Topic 14: Digestive System

Question 1

What are the 4 basic processes involved in digestion?

Answer 1

1. Digestion: breakdown food into molecules
   a) mechanical – motility
   b) chemical – enzymes with acid
2. Absorption
3. Motility
   - smooth muscle contractions in tract:
   > mix food/chyme
   > move food/chyme (peristalsis)
4. Secretion
   - digestive juice into lumen

Question 2

What are the processes of chemical digestion in the mouth?

Answer 2

i. Carbs ~~> disaccharides + oligosaccharides

ii. triglycerides ~~> FAs + monoglycerides

Question 3

What do salivary glands do?

Answer 3

> secrete saliva that moistens + prevents caries (contains lysozyme, antibodies).
PSNS – stimulates secretion of thin, enzyme rich saliva
SNS – mucus rich saliva

Question 4

Swallowing occurs in 3 phases. What are they?

Answer 4

1. Buccal Phase (voluntary)
   - food is compacted by the tongue into a “bolus” -moves to pharynx by tongue pressing on hard palate
2. Pharyngeal Phase (involuntary)
   - swallowing center (medulla) initiates swallowing reflex
3. Esophageal Phase
   - food moves down esophagus via peristalsis (wave-like contractions of smooth muscle).

Question 5

What is the breakdown of muscle control during swallowing?

Answer 5

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

Question 6

Food cannot re-enter the mouth, nasopharynx, and trachea. Why?

Answer 6

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)

Question 7

How does the STOMACH chemically digest carbs, lipids, and proteins respectively?

Answer 7

1. carbs: salivary AMYLASE remains active in the stomach until pH < 7
2. lipids: lingual LIPASE (tongue), gastric LIPASE (chief cells) are active until pH < 5-6
3. proteins: HCl denatures proteins and acts on inactive pepsinogen to create the active enzyme PEPSIN ~> breaks down proteins into peptides & AAs

Question 8

The small intestine is the primary site for digestion and absorption. Chemical digestion involves secretions from what?

Answer 8

i. pancreas
ii. gall bladder
iii. intestinal wall

Question 9

What is the first step in chemical digestion in the small intestine?

Answer 9

NEUTRALIZE ACID

Acidic chyme in duodenum triggers release of secretin:
• ⬆️ HCO3 (neutralize acid)
• ⬇️ gastric acid secretion

Question 10

What stimulates the secretion of the hormone cholecystokinin (CCK) from enteroendocrine cells of the small intestine? What are its effects?

Answer 10

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

Question 11

What does a deficiency in lactase cause?

Answer 11

Causes lactose intolerance: lactose remains undigested and passes into large intestine where bacteria will ferment it, resulting in symptoms like gas & diarrhea

Question 12

How do intestinal wall enzymes breakdown proteins?

Answer 12

Peptides

(*aminopeptidases & dipeptidases) ~~~~>

smaller peptides & AAs

Question 13

How do intestinal wall enzymes breakdown carbs?

Answer 13

di- + oligosaccharides
(*di./oligo. -ases) ~~~>
monosaccharides

eg. lactose
(*lactase) ~~~>
glucose & galactose

Question 14

Why is the small intestine the main site of absorption?

Answer 14

a) large surface area (folds, villi, microvilli)

b) nutrients digested into molecules that are small enough to be absorbed

Question 15

Approximately how much H2O is absorbed at the small intestine every day?

Answer 15

~8500 ml/day absorbed

Question 16

Where do the nutrients absorbed in the small intestine go to?

Answer 16

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)

Question 17

Fat digestion & absorption requires bile from the liver (synthesizes) and the gall bladder (stores). What does bile consist of?

Answer 17

• water
• bile salts (derived from cholesterol)
• bile pigments (bilirubin from RBC breakdown)
• ions (HCO3-)
• detoxified/inactivated drugs, toxins, hormones, etc.

Question 18

What are the functions of bile salts?

Answer 18

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

Question 19

What is the rectal (defecation) reflex of the large intestine?

Answer 19

• 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

Question 20

Describe the motility of the large intestine

Answer 20

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

Question 21

Describe the digestion in the large intestine

Answer 21

• NO digestion, but bacterial fermentation of undigested nutrients
• bacteria synthesize some vitamins (B6, B5, K, folate, biotin)

Question 22

What does the large intestine mainly absorb?

Answer 22

• H2O ~ 400 ml/day via osmosis (~100 ml/day lost in feces)
• some fermentation products - gases, vitamins
• drugs (rectally) – e.g. anaesthetics

Question 23

What happens to glucose once it is absorbed?

Answer 23

• 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

Question 24

What happens to lipids once they are absorbed?

Answer 24

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)

Question 25

What happens to amino acids once they are absorbed?

Answer 25

- enter cells by secondary active transport or facilitated diffusion - GH ⇑ entry into most cells - insulin ⇑ entry into skeletal muscle cells - in cells, AAs used mainly for protein synthesis - AAs are NOT stored – excess is converted to fat - can be used for energy (ATP synthesis) when glucose ⬇️

Question 26

The metabolic rate is the the rate of energy use in the body. It is the sum of all chemical reactions and mechanical work. What are factors affecting the metabolic rate?

Answer 26

1) SNS: ⬆️M.R. 2) hormones (e.g. epinephrine, thyroid hormone): ⬆️M.R. 3) body temp - 1°C rise ⇒ 10% ⬆️M.R. 4) exercise: ⬆️M.R. 5) food ingestion: ⬆️M.R. 6) sleep: ⬇️M.R.

Question 27

What is the basal metabolic rate (BMR)?

Answer 27

Energy body needs for essential activities (e.g. HR)

Question 28

Summarize the hormones from the stomach and duodenum

Answer 28

1. Gastrin: a) ⇑ acid, gastric enzyme secretion (lipase & pepsinogen) 2. Secretin: a) ⇑ alkaline fluid secretion (duct cells of pancreas, gallbladder) b) ⇓ gastric secretion 3. CCK: a) ⇑ pancreatic enzyme secretion b) ⇑ contraction of gallbladder c) ⇓ gastric motility, secretion

Question 29

In the ENTEROCYTES of the SI, products of fat digestion (FAs & monoglycerides) undergo reassembly into triglycerides and packaging to make them water soluble for transport in the lymph/blood. Describe the steps

Answer 29

1. reassembly of triglycerides ~> in smooth ER, monoglycerides & FAs acted on by enzymes that convert them back into triglycerides 2. *chylomicron* formation ~> in golgi, the triglycerides are packaged w/ cholesterol, phospholipids, and proteins to form water soluble package 3. exocytosis of chylomicrons ~> chylomicrons packaged into vesicles and released into ISF 4. transport into lacteal ~> chylomicrons diffuse into lymph lacteal (too large for blood capillaries)

Question 30

Once stomach acid is neutralized in the small intestine, pancreatic enzymes are secreted into the intestinal lumen to continue digestion. What enzymes help digest carbs, proteins, and lipids, respectively?

Answer 30

Carbs: *pancreatic amylase* Proteins: i. trypsinogen > *enteropeptidase* > trypsin ii. trypsin activates proteases iii. proteases break down proteins Lipids: *lipase (w/ bile salts)*