The digestion process

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The digestion process
Extension
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Learning objectives

• To recognise the organs involved in digestion
• To understand the major functions of each body
  part in the digestive process
• To understand the four major phases of digestion
• To understand the functions of different enzymes.

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Organs involved in digestion

• What is the route food takes through the body?

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Organs involved in digestion
The gastrointestinal (GI) tract involves the
stomach, small intestine and colon.
Mouth
Oesophagus
Stomach
Small intestine
Colon
Rectum
Anus
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Mouth

• Mastication is the action of the teeth and the
  jaws working together to break food down. Food
  needs to be chewed to be broken down into pieces
  small enough to swallow.
• Breaking the food down also gives it a larger
  surface area for the digestive enzymes to work
  on.
• There are two different types of teeth in the
  mouth
• - Incisors to tear food e.g. meat
• - Molars to grind the food.

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Saliva

• Saliva contains the enzyme amylase which breaks
  down starch into simple sugars.
• It also moistens the food allowing easier
  passage through the gastrointestinal tract.
• Saliva is secreted from glands found under the
  tongue and at the back of the mouth.
• The sight, smell, taste or even the thought of
  food will start to increase the amount of saliva
  secreted.

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Moving on from the mouth

• Food is masticated and mixed with saliva in the
  mouth.
• The tongue and cheeks help to push the food into
  the teeth and also shape the food into a ball or
  bolus before being swallowed.
• The bolus is passed through to the oesophagus.

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Oesophagus

• The oesophagus is similar to a conveyor belt as
  it transfers the food from the mouth to the
  stomach in 3-6 seconds.
• Circular muscles in the wall of the oesophagus
  relax in front of the bolus whilst circular
  muscles behind the food contract, pushing the
  bolus onward.
• This is called peristalsis.
• People do not have conscious control over the
  muscles in the oesophagus. Even if someone is
  upside down, the food will be passed on to the
  stomach.

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Stomach

• The stomach is an expandable sack made up of
  three different layers of muscles where the bolus
  will be churned for a few minutes or up to 2 or 3
  hours.
• The bolus is mixed with hydrochloric acid which
  helps to kill any bacteria present.

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Stomach

• The enzyme pepsin is also active in the stomach.
  It starts to break down protein to form peptides
  and amino acids.
• Alcohol and a little water is absorbed by the
  stomach. Alcohol is absorbed through the stomach
  wall and taken to the liver where it is
  metabolised.
• When the food has been churned into a creamy
  mixture known as chyme, the pyloric sphincter (a
  ring of muscles) opens and chyme is released
  gradually into the small intestine.

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Small intestine

• Chyme passes out of the stomach through the
  pyloric sphincter into the small intestine.
• The small intestine is a tube about 6 metres
  long.
• The small intestine is divided into three
  sections, the
• duodenum, jejunum and the ileum.
• The first section of the small intestine is the
  duodenum.

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Duodenum

• The duodenum receives about 12 grams of chyme
  each time the pyloric sphincter opens.
• The duodenum is about 25 centimetres long and in
  the shape of a horse shoe.
• In the duodenum, chyme is diluted with bile
  salts (from the gall bladder) and pancreatic
  juices (from the pancreas).

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Bile

• Bile is produced in the liver and stored in the
  gall bladder. This contains bile salts which
  emulsify fat.
• Fat is normally insoluble in water. The bile
  salts are released into the duodenum making it
  easier to break down fat.
• The bile allows the fats to mix in with the
  watery digestive juices, and allows the enzyme
  lipase to digest the fats efficiently.
• Fat can take from 3 to 5 hours to be broken down
  and absorbed.

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Pancreatic juices

• The pancreas provides alkaline pancreatic
  juices.
• These juices contain sodium bicarbonate to
  neutralise the hydrochloric acid mixed into the
  chyme from the stomach.
• Pancreatic juices also contain digestive enzymes
  such as
• ?Trypsin and chymotrypsin break down protein
  to peptides and amino acids.
• ?Pancreatic amylase breaks down starch and
  glycogen to maltose
• ?Lipase breaks down fat to fatty acids and
  glycerol.

