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 major nutrients the body requires for life
• The major risks associated with excess and
  deficiencies of food

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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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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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Vital Nutrients

• The small intestine absorbs all the nutrients
  required for life
• Some nutrients are required for energy and for
  building blocks for more cells and cell parts
• Carbohydrates
• Proteins
• Lipids
• Other nutrients are required for homeostasis but
  not energy
• Water
• Vitamins
• Minerals

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Carbohydrates

• A single carbohydrate molecule is called a simple
  carbohydrate (such as glucose)
• Starches are made of many bound together and are
  called complex carbohydrates
• Some are more complex than others, and the more
  complex the branching, the longer the digestion
  takes
• White starches such as white rice and pasta
  contain simpler chains
• Complex carbs, since they break down more slowly,
  are less likely to cause blood sugar to spike

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Proteins

• Proteins must be broken down to the individual
  amino acids
• The human body can synthesize many of the amino
  acids from other amino acids (so they are
  nonessential), but there are certain ones that
  cannot be synthesized
• These are called essential amino acids
• If even one of the essential amino acids is
  missing, the diet is incomplete and no new
  protein can be made

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Lipids

• There are many classes of lipids
• Some vitamins and cholesterol are both lipids,
  but cannot be burned for energy
• The energy-storing lipids are fatty acids, and
  these fats can be saturated or unsaturated
• Saturated fats have no double bonds and so pack
  together more tightly than unsaturated fats
• Treating unsaturated fats with hydrogenation can
  create trans fats, which are very difficult to
  metabolize and therefore very unhealthy

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Vitamins and Minerals

• Vitamins are organic compounds that serve key
  purposes in cell mechanisms but only in small
  amounts
• Example vitamin A used to make retinal
• Minerals are inorganic elements and compounds,
  such as sodium, potassium, calcium, iron, and
  iodine
• Also necessary for life

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Hopefully that wasnt too much to digest

• Enjoy your Thanksgiving weekend!
• See you next week for the last week of new
  lessons!
• Tuesday Urinary system
• Thursday Reproductive system