DIGESTION IN MAN

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DIGESTION IN MAN

• By Domnic Otieno
• Senior biology teacher
• Sega girls-Siaya

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KNEC specific objectives

• Relate the structures of the mammalian (human)
  alimentary canal to their functions
• Explain the role of enzymes in digestion in a
  mammal (human

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Digestive system in man
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Def.

• This is the process by which large, complex and
  insoluble organic food molecules are broken down
  to small soluble molecules. It takes place in the
  alimentary canal. Alimentary canal is a muscular
  tube running from the mouth to the anus. Small
  molecules like vitamins, minerals salts and water
  do not undergo digestion.

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Types of Digestion

• 1. Mechanical digestion
• This is achieved by the cutting and grinding
  action of teeth. This breaks the food into small
  particles that can be swallowed.
• 2. Chemical Digestion
• This is achieved by the digestive juices. Juices
  come from two sources
• Glands that are located in the alimentary canal
• Examples - The gastric glands in the stomach
• The intestinal glands of the small intestine
• Glands located outside the alimentary canal such
  as the salivary glands, pancreas and liver.
• These are called accessory glands and are
  connected to the alimentary canal by small tubes
  called ducts.

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The digestive system

• Consists of the alimentary canal and the
  accessory glands.
• It is divided into various regions
• i. The mouth
• ii. Oesophagus
• iii. Stomach
• iv. Small intestine i.e. duodenum and ileum
• v. Large intestine i.e. caecum, colon and
  appendix.

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Digestion in the mouth

• The food is chewed in the mouth by the action of
  teeth. This reduces it to small particles
  suitable for swallowing. This also increases the
  surface area of food enabling the enzymes to act
  on it more effectively.
• Food is mixed with saliva which is secreted by
  three pairs of salivary glands namely sublingual,
  sub-mandibular and parotid salivary glands.
• .

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Functions of saliva

• (a) It contains salivary amylase/Ptyalin that
  begins the digestion of starch into maltose.
• (b) It lubricates food making it suitable for
  swallowing. Saliva is neutral or slightly
  alkaline that is optimum pH for the action of
  salivary amylase.

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cont

• The tongue rolls the food into small rounded
  masses called boluses. The tongue then pushes the
  bolus into the pharynx i.e. back of the mouth.
  This is the beginning of swallowing. The soft
  palate is raised to open the gullet and close the
  nasal cavity while epiglottis relaxes to close
  the wind pipe or trachea.
• The food then passes down into the oesophagus or
  gullet. The gullet wall contains circular and
  longitudinal muscles. The passage of the bolus
  down the gullet is aided by a wave of the muscle
  contractions known as peristalsis. This produces
  a series of constrictions that squeeze the bolus
  down the gullet and into the stomach.
• .

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peristalsis

• Peristalsis-is the wave-like contractions of the
  muscular walls of the alimentary canal. It churns
  and pounds the food, mixing it with digestive
  juices. It is also responsible for pushing food
  along alimentary canal

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Digestion in the stomach

• Stomach is a sac-like expansion of the alimentary
  canal. The stomach walls contain layers of
  muscles and gastric glands that produce gastric
  juice. Production of gastric juice is stimulated
  by gastrin.

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Contents of the gastric juice

• (i) Hydrochloric Acid It is produced by
  parietal cells or oxyntic cells
• Functions of Hydrochloric acid
• -It coverts pesinogen to pepsin
• -It destroys certain bacteria that may be present
  on food
• - It folds proteins enabling pepsin to act on
  them
• - It increases acidity of the stomach contents to
  a pH of 1.5 2.5 which is the optimum pH for the
  action of pepsin
• (ii) Pepsin - Secreted by peptic cells. It is a
  proteolytic enzyme that breaks proteins to
  polypeptides. It is produced in an inactive form
  called pepsinogen that is them converted to the
  active form by hydrochloric acid.
• Production of pepsin in an inactive form
  prevents it from digesting the proteins of the
  cells
• that produce them and the lining of the
  gastric glands.

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cont

• (iii) Renin
• Secreted by peptic cells. It is produced in young
  mammals. It is responsible for the coagulation of
  the milk, protein, Casein
• (iv) Mucus-
• Secreted by goblet cells of the gastric gland
• Forms a layer over the stomach lining
• It lubricates food and also protects the stomach
  lining from being digested by pepsin and
  from being eroded by Hydrochloric acid.
• NB/ Food is churned by the rhythmical
  contractions of the muscular wall of the stomach.
  Thisforms a semi-solid mass called chyme.

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• 12 of pupils had used illegal drugs in the last
  month and 20 had used illegal drugs in the last
  year
• 13 of boys reported taking drugs in the last
  month compared to 11 of girls
• Use increased with age. Six percent of
  11-year-olds had used drugs in the last year
  compared to 39 of 15-year-olds

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Digestion in the Small Intestine

• Structure
• It is a long tube with a length of 12 16 feet
  (4-5m). It is highly coiled in order to fit in
  the abdominal cavity. It is divided into two main
  regions
• i. Duodenum It s only about 1 foot (30cm) long
  and most digestion occurs here
• ii. Ileum can be divided into Jejunum and Ileum
  proper

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Digestion in the Duodenum

• The chyme is let out into the duodenum through
  the pyloric sphincter. The sphincter muscle
  allows small quantities of chyme to pass through
  at a time.
• It receives the secretions from the following
  organs
• i.The liver has specific cells that secrete bile
  into the gall bladder to be stored.The gall
  bladder releases the bile into the duodenum
  through bile duct.
• ii.The pancreas lies just below the stomach. It
  is a thin flat and cream coloured gland. It
  secretes hormones and digestive juices.
• The arrival of food into the duodenum stimulates
  secretion of the hormone secretin from the
  pancreas and cholecystokinin from the duodenum
  whereas cholecystokinin stimulates the secretion
  of bile from the gall bladder.