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Peristalsis

• Peristalsis is the action of waves of muscular
  contractions which moves food along the digestive
  system.
• Dietary fibre aids peristalsis because it
  increases the bulk of the bolus or chyme being
  moved along.

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Wall of the small intestine

• The inner surface of the small intestine is
  folded into finger-like structures called villi,
  which greatly increase the surface area available
  for absorption.
• The villi have a surface area of about 30m2,
  this is equivalent to the size of a tennis court!

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Digestion in the wall of the small intestine

• Protease breaks down peptides to amino acids.
• Maltase breaks down maltose to glucose.
• Sucrase breaks down sucrose to glucose and
  fructose.
• Lactase breaks down lactose to glucose and
  galactose.
• Lipase breaks down fats to fatty acids and
  glycerol.

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Substances absorbed in the small intestine

• Water
• Alcohol
• Sugars
• Minerals
• Water soluble vitamins
• Peptides and amino acids
• Fatty acids, glycerol and fat soluble vitamins.

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Absorption

• The villi in the small intestine have a high
  blood supply. There are two types of absorption
  which occur here.
• Passive through the process of osmosis the
  nutrients pass through the wall of the small
  intestine and into the blood supply.
• Active a carrier transports the nutrient
  through the wall of the small intestine into the
  blood supply. This type of absorption requires
  energy.
• Once in the blood the nutrients are transported
  to the liver via the hepatic portal vein. The
  liver filters, converts the nutrients into
  substances that can be used by the bodys cells
  for energy and growth.

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Absorption

• Most fatty acids and glycerol pass into the
  lymphatic system, and then the bloodstream.
• Once in the blood, nutrients are carried to all
  the cells of the body. Some are oxidised to
  produce energy and other are used to repair the
  cell or to build new cells.
• Once the nutrients have been absorbed, the
  remaining chyme passes into the large intestine
  or the colon.

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Colon

• The colon is a tube just over one meter long,
  which is inhabited by bacteria.
• The main function of the colon is to absorb
  water into the bloodstream.
• Bacteria in the colon ferment dietary fibre
  (NSP) and produce fatty acids and gas.
• Other bacteria produce vitamin K, which is also
  absorbed.
• The products of bacterial digestion, along with
  water and any remaining minerals are absorbed
  leaving a residue behind.

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Colon

• The watery residue moves along the colon, and
  the faeces are formed and stored in the rectum
  before being excreted through the anus.
• Young children gradually learn to control this
  action.
• It may take 12-24 hours for the faeces to pass
  through the colon. This time can be reduced if
  the diet is high in fibre.

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Gut Bacteria

• The gut contains bacteria
• 400 500 different species
• includes potentially pathogenic (e.g.
  Clostridia) potentially beneficial (e.g.
  Bifidobacteria Lactobacilli) bacteria.
• These are measured in colony forming units per
  millilitres (cfu/ml).

Stomach 101 103 cfu/ml
Duodenum jejunum 102 105 cfu/ml
Colon 1010 1012 cfu/ml
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Summary of the phases of digestion

• Ingestion - this is the physical intake of
  foodstuffs into the gastrointestinal tract.
• Digestion - a series of physical and chemical
  processes which begin in the mouth, but take
  place mainly in the stomach and small intestine.
• Absorption - the passage of the digested food
  substances across the gastro-intestinal lining,
  or mucosa, into the blood and lymph.
• Elimination - the excretion, or elimination, of
  those food substances that cannot be digested
  (such as cellulose) or without any nutritional
  value in the faeces.

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Describe the digestion of

• Fish and chips Hamburger

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Review of the learning objectives

• To recognise the organs involved in digestion
• To understand the major functions of each body
  part in the digestive process
• To understand the four major phases of digestion
• To understand the functions of different enzymes.

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