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Mammalian physiology > Digestive system - Lectures 21-22-23 > Flashcards

Digestive system - Lectures 21-22-23 Flashcards

1
Q

What are the 4 functions of the digestive system?

A
  1. motility: movement of material through the GI tract as a result from muscle contraction
  2. secretion: movement of material form cells into lumen or ECF
  3. digestion: chemical and mechanical breakdown of food into absorbable units
  4. absorption: movements of material from GI lumen to ECF
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2
Q

what are the 2 parts of digestive system?

A
  1. tubing/elementary canal: all the tubing from mouth to anus –> directly open to external environment so no homeostasis
  2. accessory organs: ie liver, pancreas, salivary glands
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3
Q

Is the length of all your digestive tubings the same when alive vs dead?

A
  • alive: around 5-7 m –> lots of tonic contraction
  • dead: less contraction: length can increase to 10m
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4
Q

difference between chyme and bolus

A
  • bolus: ball of food that is mixed with saliva –> moves from mouth to oesophagus
  • chyme: when food gets into stomach + mixed with gastric secretions + starts to get digested
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5
Q

products of digestion are absorbed from tubing into what into what into what
- waste is excreted from where?
- digestive system is exposed to ______ ________ –> so, it is home to __________ __________

A
  • absorbed from tubing into epithelium into interstitial fluid into blood/lymph
  • anus
  • external environment –> GI tract is home to commensal microorganisms
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6
Q

digestive system tubing: from oral cavity where there is _______(action) and ______ glands –> into (5) (un à la suite de l’autre) + different parts

A
  • oral cavity: chewing + salivary glands
  • mouth –> pharynx –> esophagus –> stomach (fundus –> body –> antrum) –> small intestine (duodenum –> jejunum –> ileum) –> large intestine (colon –> rectum –> anus)
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7
Q

which part of GI tract has skeletal muscle? vs smooth muscle?

A
  • skeletal: mouth, pharynx and beginning of esophagus
  • smooth: everything else (ie esophagus, stomach, small intestine, large intesting)
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8
Q

what are the 4 layers of the digestive system?

A
  1. mucosa
  2. submucosa
  3. muscularis externa
  4. serosa
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9
Q

what are the 3 layers of mucosa?

A
  1. MUCOSAL EPITHELIUM:
    - 1 layer of epithelial cells
    - has goblet cells (secrete mucus) and enteroendocrine cells (secrete hormones)
  2. LAMINA PROPRIA
    - connective tissue + lymphatic tubules + small blood vessels + lots of immune cells
    - peyer’s patches of gut-associated lymphoid tissues (GALT) –> group of immune cells with protective role
  3. MUSCULARIS MUCOSAE
    - smooth muscle cells with mucosa
    - small contraction forms folds (rugae and plicae) to increase surface area
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10
Q

what is in the submucosa? (2)

A
  1. connective tissue + blood vessels + bigger lymphatic tubules (compared to the ones in mucosa)
  2. contains submucosal plexus of the enteric nervous system!
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11
Q

what is in the muscularis externa? (2)

A
  • consists of 3 layers of smooth muscle:
    1. oblique (only in stomach!): contraction will make stomach twist (like squeezing a towel)
    2. circular: decrease diameter of tubing on contraction
    3. longitudinal: decrease length of tubing on contraction
  • contains myenteric plexus of the enteric nervous system btween the circular and longitudinal muscles –> has afferent, integrating and efferent neurons!
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12
Q

what consists of the serosa?

A
  • outer covering of the digestive tract –> 1 layer of connective tissue
  • continuation of the peritoneal membrane, which forms sheets of mesentery
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13
Q

difference between rugae and plicae?

A

RUGAE:
- in stomach
- before eating, stomach is super small/folded –> around 50 mL VS folds disappear when you start eating to allow volume to increase to 1.5L
PLICAE:
- folds that are always there, in small and large intestines

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14
Q

what are the modifications in GI tract to increase surface area?

A
  1. rugae and plicae = foldings
  2. villi: further projections on the folds + microvilli (mostly in small intestine where most absorption and digestion)
  3. gastric glands, crypts (envaginations = opposite of villi) and submucosal glands
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15
Q

what are the 2 things from the enteric nervous system in layers of GI tract? in which layers?

A
  1. submucosal plexus in submucosa
  2. myenteric plexus in muscularis externa
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16
Q

short vs long reflex?

