Summary
Summary on Excretion (including kidney and liver function)
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A-level notes for excretion
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Excretionexcretion – the removal of metabolic waste from the body
The importance of excretion
Metabolic products (e.g. CO2 and ammonia) are toxic and may interfere with cell processes by
altering the pH, so normal metabolism is prevented, or they may act as inhibitors and reduced the
activity of essential enzymes.
Carbon dioxide
CO2 is transported in the blood as hydrogen carbonate ions
CO2 + H2O H2CO3 (catalyzed by carbonic anhydrase)
H2CO3 H+ + HCO3-
The H+ ions reduce the pH of the cytoplasm of RBCs and interact with ionic bonds within
hemoglobin, changing its tertiary structure reduces the affinity of hemoglobin for O 2
reduced O2 transport
H+ ions combine with hemoglobin to from hemoglobinic acid less hemoglobin for O2 transport
In the blood plasms, excess H+ ions can reduce the pH of the plasma and could alter the structure
of many proteins in the blood. Proteins in the blood acts as buffers to resist the change in pH.
If the change in pH is small then the extra H + ions are detected by the respiratory center in the
medulla oblongata of the brain. This causes an increase in the breathing rate to help remove the
excess CO2.
Nitrogenous compounds
The body cannot store excess amino acids, so they are transports to the live and deaminated to
form ammonia (very soluble and toxic). Ammonia is immediately converted into urea, which is
transported to the kidneys for excretion.
,Liver structure
The cells, blood vessels and chambers inside the liver are arranged to ensure the greatest possible contact
between the blood and the liver cells.
The liver is divided into lobes which are further divided into cylindrical lobules.
, The hepatic artery and hepatic portal vein split
into smaller vessels – inter-lobular vessels –
that run between lobules.
The blood from the 2 blood vessels is mixed and
passes along a chamber, called a sinusoid,
which is lined with liver cells.
The blood is in close contact with the liver cells, so substances can move between the blood and cells.
Specialized macrophages, called Kupffer cells, move within the sinusoids.
They breakdown and recycle old red blood cells.
One of the products of hemoglobin breakdown is bilirubin, which is one of the bile pigments excreted as
part of the bile.
Bile is made in the liver cells and released into the bile canaliculi, which join together to form the bile
duct, which transports the bile to the gall bladder.
At the center of each lobule is a branch of the hepatic vein known as the intra-lobular vessel. The
sinusoids empty into this vessel.
The branches of the hepatic vein, from different lobules, join together to from the hepatic vein, which
drains blood from the liver.
Liver cells / hepatocytes
simple cuboidal shape with many microvilli on their surface
their metabolic functions include: protein synthesis, transformation and storage of carbohydrates,
synthesis of cholesterol and bile salts, detoxification
their cytoplasm if very dense and specialized in the numbers of certain organelles it contains
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