Lecture 12: GI Secretory Functions,digestion, absorption

Question 0

Mechanisms of Stimulation of Glands (What causes these glands to release their secretory products?)

Answer 0

1. Food contact and local epithelial stimulation
2. Autonomic stimulation (mostly parasympathetic)
3. Higher brain centers
4. Hormonal stimulation

Question 1

What are the different gland types?

Answer 1

1. Unicellular mucous glands (goblet cells)
2. Crypts of Lieberkuhn (where secretory cells in intestine are found)
3. Tubular glands
4. Complex glands: (salivary, pancreas and liver)

Question 2

What is the mechanism of secretion of organic substances?

Answer 2

pg. 774; 64-1
Glandular cell: secretion of enzymatic products (proteins synthesized on rough ER); further enhanced by passing through Golgi where they are packaged into secretory vesicles and then released by exocytosis
zymogen=precursor enzymes

Question 3

Mechanisms of secretion of water and electrolytes: What is the function of mucous?

Answer 3

1. Adheres to food and other particles
2. Spreads thin film over surfaces
3. Coats wall of gut, preventing actual contact of food
4. Causes fecal particles to adhere to one another
5. Resistant to digestion by GI enzymes
6. Has amphoteric properties making it useful for buffering small amounts of acids and bases

Question 4

Mechanisms of secretion of water and electrolytes: salivary gland secretions:

1. Parotid gland secretions are almost entirely ____.
2. In contrast, _____ and ____ secretions are mixed

Answer 4

1. serous

2. Submandibular, sublingual

Question 5

Mechanisms of secretion of water and electrolytes: first stage of salivary secretion of ions

1. Occurs in ____
2. Secretion contains _____
3. Secretion is typical of ______

Answer 5

1. acini
2. ptyalin (alpha-amylase)–carb digesting enzyme
3. extracellular fluid

Question 6

Mechanisms of secretion of water and electrolytes: salivary gland secretions: Second stage of salivary secretion of ions:

1. Occurs in ____
2. Active reabsorption of ____ and active secretion of ____
3. Active/passive secretion of ____
4. Passive reabsorption of chloride ions due to ____mv in ducts

Answer 6

1. salivary ducts
2. sodium, potassium
3. potassium
4. -70mv

Question 7

Mechanisms of secretion of water and electrolytes:

1. Nervous regulation of salivary secretion is done via the _____system
2. Other factors that stimulate or inhibit salivary secretions

Answer 7

1. Parasympathetic

2. Autonomic NS makes mouth dry during tense situations? Look in book

Question 8

Figure 64-2:

Answer 8

shows salivary secretions and remodification occurring in duct. Acini secretes ptyalin, mucous, extracellular fluid; as you travel down the duct we get active absorption of sodium ions, passive absorption of Cl-, active secretion of K+, and both active and passive secretion of bicarbonate ions

Question 9

64-3: Parasympathetic regulation of salivary secretion

Answer 9

1. 3 main groups of salivary glands: parotid, submandibular and sublingual glands which have similar innervation pattern.
2. There are 2 main cranial nerves involved in this. In parotid gland, we have glossopharyngeal and facial nerve. Facial nerve has portion that is passing through the middle ear (called chordae tympani);
3. motor innervation to submandibular and sublingual gland is via the facial nerve. Motor innervation to parotid gland is via glossopharyngeal nerve. Both come from the superior and inferior salivatory nuclei–superior goes to submandibular and sublingual; the inferior goes to parotid. Glossopharyngeal activity also comes from tongue (sensory neurons) and they end up in tractus solitarius and have feedback to motor nucleus to salivary glands so when food hits tongue, it sets up feedback loop to cause saliva to flow. Because this is parasympathetic, facial and glossopharyngeal are cranial nerves that carry parasympathetic functions. We have ganglia (submandibular ganglia for submandibular and sublingual gland and the otic gland for parotid gland)–these represent the area synapse between the pre-ganglionic fiber and the post ganglionic fibers

Question 10

Gastric Secretions: What are the secretory cell types in gastric glands (complex glands)?

