Excretion notes

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  • Created on: 10-03-16 22:55
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Learning objective(s)
Explain the importance of removing metabolic wastes from the body, including
carbon dioxide and nitrogenous waste
Why must metabolic wastes be removed?
Carbon dioxide
Excess carbon dioxide is toxic. A high level of carbon dioxide has three main effects:
The majority of CO 2 is carried in the blood as hydrogencarbonate ions . The formation of
hydrogencarbonate ions also forms hydrogen ions . This occurs inside the red blood cells,
under the influence of enzyme carbonic anhydrase . The hydrogen ions combine with
haemoglobin . They compete with oxygen for space on the haemoglobin . If there is too
much carbon dioxide in the blood it can reduce oxygen transport .
The CO 2 also combines directly with haemoglobin to form carbaminohaemoglobin . This
molecule has a lower affinity for oxygen than haemoglobin .
Excess CO 2 can also cause respiratory acidosis . The C O2 dissolves in the blood plasma
Once dissolved it can combine with water to produce carbonic acid :
CO2 + H2
H2 CO3
The carbonic acid dissociates to release hydrogen ions :
H + HCO3
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 this change is small then the extra H+ ions are
detected by the respiratory centre in the medulla oblongata of the brain, causing an
increase in breathing rate to help remove the excess carbon dioxide. However, if the blood
pH drops below 7.35 it results in slowed or difficult breathing, headache, drowsiness,
restlessness, tremor and confusion. There may also 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.
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.
Instead they are transported to the liver and the potentially toxic amino group is removed by
deamination . The amino group initially forms the very soluble and less toxic compound called
urea , which can be transported to the kidneys for excretion . The remaining keto acid can be used
directly in respiration to release its energy or it may be converted to a carbohydrate or fat for
Deamination: amino acid + oxygen keto acid + ammonia
Formation of urea: ammonia + carbon dioxide urea + water
2NH3 + CO2 CO(NH2)2 + H2O

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1. Explain the difference between excretion and egestion.
Excretion is the removal of metabolic waste from cell processes. Egestion is the removal of
undigested food from the digestive system ­ it has never been in a body cells so it cannot
be classified as excretion.
2. Explain what causes respiratory acidosis.
Respiratory acidosis is the result of high carbon dioxide levels in the blood. Carbon dioxide
dissolves in the watery plasma to form carbonic acid.…read more


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