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Digestive System Review

Updated on April 30, 2014

Digestion System


Breaking of food particles into smaller and smaller molecules (monomers).

  • Specialized digestive enzymes are needed.
  • These monomers will then be absorbed and then utilized for fuel, structural elements or stored for later use.

The Mouth
The Mouth | Source

The Journey of Digestion

The Mouth

Mechanical Digestion

  • Oral cavity with teeth
  • Different teeth : Different functions

Chemicals in the mouth begin the digestion of carbohydrates

  • Salivary glands secrete the enzyme salivary amylase which breaks down starches into disaccharides.
  • Cleaves internal α-1,4 linkages of starches. (Amylose and amylopectin)



The tongue pushes a bolus of food to the soft palate

  • This initiates swallowing
  • Then the food passes into the esophagus.
  • Food is kept out of the trachea by the epiglottis.


The esophagus is comprised of skeletal muscle and smooth muscle.

Skeletal muscle

  • Coordinates the swallowing reflex.

Smooth muscle

  • Pushes the food toward the stomach via peristalsis.

Peristalsis: waves of coordinated smooth muscle contractions.

Esophagus and Stomach: Sphincters

A sphincter:

  • A ring of muscle.

The lower esophageal sphincter

  • Located at the junction of the esophagus and the stomach.
  • Prevents food from moving backward into the esophagus.

The pyloric sphincter

  • Located at the junction of the stomach and the small intestine.
  • Controls the passage of food into the intestine.

The Stomach
The Stomach | Source


Stomachs are muscular storage chambers that allow for gradual digestion.

  • It churns via muscular contractions in order to mix food with digestive juices.
  • The digestive juices are produced and secreted by specialized cells that line the organ.

Gastric pits in the stomach are lined with three types of secretory cells:

Chief cells

  • Secrete pepsinogen
  • The inactive form (zymogen) of pepsin.
  • Pepsinogen is ‘activated’ by HCl

Parietal cells

  • Secrete HCl
  • Mucus-secreting cells
  • Which protect the stomach from the low pH.

Protein Digestion

Begins in the stomach

Entry of food into the stomach stimulates the gastric mucosa to secrete the hormone gastrin (g-cells).

This stimulates the secretion of:

  • HCl –hydrochloric acid: Denatures the proteins 3D structure making the peptide bonds of the 1° structure more accessible to enzymes.
  • Pepsin: Cleaves the peptide bonds of certain amino acids to make short polypeptides.

Parts of the digestive system shown with blood vessels.
Parts of the digestive system shown with blood vessels. | Source

Off to the Small Intestines...

This mixture of gastric juice and partly digested food is called chyme.

  • So far we have partially digested starches and proteins.

Now the stomach walls contract and move the chyme to the bottom of the stomach.

  • Here the pyloric sphincter allows small amounts to enter the small intestine.

The small intestine has three sections:


  • The first section
  • Primary intestinal site of digestion.
  • Secretions from multiple digestive organs enter the duodenum.


  • The middle section.
  • Site of absorption.


  • The last section.
  • Also a site of absorption.

Accessory organs associate with the small intestine at the duodenum:

  • Liver
  • Pancreas
  • Gall bladder

Image shows positions of duodenum and pancreas
Image shows positions of duodenum and pancreas | Source


The pancreas is an endocrine and exocrine gland.

Endocrine functions include hormone release:


  • Released when blood sugar is high


  • Released when blood sugar is low

Exocrine functions include:

Secretions into the duodenum

  • Digestive enzymes for protein, fat and carbohydrate digestion.
  • Bicarbonate secretion
  • Increases the pH of the chyme

Duodenal fossae
Duodenal fossae | Source

Back to Digestion

In the duodenum:

The acidic chyme is released into the duodenum this stimulates the release of the hormone secretin.

  • Secretin will stimulate the exocrine pancreas to release bicarbonate.
  • Bicarb will increase the pH of the chyme.


The hormone cholecytsokinin (CCK) will be released

CCK will stimulate:

  • The exocrine pancreas to release digestive enzymes.
  • Contraction of the gall bladder. (This is important for fat digestion….)

Pancreatic Protein Digesting Enzymes

Protein digesting enzymes

  • Chymotrypsin
  • Trypsin
  • Carboxypepsidase

All three will continue to digest proteins by hydrolyzing peptide bonds.

