Salt and Water Transport in the GI Tract

Question 1

Osmolality

Answer 1

concentration of dissolved particles of chemicals and minerals within a solution

Question 2

Hypertonic solution

Answer 2

a solution with osmolality is more than that of plasma
water moves in

Question 3

hypotonic solution

Answer 3

a solution with osmolality less than that of the plasma
water leaves the solution

Question 4

Purpose of regulation of water and electrolyte transport mechanisms

Answer 4

1) a medium for digestion and waste elimination in the
lumen
2) a replacement of daily loss of body fluids in sweat,
urine, lungs and faeces

Question 5

Regulation of water and electrolyte transport mechanisms:

Answer 5

• gut lumen
• enteric nervous system
• autonomic neural mechanisms
• hormonal and immunogenic signs

Question 6

Regulation of water and electrolyte transport mechanisms:

Answer 6

Question 7

Disruption to absorption leads to

Answer 7

secretory diarrhoea, potential for dehydration and electrolyte imbalance

Question 8

Electrolytes table

Answer 8

Na+, K+, Cl-

Question 9

Water moves down an osmotic gradient

Answer 9

Question 10

Movement of ions require membrane proteins

Answer 10

Question 11

Principles of enterocytes transport: adaptations of gut lumen:

Answer 11

• polarised with an apical and basolateral membrane
• tight junction sprovide a barrier to free flow of gut
  lumen contents, although these tight junctions are
  more permeable in proximal small intestine

Question 12

Tonicity definition

Answer 12

measure of the osmotic pressure gradient (i.e. concentration of a solution)

Question 13

Tonicity of chyme entering the duodenum affects

Answer 13

bidirectional fluid flux

Question 14

principles of enterocyte transport: 2 routes:

Answer 14

• transcellular
• paracellular

Question 15

Principles of enterocyte transport: transcellular routes:

Answer 15

transcellular absorption may be against the conc grad and requires ATP

Question 16

Principles of enterocyte transport: paracellular routes:

Answer 16

do not require additional external energy

Question 17

Principles of enterocyte transport: transcellular and paracellular routes:

Answer 17

para = between cells, no energy

Question 18

Types of enterocyte transport (3):

Answer 18

• passive
• solvent drag
• active

Question 19

Types of enterocyte transport: passive:

Answer 19

down an electrochemical gradient through ion channels or carriers or permeable tight junctions

Question 20

Types of enterocyte transport: solvent drag:

Answer 20

water moves to reach osmotic equilibrium and takes ion (i.e. proximal small intestine, where tight junctions are more permeable)

PARACELLULAR ROUTE

Question 21

Types of enterocyte transport: active:

Answer 21

requiring ATP
- Na+/K+ ATPase pump depletes cellular Na+ and
draws K+ across apical membrane via channel and
cotransporter

Question 22

What type of enterocyte transport is depicted below?

Answer 22

solvent drag: paracellular route

Question 23

Differences between enterocytes in crypts and villi:

Answer 23

• villi: enterocytes are absorptive and dominate
  nutrient transport
• crypts: enterocytes are secretory with minimal
  nutrient transport

Question 24

Villi enterocytes and crypt enterocytes

Answer 24

Question 25

Which of the following structures are absent in the large intestine? - villus - crypts

Answer 25

villus

Question 26

Why do villus and crypt enterocytes have different functions?

Answer 26

the enterocytes express different combinations of transport proteins

Question 27

What does the small intestine contain?

Answer 27

water and key electrolytes Na+, K+, Cl- and HCO3- from ingested food and secretions of the GI Tract.

Question 28

How is isotonic chyme formed?

Answer 28

the duodenum acts to rapidly equilibrate the osmotic potential of the chyme through the movement of water, Na+, Cl- through permeable tight junctions using the solvent drag mechanism

Question 29

What is reabsorbed in the jejunum?

Answer 29

Na+, K+, Cl-, H2O

Question 30

What is secreted in the ileum?

Answer 30

HCO3-

Question 31

What does the large intestine reabsorb?

Answer 31

Na+, Cl-, H2O

Question 32

What does the large intestine secrete?

Answer 32

K+, HCO3-

Question 33

Na+ absorption in the small intestine

Answer 33

- drives absorption of other ions and organics - driven by basolateral active transport of Na+ via Na+/K+ ATPase pump into the interstitial fluid which creates an electrochemical gradient (basolateral side) ( 3 Na+ out, 2 K+ in) - secondary active transport occurs at apical carrier proteins for Na+ absorption (re) (Na+ in down conc grad, Cl- against but no ATP)(Na+ in down conc grad, H+ in against but not ATP)

Question 34

Cl- absorption (2 ways)

Answer 34

- in the jejunum, Na+ absorption with nutrients is electrochemical and leads to net negative charge in the lumen and net positive charge in the paracellular spaces - this provides an electrochemical gradient for Cl- absorption (symporter of Na+, Cl- (Na+ down))??? - in the jejunum and distal ileum Cl- absorption occurs passively through the apical membrane Cl- ion channels and in exchange for HCO3- (hence secreted)

Question 35

NaCl absorption in the large intestine

Answer 35

- driven NA+/K+ ATPase pump - Sodium entry by: - f.d Na+ channels - diffusion under aldosterone control; increases Na= channels and absorption at the expense of K+ - K+ secretion when lumen conc low - Cl-/HCO3- provide buffer for acid produced by bacteria and moves Cl- - tight junctions ensure no ion backflow; less permeable than in proximal duodenum

Question 36

Absorption of water

Answer 36

- totally dependent on the absorption of solutes, especially Na+ - Na+/K+ ATPase pumps lead to a build up of NaCl in the paracellular spaces between enterocytes - osmotic gradient draws H2O into enterocytes and the paracellular spaces

Question 37

Water moves to reach osmotic equilibrium and takes ions along with it. What is this process called?

Answer 37

solvent drag

Question 38

What is the primary function of crypts of Lieberkhun? - excretory - absorptive - secretory - facilitative

Answer 38

secretory

Question 39

Na+/glucose symporter is an example of: - diffusion - primary active transport - secondary active transport - osmosis

Answer 39

secondary active transport