The Digestive System

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The Digestive System

• Miss Ulrich

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Obtaining and Processing Food

• All organisms need a way to obtain essential
  nutrients
• Organisms that depend on organic molecules
  manufactured by other living things are called
  heterotrophs (organisms that lack chlorophyll
  animals, fungi)
• Survival depends on organic molecules synthesized
  by autotrophs (directly or indirectly)
• Organism that can nourish themselves using
  inorganic materials (water carbon dioxide) are
  called autotrophs (photosynthetic organisms)
• Self-sufficient
• Autotrophs produce food, will heterotrophs
  consume it.

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Feeding Devices and Behaviours for heterotrophs

• Types of Feeding
• Filter Feeding Strain food from water. Feed
  continuously and small amounts (e.g., blue
  whale, clams, oysters, mussels, ducks, flamingos,
  etc.)
• Fluid Feeding feed on the juices of plants and
  animals (e.g., mosquitoes, tapeworm)

Q How long can a tapeworm grow to be?
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Feeding Devices (Contd)

• Many animals , including a large number of
  insects, ingest large amounts of food at a time,
  and therefore not continuously.
• Great variety of feeding devices displayed
• For Example Mammals
• Grasping trunk of elephant
• Mobile lips and tongue of a cow
• Hands of human

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Digestion Essential Food Processing

• In larger organisms (e.g., mammals), cells are
  numerous and packed in tissues from the internal
  surface of the digestive tract.
• Most of digestive tract is not permeable to
  substances in food
• Solution to this problem development of
  digestive system and circulatory system
• Digestive system breaks down food masses that
  can be absorbed by circulatory system
• Circulatory System transports substances to
  individual cells where they are absorbed

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Digestion

• To fully understand digestion we need to explore
  some of the processes and anatomical structures
  involved. We will be looking at
• The digestive tract through which food travels
• Mechanical digestion how food moves
• Chemical digestion
• Accessory organs and their contribution to the
  digestive system

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The Human Digestive System
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4 Steps to Food Processing

1. Ingestion the taking in of nutrients
2. Digestion The chemical and physical breakdown of
   complex organic substances into smaller ones
3. Absorption The taking up of digested molecules
   into the cells of the digestive tract
4. Egestion The removal of wastes from the body

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The Human Digestive System

• Foods taken into the body consist of large
  complex organic compounds.
• Digestion must occur in order to release the
  nutrients contained within the food.
• Digestion will break down the large complex
  organic compounds into smaller, simpler units
  that can be absorbed and used by the cells of the
  organism.

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Two Types of Digestion

• Mechanical Digestion
• Physical breaking up of food into smaller pieces
  by the teeth.
• The tongue manipulates the food into a mass
  called the bolus
• The squishing action in the esophagus and
  intestines further break up the food mass
• The Churning action of the stomach muscles
  contracting to mix food with the digestive juices
  in the stomache

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Two Type of Digestion

• Chemical Digestion
• Separation of food into its molecular components
  using chemicals like stomach acid and digestive
  enzymes
• Begins in the mouth with salivary amylase which
  breaks down sugars into simpler sugars
• Majority takes place in the stomach with the help
  of hydrochloric acid and in the small intestine
  with the help of various digestive enzymes

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The Digestive Tract - Mouth

• Equipped with a number of teeth lined up on upper
  and lower jaws.
• Three sets of salivary glands produce saliva
  which moistens food entering the mouth making it
  easy to swallow
• Saliva production is stimulated by smell, hunger
  and taste of food
• Contains salivary amylase
• Tongue covered in papillae that contain taste
  buds
• Uvula prevents food from going up into the
  pharynx when we swallow

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Pharynx

• Made up of muscular walls containing
• Trachea tube going to the lungs
• When you swallow the epiglottis closed over the
  glottis preventing food from entering the
  trachea
• Esophagus Muscular tube going to the stomach
• Muscles expand and contract pushing food down to
  the stomach Peristalsis
• No chemical digestion takes place here