  • Until all that is left is amino acids.
  • Remember that protein digestion began in the stomach with HCl and pepsin

Pancreatic and Duodenal Carbohydrate Digesting Enzymes

Pancreatic amylase

  • Will continue to hydrolyze the glycosidic bonds of starches.
  • Until we are left with disaccharides.

Disaccharidases (‘brush border enzymes’)

  • Located in the mucosa of the duodenum.
  • Maltase, lactase, sucrase.

All we are left with now are monosaccharides.

  • Glucose, fructose, galactose.

Protein and Carbohydrate Digestion Complete!

The resultant amino acids and monosaccharides will be transported (absorbed) through the intestinal epithelium.

Utilizing sodium co-transporters:

  • They combine the monomers with Na+ as it diffuses.
  • Monomers then enter the villus circulation and then the liver for further modifications.

Everyone to the Liver!!

Liver cells (hepatocytes) absorb nutrients and will:

Convert them for use or storage

  • Amino acids and other monosaccharides can be converted to glucose via gluconeogenesis
  • To make ATP or Liver glycogen

Send them off for use or storage

  • To make ATP, Muscle glycogen or fat

Detoxify any potential deadly substances that come through the digestive tract.

The liver and gall bladder
The liver and gall bladder | Source

Where the Fats at?

Dietary fats are first broken down into micelles (smaller fat droplets) by bile.

Bile is synthesized from cholesterol in the liver and stored in the gall bladder.

  • Fat entering the duodenum signals the gallbladder to contract.
  • Via CCK
  • It is then released and flows via the common bile duct to the duodenum.

Then pancreatic lipase breaks down the triglycerides of the micelles into its monomers

  • Fatty acids and glycerol.

Fatty Acids

Diffuse through the intestinal mucosa

Reassemble into triglycerides

Get packaged with proteins and cholesterol: Chylomicrons

  • Will enter the lymphatic system via the lacteals
  • Then the bloodstream to their target tissue.

Image shows front view of the iliac, sigmoid, and pelvic colons, and rectum.
Image shows front view of the iliac, sigmoid, and pelvic colons, and rectum. | Source

The Large Intestine

What ever is not digested or absorbed in the SI passes into the large intestine, or colon.

  • The colon absorbs water and ions and produces feces.
  • Feces are comprised of bacteria, water and undigested foodstuff.
  • Too much water absorption in the colon leads to constipation
  • Too little leads to diarrhea.

Feces will be ‘stored’ in the rectum and evacuated via the anus.


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    • annajazz profile image

      Anna Marie 3 years ago from New Mexico


      Low production of stomach acid is also known as Hypochlorhydria.

      Acid reflex is actually a very common symptom of Hypochlorhydria.

      The public is often led to believe that this is only true for over acid production, which is simply not true. Acid reflex and GERD are much, much more likely to be from low acid (for some reason the population is lead to believe that it is caused by high acid production). The burning that people feel because of this disorder is due to the fumes/chemical break down of the food that is basically rotting in the stomach because it has yet to be broken down all the way and sent to the intestines.

      Other symptoms of this disorder include: Feeling bad after eating meat (as the low acid is unable to properly digest it), and burping or bloading after eating.

      High Acid levels on the other hand:

      Hyperchlorhydria or too much stomach acid is rather rare actually.

      Most symptoms of this include:

      Abdominal pain



      Vomiting of sour liquid or "water brash"

      Gas and Bloating




      Now for poor digestion, this is normally not a problem for those with

      an over production of stomach acid as everything gets broken down. Though, these individuals do tend to be on the thinner side.

      For the actual pH levels:

      pH levels and acidity can be associated with heartburn. The contents of the stomach are highly acidic, with a pH level of 2.0. It can cause symptoms of gastroesophageal reflux disease such as acid regurgitation, heartburn and chest pain. Excess acidity in the gastric juice in the stomach can lead to heartburn discomfort. The main cause of heartburn is irritation of the lower esophagus due to stomach acid. Generally, the esophagus has higher pH than the stomach.

      Hope this helps, if you would like me to elaborate, just let me know.


    • someonewhoknows profile image

      someonewhoknows 3 years ago from south and west of canada,north of ohio

      Could you tell me if you have any info on poor digestion relating to too high P H. I've been told that there are individuals who get the symptoms of acid reflux when in fact they have low acid rather than too much acid in their stomach.