Chapter 33: The human digestive system

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Chapter 33 The human digestive system

• Leaving Certificate Biology
• Higher Level

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Nutrition

• Autotrophic
• Green plants use light
• Some photosynthetic bacteria use light
• Some chemosynthetic bacteria use chemicals
• Heterotrophic
• Organisms that have to obtain their energy from
  preformed food in the environment (animals
  fungi most bacteria)

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Heterotrophic Nutrition

• Omnivore
• Eat both plant and animal material e.g. human,
  bears, badgers
• Carnivore
• Eat only animal material e.g. dogs, cats
• Herbivore
• Eat only plant material e.g. cattle, sheep

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Human Nutrition

• Digestion is the physical and chemical breakdown
  of food into its constituent molecules so that
  they can pass into cells
• Need for digestion in heterotrophs they acquire
  preformed food from environment. Preformed food
  particles, such as carbohydrate, protein, and
  fats are too big to pass directly into the bodys
  cells and so must be broken down into smallest
  possible particles in order to travel across cell
  membranes

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Digestion and Digestive System

• Need for digestion and digestive system
• Digestion heterotrophs acquire preformed food
  from environment, such as carbohydrate, protein,
  and fat particles that are too big to pass
  directly into the bodys cells and so must be
  broken down into smallest possible particles in
  order to travel across cell membranes
• Digestive system heterotrophs have sophisticated
  digestive systems that break down food particles
  and are designed to extract all the nutrients
  from ingested food and transport them into the
  bloodstream, while at the same time ensuring that
  microorganisms are killed and do not infect the
  body

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Human Nutrition

• Ingestion taking in of food into the mouth
• Digestion physical and chemical breakdown of
  food particles into their constituent molecules
• Absorption constituent molecules of the food are
  absorbed into cells that line the digestive tract
  and transported into the bloodstream for
  transport to the liver via the hepatic portal
  vein
• Egestion removal of undigested and unabsorbed
  material from the digestive system

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Digestive System Anatomy
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Macrostructure of Digestive System

• Mouth
• Physical/mechanical (teeth) digestion of food
• Teeth
• Human dental formula 2(I2/2 C1/1 PM2/2 M3/3)
• Incisors (8 in total) cutting food
• Canines (4 in total) tearing food
• Premolars (8in total) grinding/crushing food
• Molars (12 in total) grinding/crushing food
• Chemical digestion of food salivary glands
  secrete amylase which has an optimal pH of 7
• Starch Amylase pH7 ? Maltose Amylase
• Mouths forms a bolus of food that is then
  swallowed

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Dental Decay

• Dental decay is the destruction of tooth structure

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Dental Decay

• Symptoms
• Small holes (caries and/or cavities) appear in
  the teeth especially in areas that are
  difficult to clean with a tooth brush such as the
  grooved surfaces of the premolars and molars and
  between the teeth
• Toothache only occurs when the cavity has
  reached the dentine and pulp areas of the tooth
  where a bacterial infection of the tooth occurs
  and the tooth has to be removed

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Dental Decay

• Cause
• occurs when foods containing carbohydrates
  (sugars and starches) such as breads, cereals,
  milk, fizzy drinks, fruits, cakes, sweets are
  left on the teeth
• Bacteria that live in the mouth digest these
  foods, turning them into acids
• The bacteria, acid, food debris, and saliva
  combine to form plaque, which clings to the teeth
• The acids in plaque dissolve the enamel surface
  of the teeth, creating holes in the teeth called
  cavities, or caries

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Dental Decay

• Treatment
• Depends on how bad the tooth decay is
• Use of fluoride can prevent further decay within
  a cavity
• Fillings cavities can be filled with a
  substances such as metals, metal alloys, plastic,
  porcelain, or a combination of materials
  (composite fillings)
• Severe tooth decay may need a crown or root canal
• Extreme tooth decay may result in the tooth
  having to be removed

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Dental Decay

• Prevention
• Regular brushing (at least 2 times/day) with
  fluoride toothpaste
• Regular flossing (at least daily)
• Healthy diet
• Avoid snacking on high sugar snacks
• Avoid fizzy drinks and alcohol
• Include foods rich in calcium and vitamin D
• Chew gum after a meal

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Macrostructure of Digestive System

• Oesophagus
• Carries food (bolus) from the mouth to the
  stomach by a process called peristalsis
• Peristalsis is the rhythmical contractions of
  smooth muscle of the alimentary canal that has
  the effect of moving food along
• Peristalsis is also a form of mechanical
  digestion
• Fibre stimulates peristalsis thereby preventing
  constipation
• Good movement of food along the alimentary canal
  is important in decreasing the chance of
  contracting colon cancer

