Absorption of Ions and Water Flashcards
Which part of the digestive system is exclusively responsible for the absorption of dietary nutrients?
The small intestine.
List 5 cells that can be found in the small intestine (in the crypts of Lieberkuhn).
1 - Enterocytes (absorptive cells).
2 - Goblet cells.
3 - Enteric endocrine cells.
4 - Stem cells.
5 - Paneth cells.
What are the two main intestinal glands?
Crypts of Lieberkuhn
Brunner’s Glands
Where are Brunner’s glands found and what is their function?
- Only in the first few centimetres of the duodenum.
- Antacid function (alkaline secretion).
Where are crypts of Lieberkuhn found?
All over the surface of the small large intestines.
What is the function of Paneth cells?
Provide antimicrobial defence.
How do villous enterocytes differ from crypt enterocytes?
- Villous enterocytes are mostly responsible for nutrient and electrolyte absorption.
- Crypt enterocytes are mostly responsible for secretion.
List 5 differences between the histology of the large intestine and the small intestine.
1 - There are no villi lining the large intestine epithelium.
2 - Goblet cells are more prominent in the large intestine.
3 - Paneth cells are not usually present in the large intestine.
4 - The muscularis mucosae is more prominent in the large intestine.
5 - The outer longitudinal layer of the muscularis externa of the large intestine varies in thickness and forms three thick longitudinal bands, the taeniae coli.
What type of epithelium lines the small and large intestines?
Simple columnar epithelium (enterocytes).
What two layers comprise the muscularis externa of the small intestine?
An inner circular layer and an outer longitudinal layer.
List 3 features of the small intestine that increase surface area.
1 - Macroscopic folds of kerckring (plicae circulares).
2 - Villi.
3 - Microvilli.
What are plicae circulares?
Large circular folds of the small intestine’s mucous membrane that increase surface area.
Describe transcellular and paracellular movement.
- Transcellular: the solute crosses the cell membranes and active transport implicated.
- Paracellular: the solute moves passively between adjacent epithelial cells via tight junctions.
What does the absorption of water into enterocytes depend on?
Absorption of ions:
- Transport of Na+, Cl- and HCO3 - into the intercellular spaces near the apical end.
- Resulting hypertonicity causes osmotic flow of water from the lumen via tight junctions into intercellular spaces.
In the jejenum, ilium and colon, is Na+ absorbed or secreted?
Active or passive?
Actively absorbed in all three.
In the jejenum, ilium and colon, is K+ absorbed or secreted?
Active or passive?
Passively absorbed in the jejenum and ileum.
Both absorbed and secreted in the colon. Net secretion.
In the jejenum, ilium and colon, is Cl- absorbed or secreted?
Absorbed in all three.
In the jejenum, ilium and colon, is HCO3- absorbed or secreted?
Absorbed in the jejenum.
Secreted in the ileum and colon.
List 4 transport routes of entry for sodium.
1 - Na/Glucose transport or Na/Amino acid transport.
2 - Na-H exchanger.
3 - Parallel Na-H and Cl-HCO3- exchange.
4 - Epithelial Na+ channels.
Which protein is responsible for the cotransport of sodium and glucose into enterocytes?
SGLT-1
How is sodium ejected from enterocytes at the basal and lateral membrane?
How does this aid sodium absorption into enterocytes?
Via sodium-potassium pumps (3 Na+ out for every 2 K+ in).
- Creates electrochemical gradient across enterocyte membrane.
- Facilitates cotransport of Na and glucose / galactose / amino acids.
- Facilitates Na / H exchange.
Where is the rate of absorption of Na+ highest?
Why is the rate of absorption high here?
- In the jejunum.
- Na+ absorption in the jejunum is enhanced by the presence of glucose, galactose and neutral amino acids in the lumen.
What increases the rate of Na-H exchange?
Low intracellular pH or high luminal pH.
What can cause a high luminal pH?
HCO3- secretion by pancreatic, biliary or duodenal tissues.
Where does most Na / H exchange occur?
Where does it also occur to a lesser extent?
Mostly occurs in jejenum and to a lesser extent in duodenum.
What is the primary methods of Na+ absorption between meals (when fasting)?
Parallel Na-H and Cl-HCO3- exchange.
Where does parallel Na-H and Cl-HCO3- exchange occur?
In the ileum and proximal colon.
List 3 regulators of parallel Na-H and Cl-HCO3- exchange.
How does ach affect parallel Na-H and Cl-HCO3- exchange?
- cAMP, cGMP and intracellular Ca2+.
- Increases in any reduce NaCl absorption.
How do enterotoxins induce secretory diarrhoeas?
By elevating cAMP, inhibiting NaCl absorption.
List the stages of parallel Na-H and Cl-HCO3- exchange.
- H2O and CO2 enter the enterocyte through the basal membrane.
- Carbonic anhydrase catalyses CO2 + H2O -> H2CO3.
- H2CO3 -> H+ + HCO3-
- H+ exchange occurs, causing Na+ influx.
- Cl-HCO3- exchange occurs, causing Cl- influx.
What are ENaCs?
Where in the digestive tract are they found?
