GI Physio | Secretions I (Salivary, Gastric, Intestinal)

Question 1

A majority of GI secretions are _____. The 2 major contributors to secretions are what?

Answer 1

Passive;

Water (predominantly) and mucus

Question 2

The production of sugars, amino acids and peptides in the lumen ______ osmolarity. This causes water to _____.

Answer 2

Increases lumenal osmolarity;

Water gets PULLED into the lumen of the small intestine.

Question 3

Absorption of small molecules in the intestinal lumen, ______ the osmolarity. This causes water to _____.

Answer 3

Decreases lumenal osmolarity; Water DIFFUSES back into the body by the time food gets into the large intestine.

Question 4

Describe the 3 general rules of fluid secretion, pertaining to electrolytes and water.

Answer 4

1. Movement of ions requires energy at first.
2. Counter ions follow = electroneutrality
3. Water follows = isotonicity

Question 5

Define 3 key characteristics about salivary fluid

Answer 5

1. HYPOTONIC Concentrations of electrolytes change with rate of secretion.
2. Salivary Na and Cl are in lower concentration than in plasma.
3. Saliva is richer in HCO3- and K+ than in plasma.

Question 6

An extensive loss of saliva will most likely result in a decrease in which ion concentration?

Answer 6

Potassium (K+), because saliva is so rich in this ion as compared to the lower amount in plasma.

Question 7

Within the cells of the salivary glands, what channel is the primary energy source for driving ion secretions?

Answer 7

Na/K ATPase on the serosal side of the acinar and ductal cells.

Question 8

The sympathetics major contribution to salivary secretion is ___. Whereas the parasympathetics contributions to saliva are ____ and ____.

Answer 8

SNS (T1-T3) contributes mucus.
PSNS (CN IX, X and VII) contributes fluid and enzymes to saliva.
PSNS nerves offer a major trophic factor

Question 9

List 3 factors that stimulate salivary secretion. What are some factors that inhibit this?

Answer 9

Smell, taste, pressure or nausea can stimulate salivary secretion; whereas, fatigue, lack of sleep, fear or dehydration can inhibit this (leading to a dryer mouth).

Question 10

When stimulated, oxyntic cells secrete _____ ____ into the lumen and _____ into the interstitium.

Answer 10

Stimulated oxyntic cells secrete ISOTONIC HCl into the lumen + HCO3- into the interstitial area.

Question 11

What ion is highest in concentration within gastric juices at low and high rates of secretion?

Answer 11

Cl- ions are highest in concentration, even more than H+ at low and high secretory rates.

Question 12

What are the 2 major secretions of gastric juices? Briefly describe the consistency of each.

Answer 12

1. Oxyntic component consisting of HCl from parietal cells (aka oxyntic cells)
2. Non-oxyntic component composed of mucus, Na+, HCO3-, pepsinogen and diffuse interstitial fluid

Question 13

What microbes are able to resist the inactivation of HCl in the stomach?

Answer 13

Helicobacter pylori, E. coli and other bacteria that can withstand a pH = or < 2.0.

Question 14

What morphological change occurs in parietal cells from resting to secreting phases?

Answer 14

Tubulovesicles rich in H+ pumps from resting phase fuse with the intracellular canaliculi in order to secrete H+ into gastric juices.

Question 15

What is alkaline tide and how does it work? What are the 2 main causes of this?

Answer 15

This refers to an increase of pH in the gastric (portal) vein after gastric acid secretion. Because more H+ is secreted into the lumen, more HCO3- is secreted into the blood of the portal vein. This is a physiologic process after a big meal (post-prandial alkaline tide) or pathological as from metabolic alkalosis (sustained alkaline tide) after vomiting.

Question 16

Administration of a proton pump inhibitor (PPI) such as omeprazole after a major meal will have what effect? Why?

Answer 16

This will decrease the potential difference across the stomach from -70-80 mV to -30-50 mV. This is because less H+ is being secreted into the gastric lumen simultaneous to the reduced HCO3- secretion into the blood.

