Digestive System

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Digestive System
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By the end of this class you should understand

• The processes of physical and chemical digestion
• The route of food, nutrients and waste through
  the body
• The primary functions of the major organs of the
  digestive system
• The essential process of nutrient absorption

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Digestive Organs

• The digestive system is divided into two types of
  organs
• The alimentary organs that make up the alimentary
  canal
• Accessory organs that do not make up the
  alimentary canal
• The alimentary canal is the tube running through
  your body

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Alimentary Organs

• Mouth
• Pharynx (throat)
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Anus

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Movement

• Organs are moved from one to the next through a
  process called peristalsis (the pushing of food
  through the alimentary canal by smooth muscle)
• In the esophagus, swallowing is initiated by
  skeletal muscle but completed by smooth muscle
  and qualifies as peristalsis
• The stomach, small and large intestine all have
  peristalsis but it is much slower

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Digestive Function

• The digestive system has one goal only to put
  nutrients into the bloodstream so that all our
  cells can access those nutrients
• Many excess nutrients can be stored
• The digestive system has many preparation steps
  to prepare food for this process

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The Most Vital Organ

• Only one organ performs almost all the absorption
  of nutrients (excepting water) the small
  intestine
• So called because it has a small diameter, though
  it is several meters long
• If we drank liquids containing only single
  molecules of nutrients, we would need no other
  digestive organ!
• This is what they inject into your blood when
  youre in a coma

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Two Types of Digestion

• Digestion is the process of breaking down
  nutrients so they can be absorbed
• Undigested nutrients cannot be absorbed and pass
  through the small intestine to the large
  intestine
• For more information, ask someone who is
  lactose-intolerant about drinking milk
• Mechanical (physical) digestion is the chewing
  and churning of food
• Chemical digestion is the breaking of chemical
  bonds in food

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Mechanical Digestion

• The mouth (chewing) and stomach (churning)
  perform mechanical digestion of food
• The mouth accomplishes this with hard calcium
  structures called teeth and heavy muscles
  attached to the mandible chewing
• The stomach has layers of smooth muscles that
  churn the food before it moves to the small
  intestine

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Chemical Digestion

• The mouth and stomach also initiate the process
  of chemical digestion
• The mouth has salivary glands that produce
  saliva, which begins chemical digestion
• The stomach has pepsin, an enzyme that denatures
  and breaks down protein
• Note chemical digestion of fats does not begin
  until the small intestine

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The Stomach

• The stomach produces gastric juice, which
  contains pepsin as well as other chemicals
• This includes hydrochloric acid, which makes the
  stomach environment very acid
• The acid itself is not for digesting the food
• Kills bacteria
• Enables pepsin to work

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Stomach Ulcers

• Stomach ulcers were once thought to be caused by
  stress but are now known to be a bacterial
  infection (but stress does block the immune
  system)
• The scientist who discovered the bacterium that
  causes ulcers was eventually awarded the Nobel
  Prize
• No one had believed him initially so he drank a
  flask of the bacterium to gain ulcers

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Gastric Juice

• Gastric juice is the secretion of the stomach
• Gastric juice is very acidic and can damage
  tissues other than the stomach lining
• Repeated vomiting can cause gastric juice to
  erode the enamel of the teeth
• If gastric juice is produced in excess it can
  overflow into the esophagus, causing heartburn
• Once gastric juice mixes with and digests food it
  is called chyme

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Processing of Chyme

• Chyme is the partially digested food of the
  stomach mixed with acid
• Chyme is delivered into the small intestine very
  slowly
• Excess acid could damage the small intestine
• Helps the intestine absorb all the nutrients
• The small intestine has many digestive enzymes of
  its own that complete digestion

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Accessory Organs

• The small intestine has enzymes on its absorptive
  cells but also receives digestive juices from two
  accessory organs
• The pancreas produces a massive battery of
  digestive enzymes to finish all digestion of food
• Most prominent enzyme is trypsin, a protease that
  breaks down protein into amino acids
• The liver produces bile, which is stored in the
  gallbladder until chyme enters the small
  intestine

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Bile

• Bile does not contain digestive enzymes and does
  not directly perform chemical digestion
• Bile is instead an emulsifying agent, much like a
  detergent
• To emulsify is to allow oil and water to mix
• Bile is therefore vital for digesting and
  absorbing fats from food

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Small Intestine Absorption

• Since nutrients can only be absorbed through cell
  membrane, the small intestine epithelial tissue
  has adaptations for increasing surface area
• The tissue is arranged into fingerlike villi
  (singular villus) which create more surface area
• The individual cells have a brush border, or
  microvilli (singular microvillus)
• These cells expend ATP to actively transport
  nutrients into the blood

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Small Intestine Activity

• When food enters the stomach it still consists of
  starches, proteins and whole fats
• These cannot be absorbed since they are too
  large!
• The small intestine can only absorb the small
  molecules that make these up (simple sugars,
  amino acids, and fat molecules)
• Large molecules that are not digested cannot be
  absorbed!

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Lactose Intolerance

• Lactose Intolerance is one of many digestive
  disorders caused by a failure to absorb nutrients
• Lactose (milk sugar) is made of a molecule of
  glucose and galactose stuck together (too large
  to absorb)
• Most people express a small intestine enzyme
  called lactase that breaks them apart
• Failure to digest lactose (or other nutrients)
  results in the nutrients ending up in the large
  intestine
• Bacteria metabolize it and release gas and waste
  acids

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Liver Function

• Blood flows directly from the small intestine to
  the liver
• This is called hepatic portal circulation
• Protects the rest of the body from whatever you
  just ate
• The liver has metabolic enzymes to process many
  nutrients after they are absorbed
• Production of fat and glycogen
• Storage of vitamins
• Breakdown of toxins such as alcohol

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Large Intestine

• Once all nutrients are absorbed into the small
  intestine, the large intestine (which is mostly
  composed of a tube called the colon) carries
  remaining indigestible materials to the anus
• The large intestine is filled with colonies of
  bacteria that feast on our leftovers (often
  producing gas as a waste product)
• The large intestine is capable of absorbing water
  and vitamins, including those produced by bacteria

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Hopefully that wasnt too much to digest

• Wednesday is nutrition lesson!