F.2 STRUCTURES AND FUNCTIONS OF THE LOWER DIGESTIVE TRACT

The introduction of acid chyme into the duodenum from the stomach stimulates several responses.

• The duodenum is very narrow compared to the stomach and cannot accept much material at any one time, so when it begins to fill, it quickly distends (widens).

• This stimulates mechanoreceptors that signal back to the stomach to decrease gastric activity and constrict the pyloric sphincter.

• As space in the duodenum becomes available, the pyloric sphincter relaxes and allows more material to enter. This may happen only once or twice a minute allowing the passage of only a few drops of acid chyme each time.

• The inner walls of the duodenum are further equipped with endocrine glands and chemoreceptors.

  • These chemical-sensitive nerve endings detect the low pH and partially digested proteins and carbohydrates in acid chyme.

  • In response, they stimulate the release of two different hormones from endocrine tissue.

    • Secretin is released in response to the acidic nature of the material, while the release of CCK (cholecystokinin) is triggered by the biomolecules themselves.

  • These hormones target the liver and pancreas to promote the release of additional digestive secretions.

LIVER AND PANCREAS

• The liver, located posterior to the diaphragm, is the body’s largest internal organ.

• The stomach is tucked up in between its lobes.

• Richly supplied with blood, the liver has many functions, one of which has a direct relationship to the digestive system.

  • The liver produces bile, a salty green liquid, which it stores in the gall bladder until CCK signals its release along the bile duct and into the duodenum.

  • Bile is an emulsifier of lipids. It mechanically breaks fatty clusters into smaller pieces, a process called emulsification. Fats do not mix well in the watery interior of the digestive system, but in an emulsified form, they have a greater surface area, which allows the enzyme that digests them to be more efficient.

The pancreas lies just below the stomach.

• The pancreatic juice it produces is released in response to the presence of both secretin and CCK.

• This complex secretion, which enters the duodenum through the pancreatic duct, has 6 components:

  • 4 enzymes

  • A buffer

  • Water

• The buffer in pancreatic juice is sodium bicarbonate, which dissociates to release bicarbonate ions.

• These ions react with the abundance of hydrogen ions in acid chyme and drastically raise the pH and maintain it at about 8.3, an environment that will optimize the action of the pancreatic enzymes. Acid chyme is now called chyme .

Each of the 4 enzymes in pancreatic juice promotes the digestion of one of the four types of biomolecules in food.

• Of these, lipase, catalyzes the hydrolysis of lipids, simplifying them to fatty acids and glycerol molecules.

• Nucleic acids, not a large part of our diet, are broken apart into nucleotides by enzymes known as nucleases (digest DNA and RNA).

• The digestion of polysaccharides began in the mouth at a fairly neutral pH by salivary amylase. Pancreatic juice also contains pancreatic amylase, which resumes this process. The product of amylase activity is the disaccharide maltose.

• The digestion of proteins began in the stomach with their conversion into polypeptides. These chains of amino acids are broken into shorter pieces called peptides by the protease trypsin, produced by trypsinogen. The enzyme enterokinase activates trypsinogen.

SMALL INTESTINE

• The pancreatic enzymes function in the duodenum.

• The activity of these enzymes greatly reduces the complexity of the biochemicals in chyme.

• The small intestine also produces and secretes intestinal juice, which contains additional enzymes.

  • Disaccharides like maltase, sucrase and lactase complete the digestion of carbohydrates into monosaccharides that can be absorbed.

  • Peptidases break any remaining peptide bonds, producing individual amino acids.

  • The small intestine also produces and releases nucleases, which complete the digestion of nucleic acids into nucleotides.

  • Finally, nucleotidases break these monomers into their three components for absorption.

All the while, peristalsis moves chyme along the small intestine.

• By the time it reaches the ileum, the longest part, the chemical digestion processes are essentially completed.

• The mucosal layer of the ileum is lined with finger-like projections called villi, which are specialized for absorption, namely transporting the products of digestion from the chyme into the epithelial cells that make up the outer surface of the villi.

• These cells have extensive surface area due to minute cytoplasmic extensions called microvilli creating a “brush border” surface.

• As well, the membranes of these mitochondria-rich cells have specific transport proteins required for moving the various monomers from chyme into the cytoplasm.

• These cells also contain enzymes that reconstruct the products of fat digestion into neutral fats.

• Inside each villus is a bed of loose connective tissue into which extends a capillary bed and an absorptive end of the lymphatic system called a lacteal.

• The newly synthesized fat molecules slip through the cell membranes and enter the lacteals.

• The rest of the products of digestion are moved through the epithelial cells and across the membrane into the interior of the villi where they enter the blood stream.

In summary, the small intestine has several functions.

• Receives and produces secretions that further the digestion of food particles.

• Transports chyme from the stomach to the large intestine.

• Lined with villi and microvilli – specialized for absorption.

PREPARATION OF FECES AND DEFECATION

• Once the available nutrients from the food are absorbed, the remains (water plus undigestible materials like cellulose) pass from the ileum through the ileocaecal valve (another sphincter) into the colon (large intestine).

The first segment of the colon is called the caecum.

• The appendix, commonly considered a vestigial structure, extends downward from the caecum.

The major roles of the colon are to absorb a great amount of the water that was added to the food material all the way along the digestive system, and to house E.coli, which are anaerobic bacteria that metabolize the remaining organic material.

• This is a symbiotic relationship because while they are obtaining nutrients from the waste materials, their activity also releases minerals and manufactures some vitamins and amino acids, which get absorbed along with the water into the circulatory system.

• Their activity also begins the decomposition of the material and converts it to feces with a changed colour, smell and texture.

• The last part of the colon, the rectum, temporarily stores feces until defecation.

• The end of the rectum is equipped with the anal sphincter, which controls defecation through the anus.