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Excretion
Excretion is the removal of metabolic waste from the body, that is, the
removal from the body of by-products or unwanted substances from
normal cell processes.
Metabolic waste consists of substances
that may be toxic or that are produced
in excess by the reactions inside cells.
There are 2 substances produced in
large amounts: carbon dioxide from
respiration and nitrogen containing
compounds such as urea.
Carbon dioxide is transported in
the blood stream to the lungs
where it diffuses into the alveoli to
be excreted as we exhale.
Carbon dioxide has harmful effects on the body:
1. The majority of carbon dioxide is carried in the blood as
hydrogencarbonate ions. In the erythrocytes, under the
Respiratory Acidosis is the result of a large
influence of the enzyme carbonic anhydrase, the hydrogen
change in the pH of the blood. Proteins in
carbonate dissociates to form H+ ions. The hydrogen ions
the blood act as buffers to resist the
combine with haemoglobin and compete with oxygen for
change in pH. If the change in pH is small,
space on the haemoglobin. So, if there is too much carbon
extra hydrogen ions are detected by the
dioxide, it can reduce oxygen transport in the blood.
respiratory centre in the medulla
oblongata of the brain. This causes an
2. Carbon dioxide combines directly with the haemoglobin to
increase in breathing rate to help to
form carbaminohaemoglobin. This molecule has a lower
remove the excess carbon dioxide. If the
affinity for oxygen than haemoglobin.
pH drops below 7.35, it results in slowed or
3. Excess CO2 can cause respiratory acidosis. The carbon
difficult breathing, headache, drowsiness,
dioxide dissolves directly into the blood plasma. It combines
restlessness, tremor and confusion. There
with water to produce carbonic acid: CO2 + H2O  H2CO3.
may also be rapid heart rate and changes in
The carbonic acid dissociates: H2CO3  H+ + HCO3- . The H+
blood pressure.
lower the pH of the blood, making it more acidic.
The Liver
Hepatic vein: Blood leaves the liver
and re-joins the Vena Cava and blood
returns to normal circulation.
The bile duct
carries bile from
the liver to the
gall bladder
where it is stored
until required to
help with the
digestion of fats in
the small
intestine. Bile
travels in the
canaliculi.
Hepatic Artery:
oxygenated blood from
the heart travels from
the aorta via the
hepatic artery into the
liver.
Hepatic Portal
Vein:
deoxygenated
blood from the
digestive system
enters the liver via
the hepatic portal
vein. The blood is
rich in products of
digestion.
Histology of the
Liver
The sinusoid is lined by
liver cells. As blood flows
along the sinusoid, it is
in very close contact
with the liver cells which
can exchange
substances with the
blood. The sinusoids
then empty into the
intra-lobule vessel, a
branch of the hepatic
vein. The hepatic veins
from different lobules
join together to form the
hepatic vein which
drains the blood from
the liver.
The cells, blood vessels and chambers
inside of the liver are arranged to
ensure the best possible contact
between the blood and the liver cells.
The hepatic artery
(bringing blood from the
aorta) and the hepatic
portal vein (bringing
deoxygenated blood rich
in products of digestion
from the intestine) enter
the liver and split into
smaller and smaller
vessels. These vessels
run between and parallel
to the lobules – known
as interlobular vessels.
Branches from the
hepatic artery and
hepatic portal vein enter
the lobule and the blood
is mixed in a chamber
called a sinusoid.
The sinusoids are lined with
specialised macrophages known as
Kupffer Cells. They move about
within the sinusoids and are
involved in the breakdown and
recycling of old red blood cells.
One of the products of
haemoglobin breakdown
is bilirubin, which is
excreted as part of the
bile and in faeces.
The liver is very metabolically active.
Its functions include:
•Control of blood glucose levels,
amino acid levels, lipid levels
•Synthesis of red blood cells in the
fetus, bile, plasma proteins and
cholesterol
•Storage of vitamins A, D, B12, iron
and glycogen
•Detoxification of alcohol and drugs
•Breakdown of hormones
•Destruction of red blood cells
Dealing with Proteins…
Functions of
the Liver
Excess amino acids cannot be stored
because the amine group makes them
toxic. It would however be a waste to
excrete the whole molecule, as they
contain lots of energy. It therefore
undergoes ‘treatment’ in the liver before
the amino acid component is excreted.