A
  • short = only involves neurons in submucosal and myenteric plexus (to increase or decrease secretions/motility)
  • long = involve neurons outside (ie cerebral cortex, vague nerve)
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17
Q

GI smooth muscles exhibit different patterns of contraction: (3)

A
  1. MIGRATING MOTOR COMPLEX
    - series of contraction that begin in the empty stomach and end in the large intestine
    - similar to peristalsis but happening between meals –> for housekeeping
    - push indigestible food forward or whatever is remaining
  2. PERISTALSIS
    - moves bolus forward
    - circular muscles contract one by one
    - can be very strong and go against gravity (even if you’re upside down)
  3. SEGMENTAL CONTRACTIONS
    - alternate contraction and relaxation
    - major function = mixing! chyme with digestive juice
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18
Q

what causes the different patterns of contraction in GI tract to have motility?

A
  • slow-wave potentials that originate in interstitial cells of Cajal (located in submucosa and muscularis externa)
  • cells are regulated by external PSNS (stimulate contraction) and SNS (inhibit motility)
  • slow wave potential = spontaneous depolarizations that don’t always reach threshold –> if it does, creates action potential
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19
Q

what are the 3 phases in digestive system?

A
  1. cephalic phase: before food enters until right before food gets into stomach
  2. gastric phase: food is in stomach
  3. intestinal phase: food leaves stomach and enters small intestine
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20
Q

explain how the 4 functions of digestive system happen (or not) in the mouth

A
  1. motility: muscles in cheek, chewing, swallowing
  2. secretion: saliva, amylase, lyzosome to kill bacteria
  3. digestion: almost 0% bc food is short time in mouth
  4. absorption: almost 0% except nitroglycerin (put on tongue, absorbs quickly into blood vessels to increase BP)
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21
Q

what initiates the cephalic phase?

A
  • smelling, seeing or thinking about food –> sends signal to cerebral cortex –> hypothalamus (control center for eating) –> medulla oblongata –> vagus nerve –> target organs: receptive relaxation (ie rugae relax) + secretion of gastric juices (10-15% before food even enters)
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22
Q

what are the 4 functions of salivary secretions

A
  1. soften en lubricate food
  2. digestion of starch (chemical)
  3. taste
  4. defense (lysozomes)
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23
Q

what are the 3 functions of stomach?

A
  1. storage (upper stomach) = biggest function!
  2. digestion (lower stomach) of lipids and proteins
    - enzymes (pepsinogen), acids (pH = 1.0 to break down protein and kill bacteria) paracrine signal molecules and hormones
  3. defense against swallowed pathogens
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24
Q

when does the digestive activity in stomach begin? (2 steps)

A
  1. begins with long vagal reflex of cephalic phase (sight –> cerebral cortex –> hypothalamus –> medulla –> vagus nerve –> target organs)
  2. when food enters stomach –> it initiates short reflexes of gastric phase
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25
Q

do all foods stay in stomach the same amount of time?

A

no! carbs stay around 1h vs fat can stay around 5-6 hours = feel more full

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26
Q
  • gastric glands are found in ______ _______
  • what cells secrete gastrin? vs gastric acid? vs pepsinogen and gastric lipase? vs histamine? vs intrinsic factor? vs somatostasin?
A
  • gastric pits!
  • Gastrin-releasing peptide (GRP) or G cells secrete gastrin
  • parietal cells secrete gastric acid (HCl)
  • chief cells secrete pepsinogen (activated to pepsin) and gastric lipase
    PARACRINE SECRETIONS:
  • enterochromaffin-like (ECL) cells secrete histamine
  • parietal cells secrete intrinsic factor
  • D cells secrete somatostatin (SS)
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27
Q

how does stomach protect itself from autodigestion? (2)

A
  1. mucus-bicarbonate barrier (protect stomach epithelial cells from high acid)
  2. quick turnover in GI tract
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28
Q

where are mucous cells located in stomach? (2)
- secrete what? (2)
- what stimulus causes release? (2)

A
  • luminal surface and neck of gastric glands
  • mucus and bicarbonate
  • mucus secreted by tonic secretion + irritation of mucosa
  • bicarbonate secreted with mucus
29
Q

what type of cells secrete HCl and intrinsic factor?
- what are the functions of those 2 substances?
- what stimulus causes release?

A
  • parietal cells
  • HCl: activates pepsin + kills bacteria
  • Intrinsic factor: complexes with vitamin B12 to permit its absorption
  • both are stimulated by acetylcholine, gastrin and histamine
30
Q

what do enterochromaffin-like cells secrete?
- function of the secretion?
- stimulus for release?

A
  • histamine!
  • stimulates gastric acid secretion
  • acetylcholine and gastrin
31
Q

what do chief cells secrete? (2)
- functions?
- stimulus for release?

