OCR Biology A2 Liver & Excretion

2. This is the removal of metabolic waste from the body
Main substances:
Product: Carbon dioxide from respiration
Where it is produced: This is produced by all living cells in the body as a result of
respiration.
Where it is excreted: CO2 is passed from the cells of respiring tissues into the
bloodstream. It is transported in the blood (mostly in the form of hydrogen carbonate
ions) to the lungs. In the lungs the carbon dioxide diffuses into the alveoli to be excreted
as we breath out.
Product: Nitrogen containing compounds (Urea)
Where it is produced: Urea is produced in the liver from the break down of excess amino
acids. This process is called deamination.
Where it is excreted: The urea is passed into the bloodstream to be transported to the
kidneys. It is transported in solution (dissolved in plasma). In the kidneys the urea is
removed from the blood to become part of urine. Urine is stored in the bladder before
being excreted via the urethra.

3. Carbon Dioxide:
Excess carbon dioxide is toxic. A high level of carbon dioxide has three main effects:
1. Carbon dioxide is carried in the blood as hyrogencarbonate ions. This leads to the formation hydrogen ions. This occurs
inside red blood cells, under the influence of the enzyme carbonic anhydrase. Hydrogen ions combine with haemoglobin.
They compete with oxygen for space on the haemoglobin if there is too much carbon dioxide(too many H+) in the blood it
can reduce oxygen transport.
HCO3-
Carbonic Anhydrase
HCO3-
H+
HCO3-
H+
H+
2. The carbon dioxide also combines directly with haemoglobin to form
carbaminohaemoglobin. This molecule has a lower affinity for oxygen than normal
haemoglobin.
H+
H+
H+
Haemoglobin
Haemoglobin
H+
H+
H+
H+
O2
H+
O2
O2
H+
H+

4. 3. Excess carbon dioxide can also cause respiratory acidosis. Carbon dioxide dissolves in the blood plasma. Once
dissolved it can combine with water to produce carbonic acid:
CO2 + H2O  H2CO3
H20
H20
CO2
CO2
CO2
CO2
CO2
CO2
Carbonic acid dissociates to release hydrogen ions:
H2CO3 H+ + HCO3-
H20
I
I
I
Carbonic Acid
I
I
Carbonic Acid
H+
H+
HCO3-
HCO3-
H+
The hydrogen ions lower the pH and make the blood more acidic. Proteins in the blood act as buffers to resist the
change in pH. If the change in pH is small then the extra hydrogen ions are detected by the respiratory centre in the
(medulla oblongata of the brain). This causes an increase in the breathing rate to help remove excess CO2.
If the blood pH drops below 7.35 it results in slow or difficult breathing, headaches, drowsiness
, restlessness, tremor and confusion. There may be a rapid heart rate and changes in blood pressure. This is
respiratory acidosis. It can be caused by diseases or conditions that affect the lungs themselves, such as
emphysema, chronic bronchitis , asthma or severe pneumonia. Blockage of the airway due to swelling a foreign object
or vomit can also induce respiratory acidosis.

5. Nitrogenous Compounds:
The body cannot store proteins or amino acids. However, amino acids contain almost as much energy as carbohydrates. Therefore it
would be wasteful to simply excrete excess amino acids.
Amino
Amino
Acid
DEAMINATION: Amino Acid + Oxygen Keto acid + ammonia
Acid
Amino acids are transported to the liver
and potentially toxic amino group is
removed (deamination). The amino
group initially forms the very soluble
and highly toxic compound, ammonia.
Ammonia
}
Deamination
Keto
Acid
The remaining keto acid may be
used directly in respiration to
release its energy or it may be
converted to a carbohydrate or
fat for storage.
Urea
UREA FORMATION: Ammonia+ Carbon Dioxide Urea + Water
2NH3 + 2CO2  CO(NH2)2 + H20
This is converted to a less
soluble and less toxic
compound called Urea. This
can be transported to the
kidneys for excretion.

6. Blood leaves the liver and
retuning to the heart
Hepatic vein
Right lobe
Diaphragm
Left lobe
Hepatic artery: Receives oxygenated
blood from the aorta
Bringing fresh blood from the heart
Common bile
duct
Gall Bladder
Portal vein: Deoxygenated blood enters from the
digestive system.
Bringing blood from the intestines

7. The liver is involved in homeostasis it therefore requires a good blood supply. The liver is supplied with
blood from two sources:
Oxygenated blood from the heart:
Blood travels from the aorta to the
hepatic artery into the liver. This supplies
the oxygen that is essential for aerobic
respiration. The liver cells are very active
and carry out metabolic processes which
require energy in the form of ATP.
Deoxygenated blood from the digestive system:
This enters the liver via the hepatic portal vein. The
blood is rich in products of digestion, the
concentrations of various compounds will be
uncontrolled. The blood may contain toxic
compounds that were previously absorbed in the
intestine.
Blood leaves the liver via the hepatic vein, this re-joins the vena
cava and the blood returns to normal circulation.

