Topic 14: Digestive System Flashcards
What are the 4 basic processes involved in digestion?
- Digestion: breakdown food into molecules
a) mechanical – motility
b) chemical – enzymes with acid - Absorption
- Motility
- smooth muscle contractions in tract:
> mix food/chyme
> move food/chyme (peristalsis) - Secretion
- digestive juice into lumen
What are the processes of chemical digestion in the mouth?
i. Carbs ~~> disaccharides + oligosaccharides
ii. triglycerides ~~> FAs + monoglycerides
What do salivary glands do?
> secrete saliva that moistens + prevents caries (contains lysozyme, antibodies).
PSNS – stimulates secretion of thin, enzyme rich saliva
SNS – mucus rich saliva
Swallowing occurs in 3 phases. What are they?
- Buccal Phase (voluntary)
- food is compacted by the tongue into a “bolus” -moves to pharynx by tongue pressing on hard palate - Pharyngeal Phase (involuntary)
- swallowing center (medulla) initiates swallowing reflex - Esophageal Phase
- food moves down esophagus via peristalsis (wave-like contractions of smooth muscle).
What is the breakdown of muscle control during swallowing?
a) somatic motor neurons to skeletal muscle ⇒ upper 1/3
b) PSNS (via vagus nerve) to smooth muscle ⇒ lower 1/3
c) middle 1/3 = mix of both
Food cannot re-enter the mouth, nasopharynx, and trachea. Why?
Mouth: b/c the tongue is pressed up against the hard palate
Nasopharynx: b/c the uvula and soft palate are elevated, closing off the nasopharynx
Trachea: b/c the glottis and epiglottis are closed during swallowing (as a result breathing temporarily stops when we swallow)
How does the STOMACH chemically digest carbs, lipids, and proteins respectively?
- carbs: salivary AMYLASE remains active in the stomach until pH < 7
- lipids: lingual LIPASE (tongue), gastric LIPASE (chief cells) are active until pH < 5-6
- proteins: HCl denatures proteins and acts on inactive pepsinogen to create the active enzyme PEPSIN ~> breaks down proteins into peptides & AAs
The small intestine is the primary site for digestion and absorption. Chemical digestion involves secretions from what?
i. pancreas
ii. gall bladder
iii. intestinal wall
What is the first step in chemical digestion in the small intestine?
NEUTRALIZE ACID
Acidic chyme in duodenum triggers release of secretin:
• ⬆️ HCO3 (neutralize acid)
• ⬇️ gastric acid secretion
What stimulates the secretion of the hormone cholecystokinin (CCK) from enteroendocrine cells of the small intestine? What are its effects?
The presence of FAs and AAs in the duodenum stimulate release of CCK
> EFFECTS:
i. simulates (⬆️) pancreatic enzyme secretion
ii. stimulates gallbladder contractions – releases bile into duodenum
iii. decreases gastric motility (slows emptying of stomach) and ⬇️gastric secretions
What does a deficiency in lactase cause?
Causes lactose intolerance: lactose remains undigested and passes into large intestine where bacteria will ferment it, resulting in symptoms like gas & diarrhea
How do intestinal wall enzymes breakdown proteins?
Peptides
(*aminopeptidases & dipeptidases) ~~~~>
smaller peptides & AAs
How do intestinal wall enzymes breakdown carbs?
di- + oligosaccharides
(*di./oligo. -ases) ~~~>
monosaccharides
eg. lactose
(*lactase) ~~~>
glucose & galactose
Why is the small intestine the main site of absorption?
a) large surface area (folds, villi, microvilli)
b) nutrients digested into molecules that are small enough to be absorbed
Approximately how much H2O is absorbed at the small intestine every day?
~8500 ml/day absorbed
Where do the nutrients absorbed in the small intestine go to?
a) to liver via hepatic portal vein (blood)
b) into lacteals (lymph), and are carried through the lymphatic circulation (including thoracic duct) to the L subclavian vein where they enter the blood (lacteal ~> thoracic duct ~> L subclavian vein ~> blood)
Fat digestion & absorption requires bile from the liver (synthesizes) and the gall bladder (stores). What does bile consist of?
• water
• bile salts (derived from cholesterol)
• bile pigments (bilirubin from RBC breakdown)
• ions (HCO3-)
• detoxified/inactivated drugs, toxins, hormones, etc.
What are the functions of bile salts?
a) emulsification: breaks up larger globules of fat
b) micelle* formation: keeps products of fat digestion in solution
*micelle = vesicle-like structure formed from bile salts & phospholipids that is filled w/ monoglycerides and FAs
→ interior = hydrophobic
→ exterior = hydrophilic
What is the rectal (defecation) reflex of the large intestine?
- stimulus = feces in rectum (stretch receptors)
- CNS integration = in sacral segment of spinal cord (PSNS)
- Effector = smooth muscle of rectum contracts, internal anal sphincter relaxes
- external anal sphincter (skeletal muscle) = under voluntary control i.e. NOT part of reflex
Describe the motility of the large intestine
1) Haustral contractions
→ slow, weak - move material down tube (allow mixing, absorption of salts, H2O)
2) Mass Movements
→ due to food in stomach, via gastrocolic reflex = powerful waves of contractions from transverse colon to rectum
→ move fecal mass to rectum - initiates urge to defecate
Describe the digestion in the large intestine
- NO digestion, but bacterial fermentation of undigested nutrients
- bacteria synthesize some vitamins (B6, B5, K, folate, biotin)
What does the large intestine mainly absorb?
- H2O ~ 400 ml/day via osmosis (~100 ml/day lost in feces)
- some fermentation products - gases, vitamins
- drugs (rectally) – e.g. anaesthetics
What happens to glucose once it is absorbed?
- from blood into body cells (carrier mediated facilitated diffusion) ⇒ insulin ⇑ uptake into skeletal muscle cells, liver cells , fat cells (adipocytes)
- in cells glucose is used for:
a) catabolism by cells ⇒ ATP production
b) anabolism ⇒ stored glucose as glycogen
What happens to lipids once they are absorbed?
a) cholesterol used to make steroid hormones, bile salts, cell membranes
b) triglycerides in blood delivered to tissues
→ lipase on the endothelial cells of organs breaks down triglycerides into 3 FAs and glycerol
c) FAs and glycerol transported into tissue cells, where they can be:
i. used for ATP production (except by neurons)
ii. stored as triglycerides in adipose tissue
iii. used to synthesize cell parts (e.g. membranes, myelin)