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duodenum
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Pancreatic juice

• Its alkaline. It contains the following
  digestive enzymes
• i. Pancreatic amylase - speeds up the breakdown
  of the remaining starch into maltose
• ii. Trypsin speeds up the breakdown of proteins
  into peptides. This enzyme is secreted in its
  inactive form trypsinogen.
• iii. Pancreatic lipase - speeds up the breakdown
  of lipids into fatty acids and glycerol.
• Pancrease also produces sodium hydrogen carbonate
  that neutralizes the acidic chyme and creates a
  suitable alkaline medium for pancreatic and
  intestinal enzymes.
• The bile contains bile salts that include sodium
  glycocholate and sodium taurocholate. The salts
  aid in the breakdown of fats into tiny fat
  droplets to increase their surface area for
  digestion. This breakdown of fats is known as
  emulsification. The salts also provide an
  alkaline medium in which the enzymes work best.
  The salts also neutralize the acidic chyme from
  the stomach.

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Digestion in ileum

• The inner walls of the ileum contain secretory
  cells, some of which secrete muscus and others
  secrete an alkaline fluid known as intestinal
  juice or succus entericus.
• The arrival of chyme in the ileum stimulates the
  secretion of the intestinal juice.

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The juice contains four enzymes

• (i) Maltase - speeds up the breakdown of maltose
  to glucose
• (ii) Sucrase - speeds up the breakdown of sucrose
  into glucose and fructose
• Maltase and Sucrase complete the digestion of the
  carbohydrates,
• (iii) Peptidase - speeds up the breakdown of
  peptides into amino acids
• (iv) Lipase - speeds up the breakdown of
  lipids into fatty acids and glycerol
• NB/ The process of digestion is thus competed
  in the ileum. The resulting water emulsion of
• food is called chyle and contains
  soluble products of digestion ready to be
  absorbed.

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Absorption of Digested Food

• ? Absorption is the process by which the soluble
  products of digestion pass through the wall of
  the ileum into the blood stream.
• ? This occurs mainly in the ileum.

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Adaptation of ileum to its functions.

• 1. Long and coiled to allow food enough time to
  be digested and be absorbed/to increase surface
  area for digestion.
• 2. The intestinal lumen has projections called
  villi. The villi have projections called
  microvilli these lead to further increase
  surface area for absorption
• 3. The walls have glands such as crypts of
  lieberkhun which secrete digestive enzymes such
  as maltase ,Sucrase, lactase enterokinase and
  peptidase the walls also has glands called
  brunners glands which has goblet cells which
  produce mucus which protects intestinal wall from
  being digested and reduce friction against the
  wall to reduce abrasion

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Absorption of digested food in the villus

• ? Glucose and amino acids are absorbed across the
  epithelium of the villi by a combination of
  active transport and diffusion.
• ? They then pass via the capillary network to the
  hepatic portal vein to the liver.
• ? From the liver, they are distributed to other
  body parts or converted to suitable storage
  forms.
• ? Fats and glycerol pass into the epithelial
  cells and recombine to form neutral fat,which is
  passed to lympathatic system.
• ? Some vitamins and all mineral salts do not
  undergo digestion but are simply absorbed in the
  ileum through diffusion and active transport.

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Assimilation

• ?Assimilation is the process by which the body
  uses up the absorbed products of digestion.
• ?From the liver the products of digestion are
  transported in the blood stream to the rest of
  the body.
• ?

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Assimilation of glucose
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Assimilation of fatty acids
are oxidized to release energy Forms part of the
structure of the cell membrane Used as an
insulator against excess heat loss in
children/ Used to form layers around organs which
protect them from mechanical injury Act as a
source of metabolic water Stores various types of
vitamins ie. Fat soluble
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Assimilation in Proteins

• Forms body structures.
• Used in formation of enzymes.
• Forms haemoglobin found in red blood cells.
• Forms hormones used to regulate life processes.
• Forms antibodies that provides immunity against
  diseases.
• Is oxidized during starvation to release energy.

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Cont.

• 4. Have openings/ducts which allow bile and
  pancreatic juice into lumen
• 5. Have circular and longitudinal muscles whose
  contraction and relaxation causes peristalsis
  which leads to mixing of food with
  enzymes/digestive juices facilitating rapid
  digestion
• 6. Supplied with blood vessels which supply
  oxygen and transport /absorb digested food taking
  them to storage sites

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cont

• 7. Have lacteals for transport of lipids.
• 8. Have thin epithelial lining of its wall that
  reduce diffusion distance of digested
  food/facilitate rapid absorption /faster
  diffusion.
• 9. The epithelial cells of the villus have
  numerous mitochondria to produce energy inform of
  ATP for active uptake of food substance

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The End