Digestion Overview
There are two parts to the mammalian digestive system:
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Alimentary canal
- Mouth
- Oral cavity (teeth and tongue)
- Pharynx
- Esophagus
- Stomach
- Small intestine
- Large intestine
- Rectum
- Anus
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Accessory glands
- Salivary glands (3)
- Pancreas
- Liver
- Gall bladder (storage organ)
Focus on Protein Digestion
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Remember that proteins are long chains of amino acids.
- There are 20 different amino acids.
- Humans can make 12 from the raw ingredients of carbon, nitrogen, oxygen, etc.
- Therefore, we must get the remaining 8—the “essential amino acids"—from our diet.
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The essential amino acids can be obtained from any of these food items:
- Milk
- Eggs
- Meat
- In general, most animal products—because the animals themselves must already have these amino acids.
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Very few plants contain all eight.
- Therefore, vegetarians must get the appropriate mixture of food to insure that they get all eight essential amino acids.
- This requires eating complementary proteins (i.e. proteins that “complement” each other with different amino acids).
- The exceptions are oats, soybeans and quinoa, plants which do contain all eight.
A Trip Through the Digestive Tract
Pre-protein Digestive Pathway
The Oral Cavity
- Digestion begins in the oral cavity.
- Teeth aid in mechanical breakdown of food.
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The kind of dentition a mammal has indicates what it eats:
- Carnivores have large canines and sharp pre-molars and molars for ripping and tearing flesh.
- Herbivores have no canines, but have very sharp scissor-like incisors since they must cut vegetation. Their molars are flat and good for grinding.
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Omnivores have dentition that is somewhere in between.
- Humans are omnivores, and have therefore evolved to eat both meat and vegetation.
- Tell all your vegetarian friends that if humans were meant to be vegetarians, they wouldn't have any canines.
- Evolution doesn't lie—humans must be meat eaters!
Pharynx to Esophagus
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Two tubes descend from the pharynx.
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Trachea
- Location: in front of the neck
- Function: passageway to the lungs
- Rings of cartilage (which you can feel in your neck) help prevent the trachea from collapsing when a breath is taken.
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Esophagus
- Location: behind the trachea
- Function: passageway to the stomach
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Epiglottis
- A small piece of tissue that prevents food from going down the trachea.
- It temporarily covers the opening to the trachea during a swallow.
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Word derivation:
- The prefix epi means “upon.”
- The opening to the trachea is called the glottis.
- Epiglottis: “upon the glottis”
Swallowing is a Reflex
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When not swallowing…
- Esophageal sphincter is contracted.
- Epiglottis is up.
- Glottis is open—air can flow to the lungs.
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The swallow reflex:
- Triggered by a bolus of food in the pharynx.
- Esophageal sphincter relaxes: esophagus opens and the bolus of food can now enter.
- Larynx moves upward causing the epiglottis to tip closed.
- Food is prevented from entering the larynx.
- Once a swallow is started the nervous system completes it—it is a reflex.
Peristalsis: Moving Food Along
- The entire digestive tract from esophagus to anus is lined with two types of muscle: circular muscle and longitudinal muscle.
- It is the alternate contraction and relaxation of these two groups of muscles that moves food through the entire alimentary tract.
- This process is done without conscious control—it is completely automatic.
Protein Digestion Begins Here
Stomach
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Stomach contains gastric juice:
- Water
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Hydrochloric acid
- Gives the stomach a low pH.
- Denatures proteins.
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Pepsin
- An enzyme that chops up proteins into smaller pieces
- In technical terms, it catalyzes the hydrolysis of proteins.
- When digestion is done, the stomach produces chyme: digested material.
Small Intestine
- Most digestion occurs here.
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Duodenum
- The first foot of the small intestine.
- The most important part of the entire digestive tract.
- The pancreas acts as an accessory organ, as outlined below.
The Pancreas and Small Intestine: a Closer View
From the pancreas to the duodenum:
- Bicarbonate (HCO3-) to neutralize the acidic chyme.
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A number of (initially inactive) digestive enzymes, activated once inside the duodenum:
- Trypsin and chymotrypsin: break up peptide bonds at specific amino acids.
- Carboxypeptidase and aminopeptidase: break up peptide bonds one amino acid a time beginning from amino or carboxyl end.
The Remaining Length of the Small Intestine
- The rest of the small intestine is involved with absorption of the digested material.
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Note the amazing folding:
- It increases the surface area dramatically, to that of a tennis court!
- It packs the six meter long intestine into a small space.
Protein Digestion Review