Week 1: physio- GI secretion

Question 1

Overview of fluid produced by GI system

Answer 1

• ingested: 1-2L
• saliva: 1.5L
• gastric juice: 2L
• pancreatic juice: 1.5-2L
• bile: 0.5-1L
• intestinal secretions: 1L+
• Total about 10L of fluid is delivered to the small intestines/day
• small intestines absorb most of it, and large intestines do the rest
• about 100mL is secreted in stool

Question 2

Salivary secretion

Answer 2

• at low rates, produce hypotonic saliva. At high rates, glands produce saliva that is more like plasma
• initial fluid secreted is isotonic to plasma, but then duct modifies the contents
  1. low rates
• glands secrete NaCl and H2O
• duct reabsorbs NaCl and leave water behind with a little secretion of NaHCO3 and K+
  2. high rates
• as we stimulate fluid production, also stimulate more NaHCO3 secretion back into saliva and are less able to reabsorb NaCl

Question 3

pancreatic juice secretion

Answer 3

• as rate increases, higher HCO3- and low Cl- concentration
• different from salivary glands in 2 ways
  1. many more NaHCO3 transporters in ducts. Secretes much more NaHCO3-
  2. AQP in ducts for water to follow Na+

Question 4

Gastric juice secretion

Answer 4

• as rate increases, H+ concentration increases and Na+ decreases. Na concentration decreases because solution is being diluted with HCl
• gland secretes HCl and H2O (through AQP)
• epithelium produces NaHCO3 to neutralize acid at surface

Question 5

Cellular mechanism of secretion of NaCl, e.g. salivary glands

Answer 5

Basolateral
-Na/K ATPase with K+ recycled out through K channel
-NaK2Cl cotransporter
Apical
-Cl channel (Ca activated) and CFTR where Cl leaves to lumen
1. The Na gradient allows Cl- to enter the cell against its electrochemical gradient and exit through apical membrane. Na moves through paracellular pathway to give net secretion of NaCl
2. Second mechanism of NaCl secretion
-NHE and anion exchanger (HCo3-/Cl-) on basolateral side
-carbonic anhydrase converts H2O and CO2 to H+ and HCO3- which exit basolaterally and Cl and Na enter via NHE and AE
-Cl- exits apically via the 2 channels and Na+ moves paracellularly

Question 6

cellular mechanism of bicarbonate secretion: salivary ducts, pancreatic ducts, hepatic ducts, colon

Answer 6

Basolateral
-Na/K ATPase
-NBC (Na/HCO3- cotransporter)
Apical
-CFTR: HCO3- leaves through this
Paracellular: Na moves through this path

Question 7

Cellular mechanism of HCl secretion is gastric cells

Answer 7

Basolateral
-HCO3/Cl exchanger: HCO3 leaves cell
Apical
-H/K ATPase: H+secreted out (blocked by omeprazole)
-K+ and Cl- channels. Cl- leaves to balance H+ secretion
The H+ and HCO3- are from H2O and CO2 via carbonic anhydrase

Question 8

Effect of NSAIDs, e.g. aspirin, on buffering gastric acid

Answer 8

• pH sensor in cell detects H+ increases
• activates cyclooxyrgenase, which generates prostaglandin
• activates cAMP which activates Na/HCO3 cotransporter which brings HCO3 into the cell
• NSAIDS impairs this process and can cause gastric bleeds