A
  • pepsinogen –> digests proteins
  • gastric lipase –> digests fats
  • both are stimulated by acetylcholine and acid secretion
32
Q
  • what do D cells and G cells secrete?
  • function?
  • stimulus for release?
A
  • D cells –> somatostatin –> inhibits gastric acid secretion –> stimulus = acid in stomach
  • G cells –> gastrin (1 of major hormones in GI tract) –> stimulates gastric acid secretion (just like histamine)
  • stimulus = acetylcholine, peptides and amino acids
33
Q

what are the 3 main hormones in GI tract?

A
  1. gastrin
  2. CCK
  3. secretin
34
Q

what does acetylcholine stimulate release of? (5)
- where does acetylcholine come from?

A
  • stimulates release of HCl, intrinsic factor, histamine, pepsinogen and gastrin
  • from vagus nerve!
35
Q

mucus vs bicarbonate
- chemical or physical barrier?

A
  • mucus = physical barrier
  • bicarbonate = chemical barrier that neutralizes acid (pH = 7 at surface of cells vs gastric juice pH = 2.0)
36
Q

how is HCl generated/secreted in stomach?

A
  1. water dissociates into H+ and OH-
  2. H+ is pumped out of parietal cell into lumen by ATPase while K+ is pumped in (and returns to lumen via channel)
  3. OH- + CO2 becomes HCO3- via carbonic anhydrase
  4. HCO3- exists parietal cell into interstitial fluid (basolateral membrane) via antiporter (Cl- goes down [ ] gradient and enters parietal cell)
  5. Cl- flows out of parietal cell into lumen (apical surface) via channel to follow H+
37
Q

how can G cells increase HCl production?
(schéma!)

A
  1. food (aa or peptides in lumen of stomach) or cephalic reflexes (input via vagus nerve) initiate gastric secretion of gastrin (via G cells), histamine and acid
  2. gastrin + input from vagus nerve stimulates ECL cells –> product histamine
  3. histamine + gastrin + input via vagus nerve stimulate parietal cells to produce H+
  4. acid/H+ stimulates short reflex secretion of pepsinogen by chief cell via enteric sensory neuron + H+ allows conversion of pepsinogen to pepsin
  5. H+ –> negative feedback on D cells that secrete somatostatin which inhibits G cell, ECL cells, chief cells and parietal cells
38
Q

motility in small intestine is controlled by what?
- why? (2)

A

segmental and peristaltic contractions
- to ensure that small intestine is not overwhelmed by chyme + need to complete digestive process and allow for absorption

39
Q

gastrical emptying
- how?
- regulated by (3)
- what can inhibit gastric emptying?

A
  • strong contractions in lower stomach to push very liquid fragments in small intestine: every contraction = 3mL moves from stomach to small intestine
  • regulated by nervous system + CCK and secretin –> secretion of these hormones in duodenum is stimulated by chyme getting in duodenum
  • CCK will also go back to stomach to inhibit stomach functions (?)
  • fat in small intestine!
40
Q

what can be absorbed in the stomach?

A

alcohol! (can be absorbed in stomach (a little) and in small intestine)
- eating food while drinking alcohol prevents alcohol to go into small intestine and get absorbed = not much effect on brain VS drinking on empty stomach –> all of it is quickly absorbed in small intestine and you get drunk faster

41
Q

how is surface area maximized in intestine? (3)
- to which extent?

A

villi (x10), crypts and brush border/microvilli (x20)
- folds (x 3)
- TOTAL: increase surface area by 600!

42
Q

absorbed nutrients from intestinal epithelium must go through _______ system for distribution (through the what system?) except which macronutrients that uses what other circulation system?

A
  • circulatory system for distribution
  • through hepatic portal system
  • fats use lymphatic circulation (via chylomicrons) to enter circulatory system
43
Q

what can be found at the bottom of crypts in small intestine?

A

stem cells! regenerate new cells and push them up to replace new ones

44
Q

what are the 2 blood supplies of liver?

A
  1. hepatic artery/vein which bring ox blood to liver to feed hepatic cells
  2. hepatic portal vein which brings blood that’s been deoxygenated already in the capillaries of the digestive tract
45
Q

why do blood from digestive system have to pass through 2 capillary beds? which 2?

