The Digestive System

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The Digestive System

• Guts, teeth and glands!
• Images from http//www.whfreeman.com/life/update/
  

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Why Guts?

• Multicellular animals must have specialized
  structures for obtaining and breaking down their
  food.
• There are two processes feeding and digestion.
• Animals are heterotrophs, they must absorb
  nutrients or ingest food sources.

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How to dine...

• Ingestive eaters (us).
• Absorptive feeders (tapeworm)
• Filter feeders (clam)
• Substrate feeders (earthworms)
• Fluid feeders (mosquito)

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Vertebrate Digestion

• The digestive system uses mechanical and chemical
  digestion to breakdown food.

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Tube in a tube

• Vertebrates have a tube-within-a-tube system.
• digestion occurs in the lumen with the nutrient
  molecules being transferred to the blood.

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Stages of digestion

• Movement of food
• Secretion of digestive juices
• Digestion of food into molecules
• Absorption of molecules
• Elimination of undigested food and wastes

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The Digestive System

• Mouth, pharynx,
• esophagus, stomach
• small intestine
• large intestine
• anus
• salivary glands
• pancreas
• liver and gall bladder

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The Start

• In the mouth, teeth, jaws and the tongue begin
  the mechanical breakdown of food.
• Chemical breakdown of starch by amylase
• Mucus moistens food and lubricates the esophagus.
  
• The chewed food and saliva is then pushed into
  the pharynx and esophagus.
• The esophagus uses peristalsis to send the food
  to the stomach.

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Move the food

• Food is chewed and passed to the stomach through
  the esophagus. The name of the movement is
  peristalsis. (see video)

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The Stomach

• Holds 1 to 2L (folds)
• The stomach secretes mucus, hydrochloric acid and
  pepsin. HCl lowers pH of the stomach to activate
  pepsin.
• Pepsin hydrolysis of proteins into peptides.
• The stomach also mechanically churns the food.
  Chyme, leaves the stomach and enters the small
  intestine.
• Ulcers occur when mucus lining is reduced.

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The Small Intestine

• 3 m long tube with coils and folding plus villi.
  Very large surface area!
• Final digestion of all food and absorbtion.
• Villi produce enzymes which complete the
  digestion of peptides and sugars.
• The absorption process in the villi.

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Villi

• Sugars and amino acids go into the bloodstream
  via capillaries in each villus.
• Glycerol and fatty acids go into the lymphatic
  system. Absorption is an active transport,
  requiring cellular energy.

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Duodenum - busy place

• Secretions from the liver and pancreas are used
  for digestion in the duodenum.
• The pancreas secretes digestive enzymes and
  stomach acid-neutralizing bicarbonate.
• The liver produces bile, which is stored in the
  gall bladder before entering the bile duct into
  the duodenum.

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Small intestine - more

• Digestion of carbohydrates, proteins, and fats
  continues in the small intestine. See table in
  text - page 166.
• Bile emulsifies fats so that lipases can
  completely digested lipids.
• Most absorption occurs in the ileum and jejeunum
  (second third of the small intestine).

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Liver and Gall Bladder

• The liver produces bile and helps to detoxify of
  blood
• synthesis of blood proteins
• destruction of old erythrocytes
• storage of glucose as glycogen
• De-amination amino groups and ammonia. (this
  produces urea, less toxic)

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Glycogen-Glucose

• Low glucose levels in the blood cause glucagon to
  stimulate breakdown of glycogen into glucose.
• Insulin helps store glucose and glycogen in the
  liver (see page 929)
• When no glucose or glycogen is available, amino
  acids are converted into glucose in the liver.

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The Large Intestine what to do with left overs!

• The large intestine produces an alkaline mucus
  that neutralizes acids produced by bacterial
  metabolism.
• Water, salts, and vitamins are absorbed, the
  remaining contents in the lumen form feces
  (mostly cellulose, bacteria, bilirubin).
• Bacteria in the large intestine, such as E. coli,
  produce vitamins (including vitamin K) that are
  absorbed.

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Nutrition

• See the basics of nutrition in your text.
• Have a hand out!