Overview of the Digestive System

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Overview of the Digestive System
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Digestive System

• Consider for a moment a Big Mac
• The purpose in your eating a Big Mac, other than
  simple hedonism, is to assimilate the nutrients
  it represents and make them available to build,
  repair and maintain your own tissues, as well as
  provide energy for studying and occasional other
  pursuits.

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Digestive System

• "Exactly what nutrients are present in a Big Mac
  that I can assimilate?"
• A Big Mac is composed of
• "Two all beef patties (largely proteins and
  triglyceride, or neutral fat), special sauce
  (??), lettuce (cellulose, which you cannot
  assimilate to any great extent), cheese (again,
  protein and triglyceride), pickles (cellulose,
  starch), onions (cellulose, starch) on a sesame
  seed bun (starch)."

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Digestive System

• In terms of raw materials, a 200 gram Big Mac
  consists of roughly
• 25 grams of protein
• 28 grams of fat
• 47 grams of carbohydrate
• Add these all up and you get a total of 530
  Calories.

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Digestive System

• MacDonald's comes close to full disclosure in
  this regard,
• but what they don't tell you is that in order to
  take advantage of these nutrients, you have to
  provide the means to carefully break them down
  into much smaller molecules that can be imported
  into blood.
• Luckily, your digestive system takes care of this
  very complex process so efficiently that most of
  the time you don't even need to think about it.

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Digestive System

• At its simplest, the digestive system is a tube
  running from mouth to anus. This tube is like an
  assembly line, or more properly, a disassembly
  line.

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• Its chief goal is to break down huge
  macromolecules (proteins, fats and starch), which
  cannot be absorbed intact, into smaller molecules
  (amino acids, fatty acids and glucose) that can
  be absorbed across the wall of the tube, and into
  the circulatory system for dissemination around
  your body.

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• The breakdown of foodstuffs like a Big Mac is
  accomplished through a combination of mechanical
  and enzymatic processes.
• To accomplish this breakdown, the digestive tube
  requires considerable assistance from accessory
  digestive organs such as the salivary glands,
  liver and pancreas, which dump their secretions
  into the tube.
• The name "accessory" should not be taken to mean
  dispensable indeed, without pancreatic enzymes
  you would starve to death in short order.

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Digestive System

• In many ways, the digestive system can be thought
  of as a well-run factory in which a large number
  of complex tasks are performed.
• The three fundamental processes that take place
  are

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Digestive System

• Secretion Delivery of enzymes, mucus, ions and
  the like into the lumen, and hormones into blood
• Absorption Transport of water, ions and
  nutrients from the lumen, across the epithelium
  and into blood
• Motility Contractions of smooth muscle in the
  wall of the tube that crush, mix and propel its
  contents

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Digestive System

• Control of digestive function is achieved through
  a combination of electrical and hormonal messages
  which originate either within the digestive
  system's own nervous and endocrine systems, as
  well as from the central nervous system and from
  endocrine organs such as the adrenal gland.

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Digestive System

• Different parts of these systems are constantly
  talking to one another.
• The basic messages are along the lines of "I just
  received an extraordinary load of food, so I
  suggest you get prepared" (stomach to large
  intestine) or
• "For goodness sake, please slow down until I can
  catch up with what you've already given me"
  (small intestine to stomach).

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Basic Functional Anatomy of the Digestive System
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Basic Functional Anatomy of the Digestive System

• Mouth Foodstuffs are broken down mechanically by
  chewing and saliva is added as a lubricant. In
  some species, saliva contains amylase, an enzyme
  that digests starch.
• Esophagus A simple conduit between the mouth and
  stomach - clearly important but only marginally
  interesting compared to other regions of the
  tube.
• Stomach Where the real action begins - enzymatic
  digestion of proteins initiated and foodstuffs
  reduced to liquid form.
• Liver The center of metabolic activity in the
  body - its major role in the digestive process is
  to provide bile salts to the small intestine,
  which are critical for digestion and absorption
  of fats.

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Basic Functional Anatomy of the Digestive System

• Pancreas Important roles as both an endocrine
  and exocrine organ - provides a potent mixture of
  digestive enzymes to the small intestine which
  are critical for digestion of fats, carbohydrates
  and protein.
• Small Intestine The most exciting place to be in
  the entire digestive system - this is where the
  final stages of chemical enzymatic digestion
  occur and where almost almost all nutrients are
  absorbed.
• Large Intestine Major differences among species
  in extent and importance - in all animals water
  is absorbed, bacterial fermentation takes place
  and feces are formed.

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Microanatomy of the Digestive Tube
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Review of Food Chemistry

• The diet of any animal contains hundreds if not
  thousands of different molecules,
• The bulk of the ingested nutrients are in the
  form of huge macromolecules that cannot be
  absorbed into blood without first being reduced
  to much simpler and smaller forms
• Even table sugar (sucrose) cannot be absorbed
  without first being enzymatically ripped apart
  into glucose and fructose.
• The most important enzymatic reaction in
  digestion of foodstuffs is hydrolysis - the
  breaking of a chemical bond by the addition of a
  water molecule.

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Proteins

• Proteins are polymers of amino acids linked
  together by peptide bonds.
• Very short proteins, typically 3 to 10 amino
  acids in length, are called peptides.
• Although very small peptides can be absorbed to a
  limited degree, for all intents and purposes,
  proteins must be reduced to single amino acids
  before they can be absorbed.
• Enzymes that hydrolyze peptide bonds and reduce
  proteins or peptides to amino acids are called
  proteases or peptidases.

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Lipids

• Fatty acids are present in only small amounts in
  animal and plant tissues, but are the building
  blocks of many important complex lipids.
• True fatty acids possess a long hydrocarbon chain
  terminating in a carboxyl group.

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Lipids

• The most abundant storage form of fat in animals
  and plants, and hence the most important dietary
  lipid, is neutral fat or triglyceride.
• Triglycerides cannot be efficiently absorbed, and
  are enzymatically digested by pancreatic lipase

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Carbohydrates

• Monosaccharides or simple sugars are either
  hexoses (6-carbon) like glucose, galactose and
  fructose, or pentoses (5-carbon) like ribose.
• These are the breakdown products of more complex
  carbohydrates and can be efficiently absorbed
  across the wall of the digestive tube and
  transported into blood.
• Disaccharides are simply two monosaccharides
  linked together by a glycosidic bond. The
  disaccharides most important in nutrition and
  digestion are
• lactose or "milk sugar" glucose galactose
• sucrose or "table sugar" glucose fructose
• maltose glucose glucose

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Carbohydrates

• Polysaccharides are the most abundant dietary
  carbohydrate
• Starch is a major plant storage form of glucose
• Cellulose is the other major plant carbohydrate
• Glycogen is the third large polymer of glucose
  and is the major animal storage carbohydrate.

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END OF PART ONE