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Phys4 GI Lect10

Phys4 Flux of electrolytes and Water

How is water MAINLY absorbed from the intestine? PASSIVELY in the small intestine. **therefore fluid flux is dependent of food and electrolyte concentrations in the lumen as wall as osmotic pressures w/in the BL
Water volumes entering the small intestine, Large intestine, and exceted in a 24hr period 1.Small: 7-10L. 2.Large: 600mL. 3.Excreted: 100mL.
How does the osmolality of the GI lumen compare with that of the plasma? ISOTONIC
In a healthy GI tract, what undergoes a net secretion? Net reabsorption? SECRETION: 1.K+. 2.HCO3-. ABSORPTION: 1.Na+. 2.Cl-. 3.Water
Compare the gall bladder and small intestine epithelium to that in the colon? Small int and GB have LEAKY epithelium where as Colon has a relatively impermeable epithelium. **allows it to create large transepithelial gradients.
5 mechanisms of Na+ reabsorption 1.Sugar/Na or aa/Na symporters. 2.Na channels (Colon). 3.Na/Cl symporter. 4.Na/H+ exchanger. 5.Paracellular solvent drag (sig in jej and duo).
Key Na reabsorption in the Proximal bowel (Jejunum and Duodenum) 1.Glucose or aa's symptorter and Na/H+ exchanger predominate. 2.paracellular solvent drag. **HCO3- is secreted into BL on the basolateral side.
Na reabsorption in the Ileum/ Distal Bowel 1.Na/H+ exchanger (combines with HCO/Cl exchanger to form the COUNTER TRANSPORT). 2.Na/Cl- symporter. **There is also a Cl/HCO3 exchanger that secretes bicarb. This creates lots oc Cl- in the cell
Where are you more likely to see Na absroption via Glucose/Na or aa/Na symptorter? Proximal bowel. **Less prevalent in distal bowel b/c the glu & aa's have mostly been absorbed.
How is K+ absorbed in the small intestine? Large intestine? Passive diffusion paracellularly. **K+ is SECRETED in the large intestine
How is Na primarily reabsorbed in the colon? Na+ channels. **Targets of Aldosterone.
Movement of K+ in the colon Can be absorbed or secreted depending on lumenal content, but mostly secreted via movement through paracellular space.. **Basolateral membrane is permeable to K+ (only in colon).
What can happen to [K+] as a result of Diarrhea K+ secretion will inc due to the inc driving gradient from the Inc flows (K secretion has a flow rate dependence). **this can result in a hypokalemic metabolic ACIDOSIS.
4 things that Increase K+ secretion in the Colon 1.High Flow rates (diarrhea). 2.cAMP (activated by cholera toxin & VIP). 3.cGMP. 4.Ca+ (Inc by serotonin). **they affect apical membrane more than basolateral membrane.
What accounts for the high alkaline pH of stool water? The Cl/HCO3 exchanger which is present in both small and large intestine.
Water movement seen with a hypertonic (just after eating) GI lumen Initially moves into the lumen, then more distally both solutes then water move back into the BL.
Water movement seen with a hypotonic GI lumen Initially moves from the lumen into the BL, then distally there is Isotonic movement.
What is the major source of water and electrolytes found in stool suring diarrhea? Small intestine
How do Epsom salt (MgSO4) laxatives work? Draw plasma water into the lumen which loosens the stool.
Effect of Ca+, cAMP, and cGMP on the Crypt cells of both large and small intestine Open up Cl- channels at the apical membrane. **Mostly seen in small intestine. As a result of the Cl- & Na secretion, large volumes of water will follow.
List the Normal and abnormal secretagogues that Inc Cl- secretion from the crypt cells NORMAL: 1.ACh (Inc Ca+). 2.Secretin (Inc cAMP). 3.VIP (Inc cAMP). ABNORMAL: 1.Cholera Toxin (Inc cAMP). 2.E coli (Inc cAMP).
Secretory Diarrhea Diarrhea resulting mainly from an Increased volume of secretion (which then draws out large volumes of water). **Also is partially due to dysfunctioning colon.
Causes of Secretory Diarrhea 1.E.Coli. 2.Shigella Flexneri. 3.Salmonella. 4.Virbrio Cholerae (affects both large & small Int and Inc cAMP in crypts and Villus).
How is oral rehydration therapy used to treat secretory diarrhea? It utilizes the co-transport of Na+ w/ aa's or glucose to increase water reabaroption in the small int. **Inc lumenal aa and glucose will enhance absorption.
Ca+ reabsorption w/in the Enterocyte Enhanced by Vit D which stimulates Ca+ transport via CaBP (binding protein) which allows it to cross the brush boarder. Ca+ exits basolateral mem via: 1.Ca-ATPase. 2.Ca/Na exchanger.
Where is iron absorbed by enterocytes? Duodenum. MUST be in Ferrous (Fe2+) form. **Brush boarder reductases reduce Ferric Fe3+ to Ferrous Fe2+ so it can cross the apical mem via DMT-1 transporter
Created by: WeeG



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