A
  1. capillaries of digestive tract (stomach, intestine, pancreas, spleen)
  2. capillaries in liver (sinusoidal capillaries (like swiss cheese)) for 2 reasons:
    a) to process nutrients like glucose and amino acids! we don’t want glu to directly go to blood for circulation: it’s gonna increase blood glucose way to rapidly –> glu goes to liver for glycogenesis
    b) some toxins might get absorbed in small intestine –> need to be detoxified in live
46
Q

what is the motility in the stomach?

A
  • gastric emptying + mixing!
  • no typical peristalsis and segmental contractions
47
Q

is the 3mL from stomach hyper or hypotonic when it reaches small intestine?

A
  • hypertonic! bc digestion already started so lots of broken down pieces of ie maltoses/glucoses = increase osmolarity (vs 1 big starch molecule = low osmolarity)
  • affects water movement in small intestine –> eventually, becomes isotonic in small intestine
48
Q

what are the 3 sources of secretion in small intestine?

A
  1. intestinal/gut cells (around 3L)
  2. exocrine part of pancreas (around 750mL)
  3. liver –> bile (around 250 mL)
    - for a total of around 4L per day
49
Q

what are 5 intestinal secretions that promote digestion? from where are these secreted?

A
  1. digestive enzymes: mostly pancreas but also intestinal cells
  2. bile: liver
  3. bicarbonate secretions: intestinal cells and pancreas
  4. mucus: goblet cells
  5. isotonic NaCl secretion: intestinal cells and pancreas –> cystic fibrosis transmembrane conductance regulator (CFTR channel)
50
Q

How is NaCl secreted in intestine? 4 steps

A
  1. Na, K, Cl enter intestinal cell (on basolateral side) by cotransport (from Na/K/Cl/Cl channel)
  2. Cl- enters lumen through CFTR channel (on apical side)
  3. Na+ is reabsorbed: goes back into intestinal fluid through ATPase Na/K pump
  4. negative Cl- in lumen attracts Na+ by paracellular pathways: Na+ passed btw intestinal cells to go from interstitial fluid to lumen. water follows due to osmolarity!
51
Q

what are the 2 portions of pancreas and what do they secrete?

A

ENDOCRINE PORTION (islets):
- secretes insulin and glucagon
EXOCRINE PORTION:
- secrete digestive enzymes (acini) and sodium bicarbonate (duct cells)
- enzyme secretion: brush border enteropeptidase converts tyrpsinogen to trypsin
- bicarbonate secretion: neutralizes gastric acid

52
Q

2 parts of pancreatic duct?
- do they have a sphincter?
- does something merge with them?

A

MAIN part:
- most secretions go through this
- merges with bile duct from liver
- controlled by sphincter of Oddi –> opens during meals!
- when closed during meals, bile goes in gall bladder + other secretions go through accessory duct
ACCESSORY DUCT:
- how secretions leave pancreas in between meals when sphincter of Oddi is closed
- no sphincter!

53
Q

what are the pancreatic acini?

A
  • form the exocrine portion of the pancreas
  • secrete enzymes!
54
Q

is the ratio of enzymes (lipase/protease/amylase/nuclease) the same every day? or does it change depending on what you eat?

A

same ratio every day! –> can change overtime based on long term diet change (ie vegetarian)

55
Q

what are zymogens?

A

zymogens are inactive enzymes that are secreted –> secreted in their inactive form bc you don’t want them to act on the acini or on your own useful enzymes!
- zymogens will become activated by trypsin (?) in small intestine

56
Q

How is bicarbonate secreted in the pancreas/intestine? (4 steps)

A
  1. H2O + CO2 –> HCO3- + H+ through carbonic anhydrase activity in the pancreatic or duodenal cell
  2. using chloride gradient (from lumen to cell) to have energy to transport HCO3- from cell to lumen
  3. Cl- leaves cell (reestablish gradient) through CFTR channel to get to lumen
  4. leaky junctions allow paracellular movement of ions and water –> negative ions in the lumen attract Na+ by paracellular pathways. water follows
57
Q

cystic fibrosis affects secretion of (2) in small intestine/pancreas
- which one should we worry about?