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Esophagus

• Made up of circular and longitudinal muscles
  which expand and contract to move food to the
  stomach by peristalsis
• The circular muscle ring at the lower end of the
  esophagus (near the entrance of the stomach) is
  thickened to give the individual some involuntary
  control over the movement of food

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Stomach

• J-shaped sac in the middle of the digestive tract
• Has two sphincters
• Cardiac sphincter allows food into the stomach
  and keeps acid from enter the esophagus
• Pyloric sphincter regulates and releases the
  amount of food entering the small intestine

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Stomach - Mechanical Digestion

• After eating, the stomach walls begin to contract
  to mix and churn food with gastric juices
• This mixture is called chyme
• GROSS! This is the stuff that comes up when you
  get sick
• Food may remain in the stomach for 2 to 6 hours
  after eating (longer if you eat before going to
  bed)
• May stretch to hold up to 2 litres of food or
  liquid

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Stomach Chemical Digestion

• Gastric glands found in the wall of the stomach
  release gastric juices
• Production is stimulated by
• Thought, sight or smell of food
• Food entering the stomach
• Stretching of stomach wall
• Some Digestion takes place here water, some
  medicine, and alcohol

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Gastric Juice Contains

• Hydochloric acid (HCl)
• Lowers the pH to about 2, which is necessary for
  the digestion of proteins
• Pepsinogen (an inactive protein)
• Enters the stomach and comes into contact with
  HCl and is converted to pepsin (active enzyme)
• Pepsin breaks protein into polypeptides (chunks
  of protein).

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Small Intestine

• Subdivided into three parts
• Duodenum (First 25 cm after the stomach)
  U-shaped, Pancreative and bile ducts open into
  here making it a site for chemical breakdown of
  partially digested material from the stomach
• Jejunum (The next 2 metres) breaks down
  remaining proteins and carbs so the end product
  can be absorbed, more folds than duodenum
• Ileum (the last 5 metres) fewer and smaller
  villi, absorb nutrients, push remaining
  undigested material into large intestine
• This is the site of most digestion, along with
  nutrient absorption
• The small intestine has many folds and
  finger-like projections called villi, along with
  their microvilli drastically increase the surface
  area inside the small intestine increasing the
  amount of nutrient absorption that can take place.

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Villi and Microvilli

• All six essential nutrients are absorbed into
  the capillaries located in the microvilli

See page 340 in your textbook
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Small Intestine Mechanical Digestion

• Peristalsis
• Rhythmical segmentation is a form of peristalsis
  that mixes and pushes partially digested food
  through the small intestines
• Emulsification
• Fats are broken down into smaller droplets (not
  chemically changed) by bile
• Bile is produced in the liver and stored in the
  gall bladder, it enters the duodenum via the
  common bile duct
• Bile activates the lipases that will later
  chemically break down fats

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Small Intestine Chemical Digestion

• Two major glands involved in digestion in the
  small intestine
• The Pancreas
• The Intestinal glands
• More on this tomorrow..

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The Large Intestine

• Consists of the caecum, colon, rectum and anal
  canal
• Caecum
• Blind end of the large intestine
• Appendix
• Plays no role in digestion but may help fight
  infections hangs from the Caecum
• Colon
• Divided into 3 sections
• Ascending (up)
• Transverse (across)
• Descending (down)

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From Colon to Anus

• In the colon water and dissolved minerals are
  absorbed into the blood from undigested food
• Bacteria help release more nutrients by further
  breaking down food
• The mass of indigestible material left behind is
  called feces
• Feces passes into the rectum and anal canal
• The anal sphicter allows limited control over
  the elimination of waste material from the body
  via the anus

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Accessory Organs

• Pancreas secretes enzymes to break down fat
• Liver (largest internal organ) secretes bile,
  which digests fats
• also stores iron
• More on this tomorrow

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Homework

• Read Sections 10.1 and 10.2