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Macrostructure of Digestive System

• Stomach
• Receives food from mouth via oesophagus and
  stores food while the cells of the gastric glands
  secrete
• HCl (parietal cells) kills bacteria and
  denatures protein
• Pepsinogen (chief cells) is zymogen that is
  activated by HCl to pepsin (optimal pH12) which
  then digests proteins to peptides
• Mucous (goblet cells) is slightly alkaline and
  protects the stomach lining
• Cardiac sphincter found at entrance to stomach
  opens when food enters the stomach
• Pyloric sphincter controls the amount of chyme
  that leaves stomach

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Stomach (Peptic) Ulcer

• A peptic ulcer is a small lesion in the lining of
  the stomach wall
• A duodenal ulcer is a small lesion in the
  duodenal wall
• A perforated ulcer is a complete lesion in the
  wall of the stomach/duodenum where the contents
  can leak into the body cavity

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Stomach (Peptic) Ulcer

• Symptoms of an ulcer
• Persistent or intermittent pain in the upper
  abdomen that is relieved by eating or taking
  antacids
• Vomiting
• Fatigue (due to loss of blood due to internal
  bleeding)
• In the case of a perforated ulcer, internal
  bleeding and bacterial infection of body cavity
  (possibly fatal)

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Stomach (Peptic) Ulcer

• Cause
• Lack of mucous or excess HCl is likely to cause
  peptic ulcers
• Ulcers can also be caused by the bacterium
  Helicobacter pylori which if untreated can go
  on to cause stomach cancer
• Ulcers in the digestive system are more likely to
  occur in people who
• Take anti-inflammatory drugs such as aspirin,
  ibuprofen, and naproxen
• Drink alcohol
• Smoke
• Feel stressed

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Stomach (Peptic) Ulcer

• Treatment
• If bacterial, then antibiotics are administered
• If stress-related then time-out and exercise
• If diet-related, then a special diet is made out
  by a dietician
• If too much acid is being produced, then antacids
  are administered
• Stop smoking and/or drinking

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Stomach (Peptic) Ulcer

• Prevention
• Prevention of stomach ulcers would involve
  ensuring good hygiene when preparing food to
  reduce risk of bacterial infection
• Eating balanced, nutritious meals several small
  meals are generally better than 3 big meals
• Exercise as it combats the effects of emotional
  stress
• Avoid overuse of anti-inflammatory drugs
• Do not smoke
• Avoid alcohol

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Macrostructure of Digestive System

• The small intestine
• Consists of three parts
• Duodenum (25 cm) digestion
• Jejunum absorption
• Ileum absorption

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Macrostructure of Digestive System

• Duodenum
• Receives chyme from stomach
• Further digestion occurs
• Secretions from the pancreas (pancreatic juice)
  and gall bladder (bile) are released into
  duodenum
• Pancreatic amylase, lipase and bile salts are
  responsible for digestion within the duodenum

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Macrostructure of Digestive System

• Pancreas
• Secretes pancreatic juice into the duodenum
• Pancreatic juice contains
• Sodium bicarbonate neutralises stomach acid
• Lipase acts on lipids and converts it into fatty
  acids and glycerol (optimal pH of lipase 7)
• Amylase acts on starch and converts it into
  maltose (optimal pH of amylase 7)
• Starch amylase pH7 ? maltose amylase

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Macrostructure of Digestive System

• The liver and its functions in digestion
• Produces bile which is stored in gall bladder
• Bile consists of
• Bile salts secreted into duodenum and emulsifies
  lipids
• Sodium bicarbonate secreted into duodenum and
  neutralises acidic chyme
• Bilirubin and biliverdin breakdown products of
  haem from the haemoglobin of red blood cells

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Macrostructure of Digestive System

• The liver and its other functions
• Receives blood from the jejunum and ileum
  (hepatic portal vein) detoxifies toxins (e.g.
  alcohol)
• Breaks down excess amino acids into urea which
  is then excreted via the kidneys
• Converts excess glucose to glycogen which it then
  stores
• Stores fat-soluble vitamin (A, D, E, and K) and
  trace minerals (iron, copper, zinc)
• Synthesises clotting factors (e.g. fibrinogen)
• Synthesises cholesterol key component of cell
  membranes

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Macrostructure of Digestive System