A
  • cystic fibrosis = no CFTR channels
    1. affects Cl- secretion in small intestine –> dont worry
    2. affects bicarbonate secretion in pancreas –> increase bicarbonate concentration can block (?) –> so enzymes can’t go into small intestine which affects lipase/fat digestion! = need to worry about!
58
Q

Bile
- secreted by what?
- composition (3)
- 2 functions?
- stored where

A
  • secreted by hepatocytes
  • bile salts (made from cholesterol –> bile acids + Na+ = bile salts) + bile pigments + cholesterol
  • aids fat digestion and absorption + gets rid of bile pigments/bilirubin/waste
  • stores and concentrated in gall bladder –> gall bladder sends bile to duodenum through common bile duct
59
Q
  • common hepatic duct vs common bile duct
  • hepatic artery vs hepatic portal vein vs hepatic vein
  • what is sphincter of Oddi?
A

COMMON HEPATIC DUCT:
- takes bile made in liver to the gallbladder for storage
COMMON BILE DUCT:
- tales bile from gall bladder to the lumen of small intestin
HEPATIC ARTERY:
- brings ox blood containing metabolites from peripheral tissues to the liver
HEPATIC PORTAL VEIN
- blood is rich in absorbed nutrients from the GI tract and contains hemoglobin breakdown products from spleen
HEPATIC VEIN:
- blood leaves the liver
SPHINCTER OF ODDI:
- controls release of bile and pancreatic secretion into the duodenum

60
Q
  • hepatocytes of liver are organized into irregular hexagonal units called ____A______
  • each ____A____ is centered around a ______ _____ that drains blood into the ______ _______
  • along its periphery, a ___A____ is associated with branches of which 2 blood vessels?
  • between layers of hepatocytes, you find 2 different gaps/channels of different sizes + containing different things?
A
  • A = lobules!
  • centered around a central vein that drains into hepatic vein
  • associated with hepatic portal vein and hepatic artery
  • SMALL GAP = canaliculi –> small channels into which bile is secreted –> coalesce into bile ductules that run through liver alongside portal veins
  • BIGGER GAP = sinusoid capillaries (swiss cheese) where nutrients are absorbed and metabolized
61
Q

which nutrient is secreted into duodenum but mostly reabsorbed so liver doesn’t have to synthesize it all the time?

A

bile salts!

62
Q

what are the 3 types of pancreatic/intestinal endocrine cells?
- stimulated by what?
- what do they secrete?
- what effect does the secretion have?

A

I CELLS
- stimulated by protein and lipids in duodenum
- produces cholecystokinin (CCK)
1. stimulates acini = enzyme secretion
2. acts on gallbladder –> contraction/open sphincter of Oddi
3. decrease gastric emptying
S CELLS
- stimulated by HCl/acidic chyme that enters duodenum
- secretes secretin
1. stimulate duct cells to produce bicarbonate rich solution –> neutralizes acids –> negative regulation loop
2. increases bile secretion in liver
M CELLS:
- when small intestine is empty –> stimulates M cells
- secretes motilin
- increases migrating motor complex

63
Q

what are the 4 things that control bile secretion and release?

A
  1. vagues nerve stimulation –> contraction of gallbladder –> release bile into duodenum
  2. secretin produced by duodenum –> carried through circulation to liver –> stimulates bile secretion by liver
  3. CCK produced by duodenum –> carried through circulation to liver –> stimulates gallbladder to contract + sphincter of Oddi to relax –> releases bile into duodenum
  4. bile salts stimulate bile secretion –> over 90% of bile salts are reabsorbed in ileum and returned to liver where they stimulate additional secretion of bile salts
64
Q

explain motility, secretion, digestion and absorption in small intestine

A

MOTILITY:
- no peristaltis –> only segremental and migrating motor complex
SECRETION:
- by intestine, pancreas and liver
DIGESTION AND ABSORPTION:
- focus on fat!

65
Q

how do bile salts and phospholipids help fat digestion / how are fats digested ish? (3)

A
  1. emulsify! –> break down big fat droplets into small fat droplets –> which increases surface area to that lipases from pancreas can digest small droplets
  2. bile salts and phospholipids help monoglycerides and FA form micelles + help absorption of micelles into enterocytes
  3. micelles form chylomicrons in enterocytes –> go to lympatic tubes
66
Q

what are in micelles that enter enterocytes?

A

micelles are small disks with bile salts, phospholipids, FAs, cholesterol and mono/diglycerides

67
Q

which 2 enzymes digest triglycerides?

A

lipases and colipases

68
Q

how come bile salts and phospholipids help fat digestion?

A

because they have a fat soluble side and a water soluble side/side chains

69
Q

6 ish steps of fat digestion and absorption

A
  1. bile salts from liver coat fat droplets
  2. pancreatic lipase and colipase break down fats into monoglycerides and FAs stores in micelles (emulsion)
  3. monoglycerides and Fas move out of micelles and enter cells by diffusion
  4. cholesterol is also transported into cells
  5. absorbed fats combine with cholesterol and proteins in intestinal cells to form chylomicrons
  6. chylomicrons are removed by lympathic system

Mammalian physiology (13 decks)

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