8. Bile is a secretion from the liver. It has both a digestive function and an excretory function. The
bile duct carries bile from the liver to the gall bladder where it is stored until required to aid the
digestion of fats in small intestines.

9. Liver lobule
Interlobular
vessel
Liver cells are known as hepatocytes. They are relatively unspecialised and have a simple
cuboidal shape with many microvilli on their surface. The cytoplasm however is very dense and
specialised in the amounts of organelles it contains.
Their functions include:
• Transformation and storage of carbohydrates
• Synthesis of cholesterol and bile salts
• Detoxification
Cells lining the sinusoid
Intralobular vessel
• Protein Synthesis
Kupffer cells are specialised macrophages. They move around 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.

10. Cells, blood vessels an
chambers inside the liver are
arranged to ensure the best
possible contact between the
blood and the liver cells. The
liver is divided into
lobes, these are further
divided into cylindrical lobes.
As the hepatic artery and
hepatic portal vein enter the
liver they split into small and
smaller vessels. These
vessels run between and
parallel to the lobules (inter
lobular vessels).

11. At intervals, branches from the two
blood vessels mix and pass along a
chamber called a sinusoid. The
sinusoid is lined by livercells. The
sinusoids empty into the intralobular
vessel, a branch of the hepatic vein.
Hepatic vein
Sinusoid
(Hepatic vein)
The branches of the hepatic vein from
different lobules join to form the
hepatic vein which drains from the
liver.
One of the many functions of
the liver cells is to manufacture
bile. This is released into thee
bile canaliculus. These join
together to form the bile duct
which transports the bile too the
gall bladder.

12. Excess amino acids cannot be stored as the amine groups make them toxic. However, the amino acid molecules contain
a lot of energy so it would be wasteful to excrete the whole molecule.
It goes through the following processes before the amino component is excreted:
Amino acid Keto acid + ammonia Urea
Deamination
Ornithine Cycle
Deamination:
Ammonia is produced which is highly toxic.
The ammonia must not be allowed to
accumulate. It also produces an organic
compound called keto acid which can enter
respiration directly to release energy.
Ornithine Cycle:
The ammonia is combined with carbon dioxide to produce
urea. Urea is both less soluble and less toxic than ammonia. It
can be passed back into the blood and transported around
the body into the kidneys. The kidneys filter the urea of out
of the blood and concentrated in the urine. Urine can be
stored in the bladder until it is released from the body
2NH3 + CO2  CO(NH2)2 +H20
Ammonia + carbon dioxide  urea + water
Keto Acid

13. Ammonia
(NH3)
CO2
H20
}
Formation of water
from ammonia
Citrulline
Ornithine
NH3
Arginine
Urea
CO(NH2)2
H20
H20
Formation of
urea from
ammonia

14. Control of: blood glucose
levels, amino acid levels, lipid
levels.
Detoxification of:
alcohol, drugs
Synthesis of: red
blood cells in the
fetus, bile, plasma
proteins, choleste
rol
Breakdown of
hormones
Storage of:
vitamins
A, D and
B12, Iron, gl
ycogen
Destruction of red
blood cells

15. The liver is able to detoxify many compounds. Some of these compounds
such as hydrogen peroxide are produced in the body. Some
compounds, such as alcohol may be consumed as part of our diet.
Others, such as drugs may be taken for health reasons or for recreational
purposes. Toxins can be rendered harmless by
oxidation, reduction, methylation or combination with another
molecule.
Example: Liver cells contain enzymes that make toxic molecules less toxic.
These include catalase which converts hydrogen peroxide to oxygen and
water.

16. Detoxification of alcohol:
Ethanol is a drug that depresses nerve activity. In addition it contains chemical potential energy which may be used for
respiration. It is broken down in the hepatocytes by the action of the enzyme ethanol dehydrogenase. The resulting compound
is ethanal. This is dehydrogenated further by the enzyme ethanol dehydrogenase. The final compound produced is ethanoate
(acetate), which is combined with coenzyme A to form acetyl coenzyme A, which enters the process of respiration.
Ethanol
Ethanol Dehydrogenase
Ethanal
Ethanol Dehydrogenase
Ethanoate(Acetate )
Ethanol Ethanal Ethanoic acid-> Acetyl CoA
2H
2H
NAD
NAD Reduced
NAD
Hydrogen atoms released in this process
combine with another coenzyme called
NAD Reduced NAD to form reduced NAD.
Coenzyme A
This is used for
respiration.
Acetyl
coenzyme A
NAD is also required to oxidise and breakdown fatty acids for the use in respiration. If the liver has to detoxify
too much alcohol it has insufficient NAD to deal with the fatty acids. These fatty acids are then converted back
to lipids and are stored in hepatocytes causing the liver to become enlarged. This is a condition known as fatty
liver, which may lead to alcohol related hepatitis or cirrhosis.