• Jejunum and ileum
• Absorption of nutrient occurs in this 6 m section
  of the small intestine
• Monosaccharides (breakdown products of starch)
  glucose, fructose, galactose are absorbed
  directly into bloodstream as they are
  water-soluble
• Amino acids (breakdown products of protein) are
  also absorbed into bloodstream as they are
  water-soluble
• Fatty acids and glycerol (breakdown products of
  starch) are absorbed into the lacteals and
  combine with proteins to form lipoproteins which
  are then transported in the lymph to the blood

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Macrostructure of Digestive System

• Small intestine
• Small intestine is very well-adapted for
  digestion and absorption due to
• Length of small intestine (67 m long)
• Internal epithelium is structured into villi and
  microvilli to increase the surface area for
  absorption
• The surface of the villi are only one-cell thick
  in order that absorption into the bloodstream and
  lacteals is very rapid
• Small intestine has a very good blood supply

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Macrostructure of Digestive System

• Large intestine
• The large intestine is 1.5 m long and consists
  of
• Caecum
• Colon
• Rectum

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Macrostructure of Digestive System

• Caecum
• Separated from the ileum by the ileocaecal valve
• It is the beginning of the large intestine
• In humans, the caecum's main functions are to
  absorb fluids and salts that remain after
  completion of intestinal digestion and absorption
  and to mix its contents with mucous

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Macrostructure of Digestive System

• Appendix
• The appendix is a small branch of the caecum
• Its average length is 10 cm
• The appendix is rich in infection fighting cell
  (white cells) suggesting that it may have an
  immune function
• However, the appendix has no function officially

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Appendicitis

• Appendicitis is painful inflammation and swelling
  of the appendix
• Symptoms
• Severe and localised pain in the right, lower
  abdomen
• Cause
• Bacterial growth and build up of toxins and waste
  products
• Treatment
• Surgical removal of the entire appendix
• Prevention
• No known preventative measure

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Macrostructure of Digestive System

• Colon
• The colon consists of three parts
• Ascending, transverse, and descending colons
• The function of the colon is to absorb water
  converting the semi-solid waste that changes into
  faeces
• The colour of faeces is due to the bile pigments
  bilirubin and biliverdin
• Symbiotic bacteria (300 1000 different species
  of bacteria live in the colon!) produce biotin (a
  B-vitamin) and K which are then absorbed through
  the colon wall
• Symbiotic bacteria also help to break down unused
  energy substrates such as small amounts of
  cellulose

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Macrostructure of Digestive System

• Importance of dietary fibre for the colon
• Dietary fibre (also called roughage) is necessary
  to keep the gut healthy by stimulating
  peristalsis and ensuring that the undigested
  material moves along freely without becoming
  stuck
• Constipation results if there is not enough fibre
  in the diet
• Persistent constipation increases the risk of
  colon cancer later in life as the undigested
  material begins to fester due to bacterial action
  and toxins can build up

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Macrostructure of Digestive System

• Rectum
• Stores faeces as it arrives from the colon and
  until it is egested from the body via the anus
• Egestion from the rectum through the anus is not
  excretion as faeces is not a waste product of
  metabolism faeces is the undigested food
  material
• REMEMBER Excretion is defined as the removal of
  the waste products of metabolism from the body
  via the kidneys, skin, and lungs

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Vitamin A Deficiency

• Symptoms
• Night-blindness dry, scaly skin mental
  retardation and retarded growth in children
• Cause
• Inadequate intake of food high in vitamin A
• Treatment
• Increase intake of foods rich in vitamin A such
  as milk, cheese, cream, liver, cod liver oil,
  leafy green vegetable, carrots
• Prevention
• Eat adequate amounts of milk, cheese, cream,
  leafy green vegetables, carrots

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Balanced Human Diet

• A balanced diet is one in which a person receives
  all the essential nutrients in the correct
  proportions to maintain homeostasis
• A variety of foods should be include in the diet
  to maintain good health
• No one group of foods should be taken in excess

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Balanced Human Diet

• An individuals diet will depend on
• Age adolescents, adults and pregnant and
  lactating women will require the largest amount
  of nutrients
• Sex generally males require more calories than
  females (except during pregnancy and lactation)
• Occupation labourers will require more calories
  per day than someone with a desk job
• Activity sports people will require many more
  calories than a sedentary person

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Food Pyramid
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Slimming Disorders

• Anorexia nervosa
• Psychological condition found mostly in women
• They have a perception that they are fat
• They eat very small amounts of food and go on
  strict diets or even go long periods of time
  without eating
• Can be fatal
• Bulimia
• Psychological condition similar to anorexia,
  however, these people generally eat normally
• Difference is that bulimics will induce vomiting
  after a meal
• Harder to treat as they appear normal because of
  their eating patterns