Answer 10

1. Mucous neck cells
2. Chief (peptic) cells
3. Parietal (oxyntic) cells

Question 11

Figure 64-4

Answer 11

Shows a gastric (oxyntic) gland;
Main pt: these are deep glands in the wall of stomach and the actual secretory cells are stacked with the mucous secreting cells near the surface of the glands where the secretory product is actually expelled. Then we transcend into oxyntic or parietal cells and deeper down, we have peptic (chief cells); most important are parietal and chief cells

Question 12

Gastric secretions: Chief cells

1. Chief cells secrete ______ (inactive ___) which is a proteolytic enzyme (breaks down proteins) whose pH range of activity is between ___ to ___.
2. The release of pepsin is stimulated by ____ from ___ nerves or ____ plexus and the release is a response to ____
3. Chief cells also secrete ______

Answer 12

1. pepsinogen (pepsin), 1.8-3.5
2. Acetylcholine, vagus, gastric enteric nervous plexus, stomach acid
3. intrinsic factor

Question 13

1. Parietal cells produce ____ into the ____

2. What is the process by which HCl is released?

Answer 13

1. HCl, gastric lumen
2. a) Dissociation of water inside the cell into H+ and OH-
   b) H+ ion is pumped out of cell in exchange for K+
   c) K+ leaks outside the cell but is transported back in via H+-K+ ATPase pump
   d) Na+ is reabsorbed into cell due to Na+-K+ basolateral pump
   e) OH-+ CO2–> HCO3- (involves carbonic anhydrase)
   f) HCO3- is exchanged for Cl- ions
   g) Cl- ions are secreted through Cl channels into canaliculi

Question 14

Figure 64-5: Parietal Cell

Answer 14

Has canaliculi inside confines of cell; mitochondria necessary to drive pumps; canals are lined by microvilli where actual secretion occurs and then there is a pore coming out of the apical end of cell where HCl is actually released. Other cells are mucous neck cells that surround the parietal cells.

Question 15

Figure 64-6: HCl secretion (Dr. Anderson’s notes)

Answer 15

In the lumen of caniculi, we’re in outflow path into the lumen of stomach; CO2 diffuses in, CO2 combines with water and dissociates into HCO3- and H+ ions; bicarbonate is pumped out, CL- is pumped in, Na+/K+ ATPase pumps in K+ and pumps out Na+ and Cl simply diffuses in passively through Cl channels bc we’re removing negative ions so making inside more positive; gradient both electrical and diffusion gradient that draws Cl- in and water is moving in and crosses by osmosis; K+ ions moves back out through simple diffusion but K+ channels also there and it is pumped back in exchange for H+ ions and this is the main pump that puts H+ ions into the lumen and then we have Na+ ions both actively and passively moving in and out. Cl- ions are moved out passively if we bring Cl- ions in from basal side which increases concentration of Cl- and is moved out by its conc. gradient plus the high positve ion gradient set up by sodium and K+ to some extent. End result is to pump out H+ and Cl- and a little bit of K+

Question 16

What are the three things involved in gastric secretions?

Answer 16

Parasympathetic system, gastrin and histamin

Question 17

1. Gastrin is secreted by ____ cells in pyloric glands
2. What are the two forms of gastrin? Which one is more abundant?
3. Gastrin is released in response to presence of ____in ____
4. Gastrin causes _________ to release _____
5. Histamine causes secretion of ___

Answer 17

1. gastrin (G) cells
2. G-34 and G17, G-17 is more abundant
3. protein, pylorus
4. enterochromaffin-like cells, histamine
5. HCl

Question 18

What are the phases of gastric secretion? pg. 780

Answer 18

Cephalic phase (pre), gastric phase (during), intestinal phase (post-stomach)

Question 19

Figure 64-7

Answer 19

Cephalic phase: transmitted by vagus nerve; we have vagal center in medulla and vagus nerve comes down to stomach which excites pepsin and acid production
Gastric phase: local nervous secretory reflexes part of enteric system and vagal reflexes tied to vagus nerve innervation and hormone secretion by way of gastrin and histamine secretions
Intestinal phase: nervous and hormonal mechanisms but we have circuitory pathway bw intestine and stomach that carries intestinal secretions which are hormones to the stomach to help control the gastric phase
So cephalic phase initiated to control the gastric phase, gastric phase acts on intestine, and intestine acts on gastric phase (only works for proteins)

Question 20

In addition to secreting major digestive enzymes and substrates, the pancreas also releases _____.

