5.1.2 EXCRETION AS AN EXAMPLE OF HOMEOSTATIC CONTROL
a. the term excretion and its importance in maintaining metabolism
and homeostasis
Excretion – the removal of metabolic waste from the body.
Metabolic waste – waste products of cellular reactions.
The main excretory products are CO2 and nitrogenous compounds (e.g. urea).
o CO2 is made in the Krebs cycle during respiration and excreted from
the lungs.
o Urea is made in the liver and mitochondria and excreted from the
kidneys via the bladder.
o Other compounds, such as the bile pigments found in faeces.
Metabolic waste must be removed, as a build-up can inhibit enzyme activity
or become toxic.
o CO2 dissolves in water to form carbonic acid. This alters the pH and
causes enzymes to denature. This is called respiratory acidosis.
o Haemoglobin combines with CO2 to form carbaminohaemoglobin, and
with H+ to form haemoglobinic acid. This reduces the oxygen carrying
capacity of the blood.
o NH3 acts as an enzyme inhibitor so is toxic. It can also raise the pH of a
solution. It is highly soluble in water so cannot be stored and must be
removed from the body.
The excretory organs:
The lungs – CO2 is passed into the bloodstream from respiring cells, then
transported to the lungs. In the lungs CO 2 diffuses into the alveoli to be
excreted in exhalation.
The liver – has many metabolic roles, including the storage of glycogen,
detoxification and the formation of urea.
The kidneys – urea dissolved in the bloodstream is transported to the kidneys,
where it is removed from the blood to become part of the urine. Urine is
stored in the bladder before being excreted from the body via the urethra.
The skin – urea, uric acid and ammonia are excretory products that are lost
through sweat.
b. i) the structure and functions of the mammalian liver
The liver blood supply comes from the hepatic artery and the hepatic portal vein.
The hepatic artery supplies the liver with oxygenated blood from the heart.
o Hepatocytes are very metabolically active, so need a good O 2 supply
for respiration.
o They also have a role in homeostasis and add or remove certain
substances to maintain their concentration in the blood.
The hepatic portal vein transports deoxygenated blood from the digestive
system (e.g. stomach, intestines, spleen) to the liver.
o The hepatic portal vein has capillaries at both ends.
o This blood has a high concentration of the products of digestion (e.g.
glucose).
, o It may also contain toxins (e.g. drugs, alcohol) absorbed from the
intestine, which must be detoxified before the blood circulates around
the body.
The hepatic vein transports deoxygenated blood from the liver to the heart.
o It rejoins the vena cava, and the blood returns to the body’s normal
circulation.
The bile duct transports bile from the liver to the gall bladder.
o The gall bladder stores bile, which emulsifies lipids in the small
intestine.
o Bile also contains some excretory products such as bile pigments, e.g.
bilirubin.
The gross structure of the liver:
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The liver is divided into lobes, which
are further divided into cylindrical
structures called lobules.
Each lobule contains many rows of
hepatocytes, called liver cords.
At the centre of each lobule is a central
vein.
o Central veins from different
lobules join together to form the
hepatic vein.
The portal triad consists of the hepatic artery, hepatic portal vein and bile
duct. These are located on the outside of the lobules.
o The hepatic artery has thick walls and a narrow lumen.
o The hepatic portal vein has thin walls and a large lumen.
o The bile duct is lined with cells.
The hepatic artery and hepatic portal vein supply blood to the lobules.
o The blood from the two vessels is mixed and passes through sinusoids,
chambers lined with hepatocytes, before entering the central vein.
o Substances are exchanged between the blood and the hepatocytes
lining the sinusoids.
Kupffer cells are specialised phagocytic macrophages in the sinusoids.
o Their main role is to engulf and digest pathogens in the blood.
o They also break down and recycle old erythrocytes. The breakdown of
haemoglobin produces bilirubin, a bile pigment.
Bile made in the hepatocytes is released into the bile canaliculi.
o The canaliculi join to form the bile duct, which transports the bile to the
gall bladder.
The structure of hepatocytes:
Hepatocytes have a simple cuboidal shape and are lined with microvilli.
Their cytoplasm is densely packed with organelles (e.g. mitochondria, Golgi,
ER, nucleus) and glycogen granules, enabling the liver’s many metabolic
functions.
, The roles of the liver:
The liver is metabolically very active and carries out a wide range of
functions.
The main ones covered at A Level include:
o Storage of glycogen.
o Detoxification of substances – e.g. alcohol, drugs, H 2O2.
o Breakdown of excess amino acids (formation of urea).
Other functions include:
o Control of blood glucose levels, amino acid levels, lipid levels.
o Synthesis of bile, plasma proteins, cholesterol.
o Synthesis of erythrocytes in the fetus.
o Storage of iron and vitamins A, D and B12.
o Breakdown of hormones and old erythrocytes.
Storage of glycogen:
The liver can store approximately 100g of glycogen.
Glycogen is stored in the form of granules, in the cytoplasm of hepatocytes.
Detoxification of alcohol:
This occurs in the smooth endoplasmic reticulum of hepatocytes.
Ethanol is converted to ethanal by ethanol dehydrogenase, which is then
converted to ethanoic acid by ethanal dehydrogenase.
o This oxidation process produces NADH from NAD.
o The ethanoic acid then combines with CoA and enters the Krebs cycle.
If the liver has to detoxify too much alcohol, it uses up its stores of NAD. The
liver becomes unable to metabolise fatty acids, leading to fatty liver disease.
o These fatty acids are converted back to lipids and stored as fat in the
hepatocytes, causing the liver to become enlarged and the destruction
of hepatocytes.
Hepatocytes also contain many enzymes that help in detoxification.
o Catalase converts H2O2 to oxygen and water.
o Cytochrome P450 breaks down drugs like cocaine.
The formation of urea:
Excess amino acids cannot be stored, as the amino groups make them toxic.
However, amino acids are energy-dense, so it would be wasteful to
completely excrete them.
