Lecture 4

• Stomach

  • Mechanical breakdown of food continues, this includes the churning of food across the rugae (lines in the wall (muscles) of the stomach)
  • Gastric juices are released (HCl, mucus, pepsinogen)
  • HCl chemically breaks down food and kills foreign substances (eg. bacteria), HCl converts pepsinogen into active form pepsin. 
  • Pepsin breaks down proteins into shorter chains
  • The food becomes a semi-liquid called chyme
  • Mucus protects stomach from being digested by HCl
• The Pyloric Sphincter 

  • Controls the movement of food into the small intestine. Moves like a drawstring on a bag. 
  • At this point the food is semi liquid and 5% digested
  • The undigested matter is known as chyme. 
• More on the stomach…

  • The innermost mucosa of the stomach is folded into deep pits called GASTRIC PITS.
  • These pits are lined with specialized cells that secrete various substances that aid in digestion. Collectively, all of these secretions are called Gastric JUICES.
• The GASTRIC JUICES are composed of:

  • Hydrochloric Acid (HCl) - kills bacteria entering with food and converts inactive pepsinogen to active pepsin.
  • Water & Salts
  • Pepsin active form of the enzyme pepsinogen, that initiate the digestion of protein
  • Mucus - secreted by mucosa cells and a THICK layer of mucus protects the stomach’s inner layer (wall) from HCl and pepsin → as both can degrade the protein in the cell membranes of the stomach cells 
• More on the Pepsin..

  • PEPSIN breaks down PROTEINS and/or LARGE POLYPEPTIDES into SMALLER POLYPEPTIDES
  • Pepsinogen is the inactive form of PEPSIN (active form) 
  • This activation only occurs in an acidic environment (low pH_ of HCl in the stomach.
  • Protein digestion begins in the stomach, but it is not complete
  • The small polypeptides, produced after the action of pepsin, will undergo further chemical digestion in the SMALL INTESTINES, to be broken down into their amino acids subunits…
• Small Intestine

  • About 2.5 cm wide, 7 m long
  • Coiled Loop
  • There is Gastric Juice here as well to help with chemical digestion of chyme
• Three parts of the small intestine:

  1. Duodenum: first 25 cm where the majority of digestion occurs
  2. Jejunum: 3m long
  3. Ileum: 4 m long 
• Small Intestine - Duodenum

  • Chemical digestion of food
  • Has tubes (ducts) leading from liver and pancreas
  • Bile and digestive enzymes enter from the ducts and are mixed with the chyme 
  • Disaccharidases (sucrase, maltase, lactase) are released to further break down carbs 
• Small Intestine - Jejunum & Ileum 

  • Responsible for nutrient absorption 
  • Molecules of fat, protein and carbohydrates get absorbed into the bloodstream and then are transported to liver 
  • Pushes undigested material into large intestine
• Absorption in Small Intestine

  • The interior of the small intestine is covered in finger-like tube extensions called villi (singular = villus)
  • villi greatly increase the surface area of the small intestine thus increased absorption 
  • Each villus has hair like extensions called microvilli (singular = microvillus)
  • These will also increase the surface area of the interior small intestine and thus increase absorption
• Characteristics of Villi

  1 - Project into the SI, and are bathed in the digested nutrient molecules 

  2 - one cell layer thick (thin enough for absorption of nutrient molecules EASILY)

  3 - contain blood vessels (capillaries) and a lymph vessel 

  • (lacteal) inside of them to absorb the nutrient molecules 
• Label this

  A) Large Intestine

  B) Transverse Colon

  C) Ascending Colon

  D) Appendix / Cecum

  E) Anus

  F) Rectum

  G) Descending Colon

  
• Large Intestine - Colon

  • water and dissolved minerals are absorbed from undigested food

  • bacteria break food down further and produce vitamins (B12, K) and amino acids

  • forms and expels feces
• Large Intestine - Rectum

  • acts as temporary storage area for feces
• Large Intestine - Anus

  • egestion of wastes by peristalsis
  • sphincters control the release of feces