Functions of the liver

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Functions of the liver

• Option H.4

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1. Synthesis of cholesterol

• Some cholesterol is absorbed from food in
  intestines
• A larger quantity in made each day in hepatocytes
  (liver cells)
• Some used by the liver to produce bile
• Rest is transported via blood for use by cells
  eg. plasma membranes

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2. Synthesis of plasma proteins

• Rough er from hepatocytes produces 90 of
  proteins in blood plasma
• Three main types albumin, globulin, fibrinogen
• All albumin and fibrinogen is made in the liver.
• Some globulin made in the liver and some by
  lymphocytes (antibodies)

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3.Storage of nutrients

• Some minerals and fat-soluble vitamins are stored
  in the liver
• Eg iron, retinol (vitamin A), calciferol (vitamin
  D)
• Carbohydrate also stored as glycogen

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4. Regulation of nutrients in blood

• When there is an excess of certain nutrients in
  the blood, hepatocytes absorb and store them
• They are released back into the blood when levels
  are too low
• Eg high blood glucose levels triggers insulin to
  stimulate hepatocytes to absorb glucose and
  convert it to glycogen.
• Eg low blood glucose levels triggers glucagon to
  stimulate hepatocytes to break down glycogen and
  release it into the blood

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• This is necessary so as not to affect the osmotic
  concentration of the blood

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5. Bile secretion

• Hepatocytes secrete bile into narrow tubes called
  canaliculi
• Bile contains bile salts, bile pigment, hydrogen
  carbonate ions (HCO3-) and cholesterol
• It drains from canaliculi into the gall bladder
• Water is reabsorbed leaving it concentrated
  before eventual discharge into small intestine

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6. Breakdown of erythrocytes

• Erythrocytes have a short life span of 120 days
• The plasma membrane becomes fragile and ruptures
  releasing haemoglobin into the blood plasma
• Special Kuppfer cells lining the sinusoids in the
  liver are phagocytic
• They take up the haemoglobin by engulfing it

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• Inside the Kupffer cells haemoglobin is split
  into globin and heme groups
• Globin is hydrolysed into amino acids and
  released into the blood
• Heme groups are broken down into iron and bile
  pigment (bilirubin)
• Iron is carried to the bone marrow for use in
  production of new erythrocytes

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• Bilirubin is used to make bile.
• A build-up in the blood leads to jaundice (yellow
  skin colour)

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Breakdown of erythrocytes - summary
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7. Detoxification

• Plants and animals often produce substances that
  are toxic to prevent being eaten by predators
• Such toxins eg nicotine are broken down by the
  liver and excreted
• Artificial substances such as alcohol and drugs
  are also detoxified in the same way
• The liver contains several broad specificity
  enzymes for this purpose.

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8. Deamination

• The body cannot store protein or amino acids
• Excess amounts in the diet are broken down by the
  liver for use as energy sources

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Excessive alcohol consumption

• The liver has to metabolize excess alcohol taken
  into the body
• The enzyme pathway involved looks like this

ETHANOL ACETALDEHYDE ACETIC ACID FATTY ACIDS
Alcohol dehydrogenase
Aldehyde dehydrogenase
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• Acetaldehyde build up causes a hangover, along
  with dehydration
• Fatty acids accumulate in liver cells and around
  the capillaries in the liver
• This reduces blood flow and ultimately kills the
  cells (necrosis)
• Collagen fibres become deposited leading to
  cirrhosis and eventual liver failure

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Circulation of blood through liver

• The liver is supplied by 2 blood vessels
• Hepatic artery brings oxygen rich blood from
  heart
• Hepatic portal vein brings blood from the gut to
  the liver
• One vessel the hepatic vein carries blood
  away.

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• Inside the liver the hepatic portal vein divides
  up into vessels called sinusoids
• They are wider than normal capillaries and are
  very porous due to
• Single layer of very thin cells / no basement
  membrane / large number of fenestrations (pores)

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• This allows blood flowing along sinusoids to come
  into close contact with the hepatocytes
• Sinusoids drain into venules that lead to the
  hepatic vein, which removes blood from the liver
  back to the heart along the vena cava
• Branches of the hepatic artery join the sinusoids
  at certain points, providing oxygenated blood.