GI Physiology: Salivary Glands and Bicarbonate

Question 1

What are the three main roles of bicarbonate in the GI system?

Answer 1

1. Guards against toxic intracellular and extracellular fluctuations in pH.
2. Neutralizes gastric acid and provides an optimal pH environment for digestive enzymes to function properly in the duodenum.
3. Facilitates solubilization of macromolecules.

Question 2

What is the pH of the stomach? What about the rest of the GI tract?

Answer 2

Stomach pH is 2. The rest is 6-8

Question 3

What two protein exchangers are responsible for regulating pH in the GI tract? Where are they located?

Answer 3

1. HCO3-/Cl- exchanger (anywhere in the GI tract)

2. H+/K+ exchanger/ATPase pump (stomach parietal cells)

Question 4

If HCO3- is higher outside a cell than inside, will the cell still pump out HCO3-?

Answer 4

No, it will pump HCO3- in the opposite direction. This is true of most exchangers.

Question 5

Where do GI cells get bicarbonate from? How is it transported into the cell?

Answer 5

Can get it in two ways:

1. From the blood, transported in using the Na+/HCO3- cotransporter.
2. Bicarbonate can be produced in the cells with carbonic anhydrase.

Question 6

What is the chemical equation for the generation of bicarbonate in a GI cell?

Answer 6

H2O + CO2 –> HCO3- + H+

-enzyme needed is carbonic anhydrase

Question 7

What happens to the protons that are generated along with bicarbonate in a parietal cell (H2O + CO2 –> HCO3- + H+)?

Answer 7

They are pumped out of the cell on the basal side using a K+/H+ exchanger.

Question 8

What are the four stages of food processing?

Answer 8

1. Ingestion
2. Digestion
3. Absorption
4. Egestion (pooping)

Question 9

Which salivary gland does Mumps affect?

Answer 9

The parotid

Question 10

Name the three major salivary glands. Which one is the biggest?

Answer 10

1. Parotid (biggest)
2. Submandibular
3. Sublingual

Question 11

How many ducts connect each salivary gland to the mouth?

Answer 11

1

Question 12

On average, how much saliva is secreted per day?

Answer 12

1 - 1.5 liters

Question 13

Compare basal salivary flow rate to stimulated salivary flow rate.

Answer 13

Basal salivary flow rate is very low, and stimulated is high!

Question 14

Salivary flow rate from the minor salivary glands is __________ of stimulation, constituting ___ to ___% of total salivary output.

Answer 14

minor glands are independent of stimulation, constituting 7 to 8% of total salivary output

Question 15

What stimulates salivary output?

Answer 15

The act of chewing (doesn’t matter if there is a taste)

Question 16

Name five minor components of saliva and what each one does.

Answer 16

1. Alpha-amylase: begins CHO digestion and is inactivated at low pH.
2. Lipase: aids lipid digestion by converting triglycerides to FAs and monoglycerides.
3. Mucin: helps bolus formation and swallowing.
4. HCO3- helps maintain neutral pH in glands and mouth.
5. Na+ gets in there but it doesn’t need to be.

Question 17

Name the positive (4) regulators and negative (4) regulators of saliva production.

Answer 17

Positive:

1. Conditioning
2. Food
3. Nausea (bouts to throw up)
4. Smell

Negative:

1. Dehydration
2. Fear
3. Sleep
4. Anticholinergic drugs

Question 18

In what four ways do ductal cells modify saliva as it is passing by?

Answer 18

1. Active Na+ absorption
2. Passive Cl- absorption
3. Active K+ secretion
4. HCO3- secretion

Question 19

Describe how the osmolarity of saliva changes as it moves from the acinar portion of the salivary gland, through the ducts and into the mouth. Explain how this occurs.

Answer 19

As saliva moves through ducts, it changes from its initial isotonic state with plasma to a HYPOTONIC state (with respect to the plasma) when it gets to the mouth.

This occurs through the active removal of Na+ ions from the saliva. Water is UNABLE to follow because the ductal cells are impermeable to water (lacking aquaporin channels - important)

Question 20

How does flow rate affect Na+ levels in saliva? Explain why.

Answer 20

As flow rate increases, salivary Na+ levels also increase because the ductal cells that remove the Na+ can’t keep up with the increased flow.

Question 21

Can GI hormones regulate salivary secretion?

Answer 21

Nope

Question 22

Describe the relative saliva contributions of the three major salivary glands in the unstimulated and stimulated states, respectively.

Answer 22

Unstimulated:

1. Submandibular gland - majority
2. Parotid gland - some
3. Sublingual gland - little

Stimulated:

1. Parotid gland - majority
2. Submandibular gland - some
3. Sublingual gland - little

Question 23

Which salivary glands are serous, mixed, or mucous?

Answer 23

Parotid is purely serous.
Submandibular is mixed but mostly serous.
Sublingual is mixed but mostly mucous.

Question 24

Which of the major glands are innervated by the sympathetic nervous system and which are innervated by the parasympathetic nervous system? Do these work via positive stimulation?

Answer 24

The parotid and submandibular are controlled with parasympathetic nerves.

The sublingual is controlled with sympathetic nerves.

Stimulation of all results in increased saliva secretion (positive stimulation).

Question 25

Sympathetic stimulation stimulates the ________ gland to increase secretion of proteins (lipase, amylase, mucin), but is a minor contributor to an increase in _______ secretion.

Answer 25

stimulates the sublingual, only a minor contributor to fluid secretion.