Water & Electrolyte Transport (Wilson)

Question 1

Describe the SI mucosal layer & how its surface area is constructed.

Answer 1

Folds in the SI mucosa increase its SA. These folds are comprised of villi that are lined with epithelial cells and vasculature with their own microvilli. All this increases SA by 1000-fold (for water, ion, nutrient absorption)

Simple columnar epithelium

Question 2

What are the goals of transport across GI mucosa? (4)

Answer 2

Question 3

Features of GI mucosa that faciliate absorption & secretion.

Answer 3

• “leaky” tight junctions
• wide intercellular spaces
• polarized gastrointestinal cells (create gradient)
• abundant space in basolateral membrane
• proximity to vascular channels

GI mucosa

Question 4

What part of GIT play biggest roles in absorbing & secreting water/ions/nutrients?

Answer 4

Small and Large intestines

Question 5

Describe movement of water

Answer 5

• always passive diffusion
• always follows sodium (sometimes Cl-, HCO3-, glucose)
• moves into blood when chyme is hypo-osmotic, (more particles in vasculature than in the chyme –> water will move into the vasculature)
• moves into chyme when it is hyperosmotic (more particles in chyme than vasculature)

“movement into chyme” ~= movement into GIT

Question 6

Describe sodium transport/absorption in GIT

Answer 6

Na+/K+ ATPase pumps located on basolateral membrane of microvili:

• 3 sodium ions out of cell
• 2 potassium ions into cell
• –> cell beomes more negative and sodium in the GI lumen will want to move into the cell (move from high->low concentrations)
• water will also follow sodium down its concentration gradient

sodium is coupled to glucose transport

Question 7

Where is HCO3- largely secreted in the GIT?

Answer 7

in the small intestine

• HCO3- ions are absorbed from gastric parietal cells into vasculature and then secreted into SI to aid with protecting against ulcer formation from all the HCl being produced in the stomach

Question 8

where does sodium & water secretion primarily occur?

Answer 8

In crypts (mostly of the SI)

secreted out from crypts, re-absorbed in the villus

Question 9

What happens to water and ions in the large intestine

Answer 9

Almost all (that is left) gets re-absorped in the colon (and is not excreted/loss in the feces)

Question 10

Where are mature versus immature enterocytes located?

Answer 10

Mature: tips of villi
Immature: in the crypts

Question 11

What happens if villi get destroyed?

Answer 11

• Malabsorption of water & nutrients
• All the water then remains in the GI lumen –> diarrhea