Digestive System, Lecture 4

Question 1

Small Intestine - absorption (fats)

Answer 1

• large fat molecule (fat globules) not very soluble watery intestinal juices
  “pre treatment” or emulsification
• pieces of triglycerides from globule are coated in bile salts and phospholipids converting large fat molecules into smaller emulsification droplets (still have triglycerides just smaller)
• bile salts and phospholipids have a polar side (water liking) and nonpolar side (fat liking)
• surround triglycerides with polar side facing out:
  ◦ makes more water soluble
  ◦ repels other emulsion droplets (one way process)
  ◦ exposes more surface area to digestive action of
  pancreatic lipase which breaks each triglyceride to
  1 monoglyceride (glycerol bound to 1 fatty acid
  chain) and 2 “free” fatty acid chains
  without emulsion: 49-51% fat absorbed
  with emulsion: 96-98% fat absorbed

Question 2

Small Intestine - absorption (fats) micelle

Answer 2

• monoglycerides and fatty acids cross apical membrane by simple difussion
• if too much monoglycerides and fatty acids released - will reform triglycerides that cannot cross apical - so must not overload with monoglycerides and fatty acids
  micelle
• bile salts surrounding monoglycerides and fatty acids to prevent reforming triglyceride
• similar to emulsification droplet only micelle a smaller structure
• temporary stage to control amount of monoglycerides and fatty acids for diffusion across apical membrane
• also tend to migrate to apical membrane before breaking down - helps bring monoglycerides and fatty acids in contact with absorptive surface for diffusion (a little more water soluble, so there are near membrane and can release by)
• trying to prevent everything from going back the other direction

Question 3

Small Intestine - absorption (fats) epithelial cell

Answer 3

• once inside epithelial cell monoglycerides and fatty acids reform triglycerides plus combine with phospholipids and cholesterol to form a chylomicron (lipoprotein for transport) inside a vesicle (big again)
  exocytosis
• vesicle fuses with basolateral membrane
• membrane opens up and release chylomicron into interstitial fluid
  my notes:
• we had to simple diffusion get into cell by creating small structures, but once we get in a big structure forms again called chylomicron composed of triglycerides, phospholipids and cholesterol
• only gets through basolateral membrane by exocytosis

Question 4

Small Intestine - absorption (absorbed carbohydrates/proteins & chylomicrons)

Answer 4

absorbed carbohydrates and proteins (head to liver)
- move from interstitial fluid into blood capillaries
- smaller blood vessels join to form the portal vein
- circulate to liver for usage, storage or processing

absorbed fats - chylomicrons
- too large to enter blood capillaries
- move from interstitial fluid to lacteal (lymphatic capillary with large pores)
- once in lacteal flows into other lymphatic vessels eventually enters bloodstream near subclavian veins (can go through the subclavian veins)
- circulates to liver and adipose where enzyme lipoprotein lipase break down chylomicrons releasing monogylcerides and fatty acids which enter liver and adipose for usage and storage

my notes:
- carbohydrates and proteins essentially enter bloodstream (simple diffusion as they are quite small) and then head to liver
- chylomicron moves from interstitial fluid through one of the pore into the lacteal

Question 5

Large Intestine - bacterial digestion and absorption

Answer 5

secretions are minimal and lack digestive enzymes - mostly mucus and water secretion for lubrication and formation of feces
functions:
- absorbs more water and electrolytes (ex. sodium, potassium, chloride) / bacterial products (~10% of absorption in large intestine)
- storage of non-absorbed material in distal large intestine till defecation (storage until we eliminate it out of the body)
- in large intestine ~12-24 hours depending on composition
- bacterial flora (10 million discrete types): “microbiome”
◦ ferment some undigested carbohydrates - gases
◦ convert some undigested fiber to short chain
fatty acids
◦ produce small amounts of vitamins (vitamin B
complex and vitamin K)

Question 6

Large Intestine - mechanical digestion

Answer 6

haustral churning
- contractions of smooth muscle surrounding haustra produces a segmentation like motion
- filling cause distention (stretching) of haustral walls - respond by contracting - pushes material to adjacent haustra (backwards/forwards)

mass movement (pushing everything to end of system)
- 3-4x/day
- wave of intense peristalsis spreads rapidly from transverse colon towards rectum (“sweeping”)
- stimuli:
◦ food entering stomach triggers gastrocolic reflex
(neural)
◦ gastrin (hormonal)
- often leads to urge to defecate

my notes:
- pushing everything from large intestine to rectum (mass movement)
- if we get a signal up in stomach that food is incoming to large intestine, mass movement can clear the food still in stomach from before

Question 7

Defecation Reflex (final stage)

Answer 7

• rectal wall distension detected by mechanoreceptors (when food arrives in rectum)
• “urge to defecate” signal sent to medulla oblongata and spinal centers
• response: contraction rectum and relaxtion of internal anal sphincter plus initial contraction external anal sphincter
• pressure reached in rectum: relaxation external anal sphincter (done after the initial contraction) -> when pressure reaches certain level
• feces expelled
  voluntary control - learned behaviour
• higher brain centers can:
  ◦ over ride external anal sphincter relaxation (not
  allowing it to be expelled)
  ◦ feces moved from rectum back into sigmoid colon
  reducing stretch of rectal walls (lowers urges to
  defecate and pressure on rectum)
  ◦ once material back in sigmoid colon rectum
  relatively empty so “urge to defecate” signal goes
  away temporarily
• as contents come back into rectum (next mass movement)
• defecation reflex cycle would starts again
• restart “urge to defecate” signal