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Main Substances to be Excreted
CO2 from respiration
o CO2 diffuses from respiring cells into the bloodstream. It is
transported in the form of hydrogen carbonate ions to the lungs,
where it is breathed out.
Nitrogen-containing compounds such as urea
o Urea is produced in the liver from breaking down excess amino acids.
The process is called deamination. Urea is then passed into the
bloodstream to be transported to the kidneys. It is then stored in the
bladder before being excreted.
Why must these substances be removed?
Excess CO2 can be toxic. It can
Majority of CO2 is transported as
hydrogen carbonate ions. The
production of hydrogen carbonate ions
also produces H+ that combine with
haemoglobin. They compete for oxygen
and so it can reduce oxygen transport.
The CO2 combines directly to the
carbaminoheamoglobin which has a
low affinity for oxygen.
The excess CO2 can also cause respiratory acidosis. The CO2 dissolves in the
blood plasma and once dissolved it can combine with water to produce
carbonic acid: CO2 + H20 H2CO3
The carbonic acid dissociates to release H+ : H2CO3 H+ + HCO3-
The H+ lower the pH and make the blood more acidic. Proteins in the blood
act as buffers to resist the change in pH. If small, the change is detected by
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CO2 concentrations in the blood.
If the blood pH drops below 7.35 it results in slowed or difficult breathing,
headache, drowsiness... There may also be a rapid heart rate and changes in
blood pressure. This is respiratory acidosis. It can be caused by disease or
conditions that effect lungs.
The body cannot store proteins or amino acids. However, amino acids contain as
much energy as carbohydrates. They are then transported to the liver and the
potentially toxic amino acid group is removed (deamination).…read more
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1. Excretion is the removal of metabolic waste that comes from the reactions
inside cells. Whereas, Egestion is the removal of undigested food. In
Egestion, the substance never enters any cells so it cannot be Excretion.
2. Respiratory acidosis: CO2 dissolves in the blood plasma and combines to
water to produce carbonic acid. This then dissociates to release hydrogen
ions that lower the pH of the blood making it more acidic and this is detected
by the medulla oblongata. This is respiratory acidosis.