- Epithelial sodium channels.
- Found in the distal colon.
Why is sodium absorption in the distal colon highly efficient?
ENaCs are capable of absorbing sodium against large concentration gradients.
Where does Cl-HCO3- exchange occur in the absence of parallel Na-H exchange?
In the ileum and throughout the large intestine.
What is congenital chloridorrhoea?
What are the main symptoms?
What treatment is available?
- An autosomal recessive disorder causing the absence of apical Cl-HCO3- exchangers.
- Causes diarrhoea with a high stool [Cl-].
- Patients are alkalotic as HCO3- secretion is reduced.
- PPI for treatment.
Which chemical mediators promote absorption?
Absorptagogues.
Which chemical mediators promote secretion?
Secretagogues.
List 5 examples of absorptagogues.
1 - Angiotensin.
2 - Aldosterone.
3 - Somatostatin.
4 - Enkephalins.
5 - Noradrenaline.
Which axis is stimulated in response to dehydration and a drop in effective circulating volume?
What is released as a result?
- The renin-angiotensin-aldosterone axis.
- Angiotensin and aldosterone are released.
How and where does angiotensin promote absorption?
- In the small intestine.
- Upregulates NaCl absorption by upregulating Na-H exchange.
Where does aldosterone promote absorption?
List 3 ways in which this is achieved.
- In the colon.
1 - Increase in the opening of ENaCs.
2 - Translocation of ENaCs from vesicle pools to the apical membrane.
3 - Increased synthesis of apical ENaCs and basal Na-K pumps.
What are the 4 categories of secretagogues?
1 - Bacterial enterotoxins.
2 - Hormones and neurotransmitters.
3 - Products of cells of the immune system.
4 - Laxatives.
List 4 bacterial enterotoxins.
1 - Cholera toxin.
2 - E-coli toxins.
3 - Yersinia toxin.
4 - Clostridium difficile toxin A.
List 3 hormones and neurotransmitters that can act as secretagogues.
1 - VIP.
2 - Acetylcholine.
3 - Bradykinin.
List 2 immune cell products that can act as secretagogues.
1 - Histamines.
2 - Prostagladins.
What effect do bile acids have on absorption?
Act as secretagogues (decrease absorption).
Which second messenger is upregulated in response to cholera toxin?
cAMP.
Which second messenger is upregulated in response to E-coli toxins?
cAMP.
Which second messenger is upregulated in response to Yersinia toxin?
Ca2+.
Which second messenger is upregulated in response to stimulation by VIP?
cAMP.
Which second messenger is upregulated in response to stimulation by acetylcholine?
Ca2+.
Which second messenger is upregulated in response to histamine?
cAMP.
Which second messenger is upregulated in response to prostaglandins (an immune cell product)?
cAMP.
Which second messenger is upregulated in response to bile acids?
Ca2+.
What is the name of the bacterium that releases cholera toxin?
Vibrio cholerae.
How does the upregulation of cAMP cause secretory diarrhoea?
- cAMP increases Cl- and K+ secretion and inhibits parallel Na-H and Cl-HCO3- exchange.
- NaCl absorption decreases.
What is contained in diarrhoea relief medication?
Why does this work?
- A solution of sodium and glucose.
- Since enterotoxin reduces NaCl absorption by inhibiting parallel Na-H and Cl-HCO3- exchange, Na+ absorption can be increased using sodium-glucose cotransporters.
What is Verner-Morrison syndrome?
- A hormone/neurotransmitter mediated cause of diarrhoea.
- Pancreatic tumours produce vasoactive intestinal peptide.
- Causes diarrhoea through cAMP upregulation.
How do prostaglandins and histamine cause diarrhoea?
What are the sources of postraglandins and histamine in the bowel?
- Mast cells release histamine.
- Macrophages release prostaglandins.
- Both induce diarrhoea through cAMP.
How does loperamide function as an anti-diarrhoeal?
- Opioid receptor agonist which acts on the myenteric plexus in the large intestine.
- Decreases smooth muscle motility.
- Increases amount of time substances stay in the intestine.
- More water absorbed out of fecal matter.
Give an example of an anti-diarrhoeal other than loperamide.
Somatostatin analogues.
How do the transport routes of passive Ca2+ absorption and active Ca2+ absorption differ?
Passive is paracellular whereas active is transcellular.
List the three stages of active Ca2+ absorption.
What controls this process?
1 - Uptake via Ca2+ channels on the apical membrane of enterocytes (diffusion down electrochemical gradient).
2 - Cytosolic Ca2+ binds to calbindin, which acts as a buffer.
3 - A Ca2+ pump and a Na-Ca exchanger on the basolateral membrane extrude Ca2+ into itnerstitial fluid (establishes electrochemical gradient).
- Active vitamin D (D-1,25-dihydroxy-vitamin D) stimulates all 3 steps by binding to VDR.
What is rickets?
How is it treated?
- Vitamin D deficiency which leads to hypocalcaemia.
- Treatment involves increasing dietary intake of calcium, phosphates and vitamin D (exposure to ultraviolet (sunshine), fish oils are good sources of vitamin D).
