OCR F214 Excretion

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  • Created on: 30-04-16 21:05
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F214 Module 2: Excretion
4.2.1 Excretion
(a) define the term excretion
Excretion the removal of metabolic waste from the body , of byproducts or unwanted substances from normal
cell processes.
Functions of the Liver:
control of: blood glucose levels, amino acid levels, lipid levels
synthesis of: red blood cells in the fetus , bile , plasma proteins , cholesterol
storage of: vitamins A, D and B 12, iron, glycogen
detoxification of: alcohol , drugs
breakdown of hormones
destruction of red blood cells
(b) explain the importance of removing metabolic wastes, including carbon dioxide and nitrogenous waste,
from the body
Carbon Dioxide:
Carbon dioxide is produced by every living cell in the body as a result of respiration. It's passed from the cells of
respiring tissues into the bloodstream where it is transported, mostly in the form of hydrogencarbonate ions , to
the lungs where the CO 2 diffuses into the alveoli to be excreted as we breathe out.
Affecting Haemoglobin :
o CO2 is mostly transported in the form of
hydrogencarbonate ions which forms hydrogen ions .The
hydrogen ions combine with haemoglobin and they compete with oxygen for space, reducing
oxygentransport .
o CO 2 can also directly
combine with haemoglobin
to form carbaminohaemoglobin , which has a
loweraffinityfor oxygen than normal haemoglobin.
Respiratory Acidosis :
1. Excess CO 2 dissolves in the blood plasma and combines with
water to form carbonic acid .
2 + H
2O H
2CO 3
2. The carbonic acid dissociates to release hydrogen ions.
+ H+
3. The H+ ions lower the pH and make the blood more acidic.
4. If the change in pH is small then the extra hydrogen ions are detected by the respiratory centre in the
medullaoblongata of the brain . Breathing rate increases to help remove excess CO 2.
5. If the blood pH drops below 7.35 it results in slowed or difficult breathing , headache , drowsiness ,
restlessness , tremor , confusion , rapid heart rate and changes in blood pressure respiratory acidosis . It can
be caused by diseases or conditions that affect the lungs themselves such as emphysema , chronic bronchitis ,
asthma or severe pneumonia , as well as blockage of airways due to swelling , a foreign object or vomit .
Nitrogenous Compounds (Urea):
Urea is produced in the liver from excess amino acids bring broken down, called deamination (the removal of the
amine group from an amino acid to produce ammonia). It's passed into the bloodstream to be transported to the
kidneys where the urea is removed from the blood to become part of the urine.
The body can't store proteins or amino acids , but they contain almost as much energy as carbohydrates so it
would be wasteful to excrete excess amino acids.
1. Excess amino acids are transported to the liver and the potentially toxic amino acid is removed
( deamination ). The amino group initially forms the very soluble and highly toxic compound , ammonia .
Deamination: amino acid + oxygen keto acid + ammonia
2. The ammonia is converted to a less soluble and less toxic compound called urea, which can be transported
to the kidneys for excretion.
Formation of Urea: ammonia + carbon dioxide urea + water
+ CO2 CO(NH2
+ H2

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F214 Module 2: Excretion
3. 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 storage.…read more

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F214 Module 2: Excretion
Water Reabsorption:
The role of the loop of Henle is to create a low (very negative) water
potential in the tissue of the medulla . This ensures that even more
water can be reabsorbed from the fluid in the collecting duct.
As the fluid in the descending limb gets deeper into the medulla, the
water potential becomes lower (more negative) due to the increasing
concentration of sodium and chloride ions down the descending limb.…read more

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F214 Module 2: Excretion
Haemodialysi 4dialysisfree days a week. Have to plan your life around the dialysis
s sessions , e.g. holidays.
Your diet and the amount of fluid you
drink needs to be restricted ­ avoid foods
high in potassium (e.g. bananas,
chocolate, crisps) and advised not to drink
more than a couple of cups of fluid a day.
Peritoneal Regularvisits to a dialysis unit are not Needs to be carried out every day .…read more


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