Question 17

Rank the top 3 phases of control for acid secretion from the stomach in order. Briefly describe what stimulates and mediates each.

Answer 17

1. Gastric phase (50% HCl) - stimulated by food in the stomach
2. Cephalic phase (30%) - stimulated by sight, smell, chewing, tasting and swallowing of food.
3. Interdigestive phase (15%) - basal; no stimulation
   * These are all mediated by PSNS, gastrin and histamine to produce acid*

Question 18

Why does acid secretion drop significantly (10-fold) after the gastric phase? What are the mediators of the intestinal phase?

Answer 18

After chyme has been digested in the stomach, we don’t want intestinal enzymes to be destroyed by excess acid in lumen. Secretin, CCK and GIP inhibit gastrin’s effect to increase acid production.

Question 19

Define the major inhibitors or gastric acid secretion and their MOA.

Answer 19

1. Secretin, CCK and GIP indirectly reduces acid secretion by nullifying gastrin’s effects.
2. Prostaglandins and somatostatin work DIRECTLY to inhibit acid secretion by reducing cAMP activity in the parietal cells.

Question 20

Define the major trophic stimulator of parietal cells.

Answer 20

The more gastrin we have, the LARGER the parietal cells and more cAMP activity for acid production.

Question 21

How strong is the H+ gradient of acid contained within my stomach?

Answer 21

3-million fold from outside stomach to inside. The acid is so strong that it can reduce rust.

Question 22

Describe the 2 modes of containment for HCl in the stomach. What is the significance of this for anti-inflammatory drugs?

Answer 22

1. Mucosal barrier (mucus glycoproteins and HCO3-)
2. Prostaglandins - directly inhibit acid secretion. This is significant since people who take anti-inflammatory drugs, such as NSAIDs, have trouble with gastric secretion and therefore properly digesting foods.

Question 23

Pancreatic enzymes are usually secreted in ______ into the duodenum. What is the limiting factor for digestion and what anatomic structure controls it?

Answer 23

Pancreatic enzymes in Excess into the duodenum;

Rate of Gastric emptying, controlled by the Pyloric Sphincter, is the limiting factor for digestion

Question 24

Describe what occurs if the pyloric sphincter is damaged or removed from surgery?

Answer 24

Dumping Syndrome - a SEVERE condition where the body loses A LOT of fluid resulting in watery, painless diarrhea

Question 25

Explain the pathophysiology of Dumping Syndrome and list its systemic effects on the body.

Answer 25

1. Rapid gastric emptying causes a sudden INCREASE in osmotic pressure (water diarrhea).
2. This also causes distention of the lumen increasing motility (reduces absorption).
3. Fermentation of unabsorbed nutrients in large intestine leads to gas.
4. Loss of blood volume results in weakness and tachycardia.
5. Sudden increase in blood sugar causes over-secretion of insulin. This leads to hypoglycemia, diaphoresis and tachycardia.

Question 26

Define pancreatitis. What are some common causes?

Answer 26

Pancreatitis is inflammation of the pancreas from untimely activation of pancreatic enzymes that results in self-digestion.
3 Common causes include:
1. Congenital trypsin inhibitor deficiency
2. Abdominal trauma (proteases from injured pancreatic cells activate trypsin)
3. Cystic fibrosis (enzyme is stuck in pancreas)

Question 27

Why do we not have amylase as part of the pancreatic secretions?

Answer 27

By this time starch has been broken down in the mouth and stomach. Remaining peptides, amino acids and fats in the lumen explains why we have more lipases, proteases and DNAases instead.

Question 28

Explain the significance of pancreatic fluid secretion in relation to its enzymes.

Answer 28

Fluid helps to deliver the enzymes from the pancreas into the small intestine. This HCO3- rich fluid also protects enzymes by neutralizing remaining H+ in lumen.

Question 29

Why don’t we feed newborns foods high in starch?

Answer 29

The exopancreas is only partially developed at birth. Don’t give starchy foods, even if liquified.