Salt and Water Transport in the GI Tract Flashcards
Osmolality
concentration of dissolved particles of chemicals and minerals within a solution
Hypertonic solution
a solution with osmolality is more than that of plasma
water moves in
hypotonic solution
a solution with osmolality less than that of the plasma
water leaves the solution
Purpose of regulation of water and electrolyte transport mechanisms
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
Regulation of water and electrolyte transport mechanisms:
- gut lumen
- enteric nervous system
- autonomic neural mechanisms
- hormonal and immunogenic signs
Regulation of water and electrolyte transport mechanisms:
Disruption to absorption leads to
secretory diarrhoea, potential for dehydration and electrolyte imbalance
Electrolytes table
Na+, K+, Cl-
Water moves down an osmotic gradient
Movement of ions require membrane proteins
Principles of enterocytes transport: adaptations of gut lumen:
- 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
Tonicity definition
measure of the osmotic pressure gradient (i.e. concentration of a solution)
Tonicity of chyme entering the duodenum affects
bidirectional fluid flux
principles of enterocyte transport: 2 routes:
- transcellular
- paracellular
Principles of enterocyte transport: transcellular routes:
transcellular absorption may be against the conc grad and requires ATP
Principles of enterocyte transport: paracellular routes:
do not require additional external energy
Principles of enterocyte transport: transcellular and paracellular routes:
para = between cells, no energy
Types of enterocyte transport (3):
- passive
- solvent drag
- active
Types of enterocyte transport: passive:
down an electrochemical gradient through ion channels or carriers or permeable tight junctions
Types of enterocyte transport: solvent drag:
water moves to reach osmotic equilibrium and takes ion (i.e. proximal small intestine, where tight junctions are more permeable)
PARACELLULAR ROUTE
Types of enterocyte transport: active:
requiring ATP
- Na+/K+ ATPase pump depletes cellular Na+ and
draws K+ across apical membrane via channel and
cotransporter
What type of enterocyte transport is depicted below?
solvent drag: paracellular route
Differences between enterocytes in crypts and villi:
- villi: enterocytes are absorptive and dominate
nutrient transport - crypts: enterocytes are secretory with minimal
nutrient transport
Villi enterocytes and crypt enterocytes
Which of the following structures are absent in the large intestine?
- villus
- crypts
villus
Why do villus and crypt enterocytes have different functions?
the enterocytes express different combinations of transport proteins
What does the small intestine contain?
water and key electrolytes Na+, K+, Cl- and HCO3- from ingested food and secretions of the GI Tract.
How is isotonic chyme formed?
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
What is reabsorbed in the jejunum?
Na+, K+, Cl-, H2O
What is secreted in the ileum?
HCO3-
What does the large intestine reabsorb?
Na+, Cl-, H2O
What does the large intestine secrete?
K+, HCO3-
Na+ absorption in the small intestine
- 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)
Cl- absorption (2 ways)
- 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)
NaCl absorption in the large intestine
- 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
Absorption of water
- 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
Water moves to reach osmotic equilibrium and takes ions along with it. What is this process called?
solvent drag
What is the primary function of crypts of Lieberkhun?
- excretory
- absorptive
- secretory
- facilitative
secretory
Na+/glucose symporter is an example of:
- diffusion
- primary active transport
- secondary active transport
- osmosis
secondary active transport
