal tubule have many microvilli on their surface
to increase the surface area for absorption, contain many mitochondria
and carrier proteins.

It is the role of the loop of Henle to establish a low water potential, to
ensure that even more of the remaining water can be reabsorbed from
the fluid in the collecting duct.
The loop of Henle consists of two sections:

The descending limb and the ascending limb.

The way that the loop of Henle is arranged allows sodium and chloride
ions to be pumped out of the ascending limb and into the descending
limb.

This creates a high concentration gradient of salts (the sodium and
chloride ions) in the tubule fluid (the fluid inside the loop of Henle),
which causes the salts to move out of the ascending limb and into the
surrounding tissue, of the medulla, which of course decreases the water
potential.

The descending limb is the only limb to not actively pump sodium and
chloride ions into the medullary fluid.
1. The descending limb is highly permeable to water, and so water is lost
to the surrounding medullary fluid by osmosis as the fluid flows down
the limb.
2. The water which is lost is reabsorbed back into the blood by surrounding
capillaries
3. This descending limb does not actively lose any sodium or chloride ions
4. At the bottom section and the ascending limb, Chloride ions (followed
by Sodium ions) are actively pumped out of the limb into the medullary
interstitial fluid, lowering the solute concentration of the filtrate and
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adding to the solute concentration of the surrounding fluid
5. Near the top of the ascending limb, more sodium chloride is actively
transported out into the medullary fluid.
6. The walls of the ascending limb are impermeable to water.
7. Some of these salts may diffuse back into the top of the descending
limb, as this is where the concentration of salts is highest in the
medullary fluid
8. As the tubule fluid moves along the loop of Henle, the transport of
sodium chloride out of the ascending limb, coupled with the osmosis of
water out of the descending limb, establishes an osmotic concentration
in the medullary interstitial fluid which becomes increasingly hypertonic
as the loop drops in towards the medulla.
A Counter Current Multiplier System has been set up.
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19
Explain, using water potential terminology, the control of the water content of
the blood, with reference to the roles of the kidney, osmoreceptors in the
hypothalamus and the posterior pituitary gland;
ADH and the brain

There is a water potential gradient down the collecting duct to the
medullary tissue of the kidney.

This causes water to move out of the collecting duct by osmosis.

Water reabsorption in the collecting duct like this is controlled by the
levels of ADH (antidiuretic hormone) in the blood vessels in contact
with the walls of the collecting duct.

ADH is a hormone which in larger amounts will cause molecules called
Aquaporins to bind to the cell membrane of the collecting duct.

Water passes through the aquaporins more easily and so more water is
absorbed, and so urine is more concentrated and less urine is released.

The water potential of the blood is detected by sensory cells called
osmoreceptors in a region of the brain called the hypothalamus.

Normal osmosis rules apply to these cells, as in when the water
potential of the blood is low, water will move out of osmoreceptors by
osmosis causing the cells to shrivel – and so more ADH is released .
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
When the osmoreceptors shrink due to a lower water potential, they
stimulate neurosecretory cells in the hypothalamus to release ADH.

When ADH is released it travels down the axon of the cell in which it is
made and is stored in the posterior pituitary gland until needed.

When the neurosecretory cells have been stimulated, the ADH is released
into the bloodstream by the posterior pituitary gland
Renal capsule
(glomerular
filtrate)
glucose
Proximal
convoluted
tubule


amino acids
proteins
water
urea
ions
Loop of Henle



Distal
convoluted
tubule
reabsorbed
Collecting duct
not present in filtrate
reabsorbed
(only small proteins)
not present in filtrate
not present in filtrate
60%
reabsorbed
much
reabsorption
more
reabsorbed
possible
further
reabsorption




partially
reabsorbed
Urine
mostly
reabsorbed
final
reabsorption
(small
amounts)

not present in
filtrate
21
Outline the problems that arise from kidney failure and discuss the use of renal
dialysis and transplants for the treatment of kidney failure

Kidney failure does not arise from one sole cause.

The most common factors which can cause kidney failure are diabetes,
hypertension and infection.

When kidneys fail, the body is left unable to remove waste substances
and excess water from the blood, which of course includes urea, which is
harmful.

This will ultimately lead to death, and will not take too long.
Treatment with dialysis

The most common form of treatment for someone with kidney failure is
dialysis, which performs the same function as a kidney by passing the
blood over a dialysis membrane to remove excess fluids, wastes and salts.

The membrane keeps the blood and dialysis fluid separate, but allows for
exchange of materials between them.

The dialysis fluid contains optimal concentrations of substances, and
diffusion
across
concentrations
in
the
artificial
the
blood:
membrane
anything
in
restores
excess
flows
the
correct
across
the
membrane to the fluid, and anything in the blood with too low a
concentration flows from the fluid to the blood over the membrane.

Haemodialysis is the most common form where blood from a vein is
passed over a dialysis membrane in an external machine.
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
The polysaccharide heparin is added to prevent clotting of the blood, and
all bubbles are removed before the blood is returned to the body.

Haemodialysis is a big burden on someone’s life and prevents them from
doing many things they would otherwise be able to do.

It must be undergone in a clinic at least three times a week, although
many patients buy their own machines and learn how to perform it at
home.
In peritoneal dialysis, the dialysis is done internally, not via an external
machine.

A permanent catheter is inserted into the abdomen.

In this form of dialysis the filter is the patient’s own abdominal
membrane, the peritoneum.

Dialysis fluid is inserted through the catheter into the space between
the abdominal wall and surrounding organs.

The same substance exchange as with haemodialysis occurs here.

After a couple of hours, the fluid is drained from the abdomen, and this
process must be performed in several consecutive sessions daily.

An advantage of peritoneal dialysis is that it has less of an impact on
the patient’s life, as they are able to walk around whilst the exchange
occurs.

However, this must begin with surgery in order to permanently implant
the catheter into the abdomen
23
Describe how urine samples can be used to test for pregnancy and detect misuse
of anabolic steroids

Pregnancy testing A women’s urine can be used to test for pregnancy,
using commercially prepared testing strips.

The number of coloured bands which appear on a strip are used to
categorise as either pregnant or not pregnant.

It is the hormone human chorionic gonadotrophin (hCG) which is pivotal
here. hCG is produced by the developing placenta, and has important
roles during pregnancy.

However, some of it is lost to the urine, which is why it can be
detected. Women who are not pregnant do not product hCG.

An hCG test strip can detect even low levels of the hormone when
present in the urine.

Urine is drawn along the testing strip across three ‘bands’.

These represent three zones, which are often known as the reaction
zone, the testing zone and the control zone
1. The reaction zone contains a type of antibody which recognises a single
molecular feature of the hCG molecule. These are called monoclonal
antibodies and only will bind to hCG. These antibodies have blue beads
attached to them, which always remain on the antibodies. The blue
beads will later become visible, which will give an indication of pregnancy
or no pregnancy
2. The
testing
zone
contains
a
group
of
antibodies
called
polyclonal
antibodies which can bind to numerous molecular parts of hCG.
3. The control zone contains antibodies which can recognise and bind to the
antibodies which are currently in the reaction zone

By this method, when there is hCG present in the urine (i.e. in
pregnant women), there will be two blue strips.

However, when the woman is not pregnant there is no hCG so the
monoclonal antibodies will not attach to the hormone and therefore will
not bind to the testing zone, so there is only one blue strip (the
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control zone at the end)
Testing for anabolic steroids

Urinary testing can also be used to test for anabolic steroids which can
be used to enhance physical performance.

This is done using gaseous chromatography.

This is a method which separates different gases, so that various
substances can be identified within the urine sample.
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