F214 module 2 Excretion

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  • Created on: 14-02-14 12:01
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F214 Communication, Homeostasis and Energy
Module 2 Excretion
4.2.1 Excretion
a. Define the term excretion
Excretion-The removal of metabolic waste from the body which are by products
or unwanted substances from cell processes.
b. Explain the importance of removing metabolic wastes, including carbon dioxide
and nitrogenous waste from the body
Carbon dioxide
o Excess carbon dioxide is toxic and has three main effects:
The majority of carbon dioxide is carried in the blood as
hydrogencarbonate ions but in the process hydrogen ions are
made inside red blood cells with carbonic anhydrase. These
hydrogen ions compete with oxygen for space on the haemoglobin.
Therefore too much carbon dioxide can cause a reduction in
oxygen transportation.
Carbon dioxide can also combine with haemoglobin to make
carbaminohaemoglobin which has a lower affinity for oxygen.
Excess carbon dioxide can also cause respiratory acidosis by
dissolving in the blood plasma and reacting with water to make
carbonic acid which dissociates and releases hydrogen ions. This
lowers the pH by making the blood more acidic. If the change is
small is leads to an increased breathing rate but if it is larger it
can cause difficult breathing, drowsiness, headaches and
confusion. It could cause a rapid heart rate and changes in blood
Nitrogenous compounds
o The body cannot store proteins or amino acids.
o But because they contain a lot of energy, amino acids are transported to
the liver where they get the toxic amino acid group removed via
o The amino acid then forms the very soluble but highly toxic ammonium
before being converted to urea and is then transported to the liver for
o There is some remaining keto acid which can be respired or converted to
a carbohydrate or fat for storage.
c. Describe the histology and gross structure of the liver
Hepatic artery
o Supplies the liver with oxygenated blood from the heart.
o The oxygen supplied is need for aerobic respiration.

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Hepatic portal vein
o This carries oxygenated blood to the liver.
o The blood is rich in the products of digestion some of which may be toxic
Hepatic vein
o This is where blood leaves the liver which then re-joins the vena cava.
Bile duct
o Bile is secreted from the liver.
o Bile has a digestive and excretory function.…read more

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Describe the formation of urea in the liver including an outline of the
ornithine cycle
o Amino acid + O2 Keto acid + ammonia
Ornithine cycle
o 2NH3 + CO2 CO(NH2)2 + H2O
o Ammonia + carbon dioxide Urea + water…read more

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Describe the roles of the liver in detoxification
Liver cells contain enzymes that make toxic molecules less toxic.
o E.g. Catalase which converts hydrogen peroxide to oxygen and water.
Detoxification of alcohol
o Ethanol contains chemical potential energy that can be used in respiration.
o It is broken down in the hepatocytes using ethanol dehydrogenase.
o This makes ethanal which is dehydrogenated further using ethanal dehydrogenase.
o Then ethanoate is produced with coenzyme A to form acetyl coenzyme A.…read more

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Describe and explain the production of urine, with reference to the processes
of ultrafiltration and selective reabsorption
1. Blood flows into the glomerulus from the afferent arteriole which is
wider than the efferent arteriole which carries the blood away from the
2. This means that the blood in the capillaries is under increased pressure.
3. The blood pressure in the glomerulus is higher than in the Bowman's
4. So fluid from the blood is pushed in the Bowman's capsule.
5.…read more

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Loss of water by osmosis to the surrounding tissue fluid.
Diffusion of sodium and chloride ions into the tubule from the
surrounding tissue.
The lower down the descending limb the lower the water potential.
o In the ascending limb
At the base the sodium ions diffuse out.
Higher up the sodium and chloride ions are actively transported
our into the tissue fluid.
The wall is impermeable to water so it cannot leave.
The fluid loses salts as it moves up the ascending limb.…read more

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Also, the body is unable to regulate the levels of water and salts in the
o It removes wastes, excess fluid and salt from the body by passing the
blood over a partially permeable dialysis membrane which allows
exchanges between the blood and dialysis fluid.…read more

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The hCG antibody complex moves up the stick and attaches to a band of
immobilised enzymes forming a blue line.
Anabolic steroid testing
o Anabolic steroids increase protein synthesis in cells resulting in a
build-up of cell tissue especially in muscles.
o They have a half-life of about 16 hours and remain in the blood for
several days.
o Because they are relatively small molecules they can enter the nephron
easily.…read more


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