Amino
Acid
Ammonia
+ Keto Acid
Urea
Dealing with Proteins: Deamination
Deamination
produces
ammonia which
is very soluble
and highly toxic.
Keto
acid
The organic
compound that
remains is a keto
acid. This can be
metabolised – used
in respiration to
release its energy.
NH3
Ornithine Cycle
Ammonia must be
converted to a less toxic
substance quickly. The
ammonia is combined with
carbon dioxide to produce
urea – this occurs in the
Ornithine cycle.
2NH3 +
CO2
Urea is less soluble and
less toxic than
ammonia. It can be
passed into the blood
stream transported to the
kidneys. In the kidneys, the
urea is filtered out of the
blood and concentrated in
the urine.
CO(NH2)2
+
H2O
Detoxification
is the conversion of toxic molecules to
less toxic or non toxic molecules.
Alcohol is broken down by hepatocytes. Ethanol is dehydrogenated to ethanal by
the enzymes ethanol dehydrogenase. The ethanal is further dehydrogenated by
ethanal dehydrogenase to form acetate.
Ethanol
Reduced NAD
NAD
Ethanol dehydrogenase
Ethanal (acetaldehyde)
Reduced NAD
NAD
Ethanal dehydrogenase
Ethanoate (acetate)
Enters Krebs Cycle to make ATP
The coenzyme NAD accepts the hydrogens produced in this reaction. If
the liver has got too much alcohol to detoxify, it has insufficient NAD
to deal with the fatty acids. These fatty acids are then converted back
to lipids and stored in the hepatocytes, causing the liver to become
enlarged. This is a condition called ‘fatty liver’ which can lead to
alcohol related hepatitis or to cirrhosis.
Combines with
Coenzyme A to
form acetyl CoA
which enters
respiration.
The Kidney
The bulk of each kidney consists of tiny
tubules called nephrons. There are about 1
million nephrons in each kidney which are
closely associated with blood capillaries.
Each nephron starts in the cortex. In the cortex, the
capillaries form a knot called the glomerulus. This is
surrounded by a cup shaped structure called the
Bowman’s Capsule. Fluid from the blood is pushed
into the bowman’s capsule by ultrafiltration.
The capsule leads into the
nephron which is then
divided into 4 parts: proximal
convoluted tubule, loop of
Henle, distal convoluted
tubule, collecting duct.
Formation of Urine
1.
Ultrafiltration
in the renal
capsule
[Hydrostatic
pressure in the
glomerulus]
3. Loop of
Henle:
Water reabsorption
[Descending limb
permeable to H2O;
Ascending limb
permeable to
Na+/Cl-]
2. Proximal
Convoluted
Tubule:
Selective
Reabsorption
4. Collecting
Duct:
Osmoregulation
Bowman’s Capsule: Ultrafiltration
Fluid from the glomerulus is forced into the bowman’s
capsule by hydrostatic pressure. The fluid is known as
the glomerular filtrate.
The blood arrives at the glomerulus through a wide
afferent arteriole and leaves through a narrow efferent
arteriole. The difference in the diameters ensures that
the blood pressure in the glomerulus is higher than in
the bowman’s capsule. The pressure difference pushes
fluid from the blood into the bowman’s capsule that
surrounds the glomerulus.
There is a 3 1. Endothelium of the capillaries have narrow gaps between the Squamous
epithelial cells, through which blood plasma and the substances dissolved in
layered
it can pass.
barrier that
exists
2. Basement membrane consists of a fine mesh of collagen fibres and
between
glycoproteins. These act as a filter to prevent the passage of molecules with
the blood in
a molecular mass greater than 69000 – this means that most proteins and all
the capillary
blood cells are held in the capillaries of the glomerulus.
and the
3. The epithelial cells in the bowman’s capsule – called podocytes – have a very
lumen of
specialised shape. They have many finger-like projections called major
the
processes. These ensure that there are gaps between cells. Fluid from the
bowman’s
blood in the glomerulus can pass between those cells into the lumen of the
capsule.
