Physiology: H2O balance in the GI tract

Question 1

what drives the absorption of H2O in the GI tract

Answer 1

passive process driven by transport of solutes (especially Na) from lumen of intestines to blood stream

Question 2

describe the relationship between Na reabsorption and H2O absorption

Answer 2

reabsorption Na largely drives H2O absorption and provides local osmotic force for absorption of H2O

Question 3

what different cotransport mechanisms are involved in Na absorption

Answer 3

Na/glucose and Na/amino acid cotransport

Question 4

what different exchange mechanisms are involved in Na absorption

Answer 4

Na/H+ exchange, and

Na/H+ and Cl/HCO3 exchange in parallel

Question 5

what other mechanism that isn’t exchange or cotransport is Na absorbed by

Answer 5

epithelial Na channels(ENaC)

Question 6

describe the driving force in Cl absorption in the small intestine

Answer 6

lumen -ve potential due to electrogenic transport of Na

Question 7

describe the driving force of Cl absorption in the large intestine

Answer 7

lumen -ve potential due to electrogenic movement through ENaC

Question 8

what type of cells secrete Cl

Answer 8

crypt cells

Question 9

what 3 processes are involved in Cl secretion

Answer 9

Na/K ATPase
Na/K/2Cl cotransport(NKCC1)
K channels

Question 10

what effect does low intracellular Na have on Na, K and Cl

Answer 10

drives inward movement of them via NKCC1

Question 11

describe how Cl exits the cell

Answer 11

intracellular conc. Cl increases providing electrical gradient for Cl to exit cell, via CFTR on apical membrane