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What is excretion?
· The removal of waste products from
metabolic processes.
· Metabolic waste consists of waste substances
that may be toxic or produced in excess by the
reactions in cells.
· CO2 and Urea are the main waste products.…read more

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Why must they be removed?
Carbon Dioxide:
·CO2 combines directly with haemoglobin which lowers the haemoglobins
affinity to oxygen.
·CO2 can dissolve in the blood plasma and combine with water to produce
carbonic acid. This dissociates to release hydrogen ions that lowers the pH of
the blood. Proteins act as buffers but when the pH drops below 7.35 it results
in slowed and difficult breathing, headaches, drowsiness, tremor and
·When CO2 is converted into hydrocarbonate ions, hydrogen ions are also
formed and these combine with haemoglobin and compete for space on the
haemoglobin molecule with oxygen.
Nitrogenous Compounds:
·The body cannot store excess proteins or amino acids.
·They are transported to the liver and the toxic amine group is removed to
produce the highly toxic and soluble ammonia. This is the converted to the
less soluble and less toxic urea which can be transported to the kidneys.…read more

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The Liver
Blood flow to and from the Liver:
· Oxygenated blood from the heart. Blood travels from the aorta via the
hepatic artery. This supplies oxygen essential for aerobic respiration.
· Deoxygenated blood from the digestive system, which enters the liver via
the hepatic portal vein. It contains all the products from digestion and the
concentrations of various products will be uncontrolled and could contain
toxic compounds.
· Blood leaves the liver via the hepatic vein and joins the vena cava vein.
· The fourth vessel is not a blood vessel, it is a bile duct. Bile is a secretion
from the liver and has a digestive and excretory function. The bile duct
carries bile from the liver to the gall bladder where it is stored until
required.…read more

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Arrangement of Cells in the Liver:
· The liver is divided into lobes, which are further divided into cylindrical lobules.
· As the hepatic artery and vein enter the liver they branch into smaller and
smaller vessels. These run between and parallel to the lobules.
· At intervals, the braches enter the lobules and the blood from the two
vessels is mixed and passes along a sinusoid lined by liver cells.
· The sinusoids empty into a branch of the hepatic vein, which join
together to form the hepatic vein.
· As the blood flows through the sinusoid,
the liver cells can remove molecules from
the blood and pass molecules in
· Liver cells also manufacture bile, which is
released into the bile caniliculis.
· These join to make the bile duct.…read more

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Liver and Kupffer Cells
Liver Cells (Hepatocytes):
· Appear to be unspecialised. Are cuboidal shape and have many
· Their functions include: protein synthesis, transformation and
storage of carbohydrates, synthesis of cholesterol and bile salts and
· This means they have a very dense cytoplasm and contain certain
Kupffer Cells:
· These are specialised macrophages.
· They are found in the sinusoids and breakdown and recycle old red
blood cells.…read more

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