Bowman’s Capsule.
Selective
Reabsorption
2. Sodium ions are actively transported out of the cells
lining the proximal convoluted tubule. This lowers the
concentration of Na+ inside the cells.
4. As glucose and amino acid concentrations
build up inside the cell, these substances are
able to diffuse out of the opposite side into the
tissue fluid. From the tissue fluid, these
substances diffuse into the blood and are
carried away.
1. Glomerular Filtrate containing
water, sodium, amino acids and
glucose.
3. Sodium ions,
glucose and amino
acids are
transported down
the concentration
gradient from the
glomerular filtrate
into the cells by
facilitated
diffusion.
5. The reabsorption of salts, glucose
and amino acids reduces the water
potential inside the cells and increases
the water potential of the tubule fluid.
Water will therefore enter the cells and
then be reabsorbed into the blood by
osmosis.
Water reabsorption in the Loop of Henle
2. In the descending
limb, the fluid from the
proximal convoluted
tubule passes through
tissue into which Na+
and Cl- ions have been
pumped. There is
therefore a water
potential gradient, and
water will move from
the descending limb
into the tissue by
osmosis.
This arrangement
is known as a
hairpin counter
current multiplier
system.
1. In the upper part of the
ascending limb, sodium
and chloride ions are
actively transported out of
the nephron and into the
surrounding tissues. This
increases the water
potential of the fluid inside
the nephron and decreases
the water potential outside
of it.
3. When the fluid gets to
the base of the
descending limb, it is very
concentrated – the
concentration of the ions
is very large. As it goes
around to the ascending
limb, the Na+ and Cl- ions
diffuse out of the tubule.
By the time that the fluid reaches the top of the ascending limb in
the cortex, its water potential becomes higher. This is because at the
base of the tubule, Na+ and Cl- have diffused out of the tubule;
higher up, the sodium and chloride ions are being actively pumped
out of the tubule; water cannot leave (as the ascending limb is
impermeable to water) and so the fluid is losing salts but not water.
The longer the loop of
Henle the greater the
concentration of solutes
that can be built up at the
bottom of the loop. The
very low water potential in
the medulla helps water to
be conserved and not lost
in urine – the water
potential gets lower as you
go deeper into the medulla.
If the tissue surrounding
the descending limb has a
very high solute
concentration, it will have
a low water potential.
Water will therefore move
out of the descending
limb and into the
capillaries by osmosis.
This is advantageous for animals that live in
arid conditions, like the desert kangaroo rat,
because it minimises the amount of water lost
in urine.
Explaining Concentration Changes …
Glucose: a large drop in concentration as all of
the glucose is reabsorbed in the proximal
convoluted tubule.
Urea: concentration of urea increases as
water is reabsorbed.
Sodium: in the proximal convoluted tube,
what little reabsorption there is is balanced
by that of water. The concentration increases
in the descending limb as water is lost. The
concentration decreases in the ascending
limb as Na+ ions are actively pumped out. In
the distal convoluted tubule and collecting
duct, concentration increases as water is lost.
Potassium: the same as for sodium, except
in distal convoluted tubule, K+ ions are
actively transported in, increasing the
concentration.
Osmoregulation
The control of the water potential of the blood
and body fluids.
Water is gained by food, drink and respiration. Water is lost in
urine, sweat, faeces and water vapour in exhaled air.
When less water needs to be conserved, walls of the collecting
duct are less permeable: less water is reabsorbed so more
urine is produced.
When more water needs to be conserved, collecting duct wall
become more permeable. More water is reabsorbed and so
less urine is produced.
If it is a cool day and you
have drunk lots of fluid,
you will produce large
volumes of dilute urine. If
it is a hot day and you
have drunk little, you will
produce small volumes of
concentrated urine.
 The walls of the collecting duct respond to anti-diuretic hormone
 Cells in the walls of the collecting duct have membrane bound receptors for ADH.
 The ADH binds to these receptors and causes a chain of enzyme controlled reactions to occur
inside the cell.
 Vesicles containing water permeable channels, aquaporins, fuse with the membranes of the
collecting duct cells.
 This makes the collecting duct walls more permeable to water.