Answer 20

Bicarbonate which is broken down into salt and carbonic acid

HCl + NaHCO3—>NaCl + H2CO3

Question 21

1. Pancreatic secretions: What is/are the enzymes for proteins?
2. Trypsin inhibitors are secreted by _____
3. Trypsin inhibitors are necessary to prevent the action of trypsin on ____

Answer 21

1. Trypsin, chymotrypsin and carboxypeptidase
2. glandular cells
3. pancreatic tissues themselves

Question 22

Pancreatic Secretions: What is/are the digestive enzymes for carbohydrates?

Answer 22

Pancreatic amylase

Question 23

Pancreatic Secretions: What is/are the digestive enzymes for fat?

Answer 23

Pancreatic lipase, cholestrol esterase, phospholipase

Question 24

How does the pancreas secrete bicarbonate ions?

Answer 24

H2O + CO2 (from blood, involves carbonic anhydrase)—> H2CO3–> HCO3- + H+
HCO3- + Na+ are actively transported into duct lumen and then hydrogen ions are exchanged for sodium ions

Question 25

How is the release of acetylcholine from the pancreas regulated?

Answer 25

From parasympathetic nerves and enteric nervous system

Question 26

How is cholecystokinin secretion from the pancreas regulated?

Answer 26

Release is stimulated by presence of food in upper intestine
Secreted by duodenal and upper jejunal mucosal cells
Results in dramatic increase in secretion of pancreatic enzymes

Question 27

How is Secretin secretion from the pancreas regulated?

Answer 27

Release of secretin is stimulated by presence of acidic foods in upper intestine
Secretin is secreted by duodenal and upper jejunal mucosal cells
Secretin stimulates the release of large amounts of sodium bicarbonate

Question 28

What factors stimulate the release of bile from the gall bladder?

Answer 28

Presence of fatty food in duodenum

Question 29

1. Bile is secreted continuously by _____ in the ____ and stored in the gall bladder until needed.
2. In the gallbladder, bile is concentrated by active transport of ____ followed by secondary absorption of _____ ions, water and other diffusible constituents.

Answer 29

1. hepatocytes, liver

2. sodium, chloride

Question 30

How is bile salts synthesized by the liver?

Answer 30

Cholestrol is converted to cholic acid or chenodexycholic acid, which are then conjugated with glycine or taurine

Question 31

What are the functions of bile salts?

Answer 31

1. Emulsification

2. Complex with lipids to form micelles for absorption across intestinal mucosa

Question 32

List factors that contribute to gall stones

Answer 32

Look in text

Question 33

Figure: 64-11: Liver secretion and gall bladder emptying

Answer 33

Bile duct empties into the duodenum near where pancreatic duct opens. Bile acids by way of blood, stimulates parynchymal secretion. Secretin by way of bloodstream stimulates liver ductal secretion. Cholesystokin from the blood causes gall bladder contraction and the relaxation of sphincter of ode, which is a sphincter muscle that controls the opening and closing of the common bile duct. Choelsystokin both contracts the gallbladder and relaxes sphincter of Ode so when gallbladder contracts, it releases bile stored in gallbladder and sphincter relaxes for bile to enter duodenum. Two main things work on bile storage and secretion: secretin and cholesystokinin.

Question 34

Fig 64-12 Formation of gallstones

Answer 34

Causes of gallstones: too much absorption of water from bile, too much absorption of the acids from bile, too much cholestrol in bile, inflammation of epithelium
Normally, bile comes down from liver via the hepatic duct and then turns through cystic duct up into the gall bladder. When we need bile in SI, it basically reverses the route and comes down through common bile duct. Because of this complex pathway, if you form gallstones in gallbladder, when theyre in gallbladder, they dont cause harm but may fragment or move down into ducts, liver, or duodenum or stay in cystic duct. This is when we get excruciating pain of gall bladder attack.

Question 35

Intestinal Secretions: What are the cell types in the intestine that make up the crypts?

Answer 35

Goblet cells (produces mucous) and enterocytes in the crypts of lieberkuhn. Intestine also includes paneth cells.

Question 36

What are the secretions of paneth cells? Fig 64-13??

Answer 36

Look in text

Question 37

Fig. 64-13

Answer 37

Mucous cells (goblet cells) are at the top of the crypt just like in gastric glands. The paneth cells and other cells are towards the base.

Question 38

1. Condensation reactions are used to remove ____ions and ___ ions fom building blocks in order to allow the bonding of monomers into polymers
2. _____ is the reverse of condensation which incorporates water molecules in such a way that polymers are broken into monomers.

Answer 38

1. hydrogen ions, hydroxyl ions

2. hydrolysis