Electrolytes and Motility Physiology Flashcards
What is Anismus?
Anal sphincter dyssynerigia
What are Haustra?
One of the pouches of the colon, produced by adaptation of its length to the taenia coli
What is Haematochezia?
Blood in the stool
What is Ileus?
Failure of forward movement of intestinal contents
What is purgative?
Substance that promotes bowel loosening and movement; cathartic, laxative
What is Tenesmus?
Feeling that you need to pass stool even when bowels are empty
What are three transmembrane ion transporter processes in the large bowel?
- Pumps
- Channels (pores)
- Carriers
How do Pumps work?
Active transport, Na+/K+ ATPase and H+/K+ ATPase.
- Uphill transport against an electrochemical gradient
- Effective at low concentrations
- Demonstrate saturable kinetics
- Require cellular energy
- Demonstrate high ionic specificity
How do channels work?
(Pores) Na+, CFTR, K+
-Passive movement along the prevailing electrochemical gradient
How do carriers work?
Movement of ion or substance against electrochemical/concentration gradient by coupling it to movement of another ion moving with electrochemical gradient (secondary active transport)
- Exchangers - substances move in opposite directions (antiporter) - NHE Na+/H+ and Cl-/HCO3-
- Cotransporters - substances move in same direction (symporter) - 2 Na+/glucose, bile salts, amino acids, Pept1 H+/peptide, 2Cl-/Na+/K+ found on basolateral import Cl- into cell
What amount of water is moved through the GI tract?
~9 liters of fluid enters the gut/day
~8.9 liters of fluid is absorbed/day by the small and large intestines
~0.1 liter is excreted by the gut/day
How fluid does the colon absorb of the fluid presented to it?
90%
If the colon is removed, the body will compensate by increasing fluid and electrolyte intake.
What does absorbed fluid replenish?
Fluid losses in sweat, urine, and lungs
How do properties of the epithelium control fluid movement?
- Water can move through cells (transcellular rout) or between cells (paracellular route)
- Tight junction permeability is regulated by cytokines, bacterial toxins, and hormones which modify claudins.
- Water movement follows osmotic gradients
- Driven by electrogenic or electroneutral ion transport processes
What are the seven main Net Ion movements in the Small Intestine?
- Electroneutral NaCl absorption
- Bicarbonate secretion
- Sodium-coupled nutrient absorption
- Proton-coupled nutrient absorption
- Chloride Secretion
- Sodium-coupled bile acid absorption
- Calcium and iron absorption (not a major determinant of fluid transport)
How does fluid absorption and excretion occur simultaneously in the small intestine?
- Cells at tips of the villi absorb fluid
- Cells in crypts secrete fluid by chloride ion secretion
- Huge surface area created by folded mucosa allows for high capacity for absorption
How does Electroneutral NaCl absorption mediate fluid uptake?
- Coupled activity of sodium/hydrogen exchanger (NHE) and chloride/bicarbonate exchanger in the apical membrane
- Water follows to maintain osmotic balance
How does Electrogenic uptake of Na+ and glucose (amino acids, bile salts) drive fluid absorption in the small intestine?
-No active transport of a counterion. Instead, anions (largely chloride) and water follow passively via the tight junctions
What do Oral Rehydration Salts contain to help facilitate fluid uptake in small intestine?
Contain NaCl and glucose to exploit these transport processes and promote water uptake.
How is Chloride Secretion in the Crypts of Lieberkuhns in the Small Intestine a major mechanism promoting fluid entry into the intestinal lumen?
- Chloride is actively taken up by the 1Na+, 2Cl- symporter (NKCC1) at the basolateral membrane
- Chloride leaves the luminal side of the cell via CFTR
- Postassium leaves basilar side via a Ca++ activated K+ channel
What mediates the opening of CFTR channels?
cAMP
Where does calcium get absorbed and how is it ingested?
Ingested mostly in dairy and 30 percent is absorbed predominantly in duodenum.
How is Calcium absorbed?
30 percent is absorbed predominantly in duodenum through paracellular and transcellular routes.
- Ca++ channel on brush border membrane inwardly driven by the electrochemical gradient.
- Intracellular calcium is bound to calbindin
- Basolateral Ca++-ATPase pumps, calcium exchanged with sodium, or calcium is exocytosed.
- Vitamin D enhances expression of Ca++ channels.
Where does iron get absorbed and how is it ingested?
3-6 percent of ingested iron mostly from meat is absorbed - primarily in duodenum.
What does excess iron absorption lead to?
Hemochromatosis and a decrease in number of DMT1 transporters (iron transporters).
What is a frequent complication of partial gastrectomy?
Iron deficiency anemia
What is the function of gastric secretions in iron absorption?
Gastric secretions dissolve the iron and aid its reduction to the Fe2+ form. So individuals deficient in acid secretion absorb less.
What things regulate ion absorption and secretion?
- Active transport of solutes, especially Na+, requires energy therefore blood flow increases during a meal to increase oxygen and glucose.
- ACh/VIP stimulate chloride secretion (long reflex)
- Stroking the mucosa releases 5-hydroxytryptamine (serotonin) from local enterochromaffin cells (short reflex)
- Mucus secretion lubricates the GI tract, binds bacteria, and traps immunoglobulins
What can VIP and cholera toxin do to secretions in the small and large intestines?
VIP and cholera toxin are agonists that elevate cAMP and promote opening of the CFTR channel resulting in copious secretion in small and large intestine.
What can a VIP stimulating tumor induce?
Secretory diarrhea
What is Guanylin and what secretes it?
Gastrointestinal polypeptides secreted by goblet and ECL cells.
What does Guanylin do?
Binds GC-C recpetor on epithelial cells to promote secretion.
-Binding raises concentration of intracellular cGMP increasing Cl- secretion through CFTR into the lumen
What is Linaclotide?
A drug that acts similarly to Guanylin. It activates guanylate cyclase type C receptors and is used to treat constipation.
What toxin also induces diarrhea in the same manner as Linaclotide/Guanylin?
Heat-stable toxin produced by pathogenic E. coli induces diarrhea in the same manner.
In a healthy adult, the volume of fluid presented to the intestine on a daily basis is approximately 8 liters. Assuming a normal diet, reabsorption of the bulk of this fluid is driven primarily by which of the following?
Nutrient-coupled electrogenic sodium absorption (SGLT1 - is the transporter for glucose and Na+)
What five processes mediate the Net Movement of Ions in the Colon (lack of nutrients)?
- Electrogenic sodium absorption
- Electroneutral NaCl absorption
- Short chain fatty acids absorption
- Chloride absorption/secretion
- Potassium absorption/secretion (not a major determinate of fluid transport)