 If more ADH is released, the walls become more permeable, so more water is reabsorbed by
osmosis into the blood. Less urine, with a lower water potential, passes out of the body.
 If less ADH is released, the cell surface membrane folds inwards to create new vesicles that
remove water permeable channels from the membrane. The walls are made less permeable,
less water is reabsorbed and more water with a higher water potential passes out of the body.
Controlling ADH levels
 The water potential of the blood is monitored by osmoreceptors ion the hypothalamus of the brain.
 When the water potential of the blood is low, osmoreceptors lose water by osmosis. This causes them to
shrink and stimulates neurosecretory cells in the hypothalamus.
 Neurosecretory cells are specialised neurones that produce and release ADH. The ADH is manufactured
in the cell body of the cells, which lies in the hypothalamus.
 ADH flows down the axon to the terminal bulb in the posterior pituitary gland. It is sored in the posterior
pituitary gland until needed.
 When the neurosecretory cells are stimulated, they send action potentials down their axons causing the
release of ADH into the blood capillaries running through the posterior pituitary gland.
 It travels in the blood stream around the body and acts on the collecting duct wall cells – these are its
target cells.
 Once the water potential of the blood rises again, less ADH is released.
Increase in
water
potential of
blood
Detected by
osmoreceptors
in
hypothalamus
Decrease in
ADH output
from posterior
pituitary
Permeability of
collecting duct
decreases
Less water
reabsorbed
into
bloodstream
Increased
volume of
dilute urine
produced
Reduced
volume of
concentrated
urine produced
More water
reabsorbed
into
bloodstream
Permeability of
collecting duct
increases
Increase in
ADH output
from posterior
pituitary
Detected by
osmoreceptors
in
hypothalamus
Decrease in
water potential
of blood
Kidney Failure
The most common causes of kidney failure are
diabetes mellitus, hypertension and infection.
Dialysis is a process in which blood passes over a partially
permeable dialysis membrane. This allows the exchange
of substances between blood and dialysis fluid. The fluid
contains the correct concentrations of salts, urea, water
and other substances in blood plasma. Substances in
excess in the blood will diffuse across the membrane and
into the dialysis fluid.
Haemodialysis: blood form a vein is passed into a machine that
contains an artificial dialysis membrane.
Dialysis fluid on the other side of the membrane passes in the
opposite direction: the fluid has the water potential and contains
the concentrations of ions and glucose that the blood should
contain if the kidneys were working properly.
Heparin is added to avoid clotting and any bubbles are removed
before returning to the body.
Peritoneal Dialysis: a surgeon implants a catheter into the patient’s
abdomen. The peritoneum is the layer of tissue that lines the abdominal
cavity. The cavity is filled with dialysis fluid, left there for some time and
then drained off.
Kidney Transplants: kidney donated by a healthy person who is a
close tissue match or a dead donor.
Treatment
Advantages
Disadvantages
Haemodialysis
More efficient at removing substances
Several hours several times a week;
between treatments, diet must be
managed carefully
Peritoneal
Dialysis
Frees patient from immovable dialysis
Higher risk of infection; has to be done
machine; continuous process so no large more often
swings in blood volume or content
Transplant
Best life extending treatment; freedom
from dialysis; better quality of life
Testing Urine Samples
Pregnancy Testing
Once a foetus is implanted, it releases human
chorionic gonadotrophin. This hormone
passes through the mother’s nephrons and
into the urine. Pregnancy tests use
monoclonal antibodies which are tagged with
a blue head marker. The antibodies will bind
with the HCG and this will form a blue line if
the woman is pregnant.
Need immunosuppressant drugs for life;
major surgery under general anaesthetic;
risk of infection and bleeding; side effects
of anti-rejection drugs.
Anabolic Steroid Testing
Gas chromatography or mass spectrometry is
used. Gas chromatography involves
vaporising the sample in the present of a
solvent. It is then passed down a long tube
lined by an absorption agent. Each substance
stays in the tube for a specific amount of
time – the retention time. Once all of the
substances have come out of the gas and are
absorbed onto the lining, this is analysed to
create a chromatogram which can be
compared to known standard samples.