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Digestive and Renal Sciences > Absorption of ions and water > Flashcards

Absorption of ions and water Flashcards

1
Q

What is responsible for the absorption of dietary nutrients?

A

The small intestine exclusively

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2
Q

How is the small intestine adapted for absorption?

A

Macroscopic folds of Kerckring (plicae circulares).
Villi.
Microvilli on the apical surfaces of the epithelial cells and crypts

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3
Q

What is the approximate total area of the human small intestine?

A

Approximately 200M^2

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4
Q

On average how much fluid flows through the digestive system per day, and how much is reabsorbed?

A

About 8.5L per day

Small intestinal absorption about 6.5L

Large intestinal absorption about 1.9L

(8.4L reabsorbed)

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5
Q

What is transcellular absorption?

A

the solute must cross the two cell membranes in series. Active transport implicated

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6
Q

What is paracellular absorption?

A

the solute moves passively between adjacent epithelial cells via the tight junctions

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7
Q

How is water absorbed and what does it depend on?

A

Transport of Na, Cl - and HCO3 into the lateral intercellular spaces. The resulting high NaCl near the apical end of the intercellular space causes it to be hypertonic which causes an osmotic flow of water from the lumen, and via the tight junctions into the intercellular spaces.

Absorption of water depends on the absorption of ions (Na and Cl)

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8
Q

What are the different transport routes of entry for sodium?

A

Na/Glucose transport or Na/Amino acid transport

Na-H exchanger

Parallel Na-H and Cl-HCO3 exchange (electromutural sodium transport)

Epithelial Na^+ channels

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9
Q

Explain how sodium is absorbed by Na/Glucose transport or Na/Amino acid transport?

A
  1. Na and glucose bind to SGLT1 transporter which brings in the ions and glucose into the cell.
  2. Na is transported in down and electrochemical gradient which is set up by a Na/K pump on the basal side of the cell.

Important postprandial

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10
Q

Explain how glucose is absorbed by Na-H exchanger?

A

The Na-H exchanger couples Na+ uptake across the apical membrane to proton extrusion into the intestinal lumen. The process is enhanced by both decreases in intracellular pH and increases in luminal pH.

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11
Q

Explain how sodium ions are absorbed by the Parallel Na-H and Cl-HCO3 Exchange (Electroneutral)?

A

Electroneutral NaCl absorption is due to two apical membrane Na-H and Cl-HCO3 exchangers closely linked

The two antiporters bring in Na+ and Cl- ions in exchange for H+ and HCO3- ions meaning that this process is electroneutral.

Occurs in the ileum and large intestine

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12
Q

Explain how sodium ions are absorbed by the Epithelial Na+ Channels?

A

Na+ entry occurs across the apical membrane via ENaC channels that are highly specific for Na+

• Na+ absorption in the distal colon is highly efficient as it is capable of absorbing Na+ against large concentration gradients

Regulated by mineralocorticoids

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13
Q

How do mineralocorticoids increase Na+ absorption?

A
  1. Increase in the opening of apical Na+ channels
  2. Insertion of preformed Na+ channels from sub-apical epithelial vesicle pools into the apical membrane
  3. Increased synthesis of apical Na+ channels and Na-K pumps
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14
Q

What are the 3 methods for Cl- transport in the small intestine?

A
  1. Passive Cl- Absorption using Cl electrochemical gradient
  2. Parallel Na-H and Cl-HCO3 Exchange (same as Na)
  3. Cl-HCO3 exchange in absence of electroneutral Na-H coupled exchange.

There is also evidence of Cl-secretion (mediated by cAMP and Ca2+) in small and large bowel crypt cells

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15
Q

What is Congenital Chloridorrhea?

A

Autosomal recessive disorder where there is congenital absence of apical Cl-HCO3 exchangers (which mediate Cl and Na absorption)

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16
Q

What are absorptagogues?

A

substances that promote absorption where as secretagogues promote secretion (diarrhoea)

17
Q

Give some examples of absorptagogues and how they work

A

Angiotensin and aldosterone.

Regulate total body Na homeostasis by stimulating Na absorption.

18
Q

What are the secretagogues?

A 
Substances that promote secretion
Broken into 4 categories:
 (1) bacterial  enterotoxins,      
(2) hormones and neurotransmitters,   
(3) products of cells of the immune system,
(4) laxatives
19
Q

What causes cholera?

A

Bacterium Vibrio Cholerae which releases cholera toxin, an enterotoxin responsible for the massive diarrhoea of the disease (patients may produce up to 20L of stool per day)

20
Q

Why and how does the cholera toxin cause sercretory diarrhoea?

A

The enterotoxin induces intracellular concentrations of cAMP which increases Cl and K secretion and inhibits electroneutral NaCl absorption.

21
Q

How can neurotransmitters/hormones induce diarrhoea?

A

Verner-Morrison Syndrome:

Pancreatic trumours producing a vasoactive intesinal peptide which through cAMP induces diarrhoea.

22
Q

What are some treatment strategies for anti-diarrhoeals?

A
  1. Loperamide is an opioid receptor agonist which acts on the myenteric plexus in the large intestines.

• Decreases the smooth muscle motility increasing the amount of time substances stay in the intestine, allowing for more water to be absorbed out of the fecal matter. 2. Somatostatin analogue

23
Q

What are some different laxatives and what do they do?

A

Bulk - Increased fibre when taken with water

Stool softners - allow water to enter stool more readily

Lubricant - lubricate the stool surface

Osmotic - hypertonic increase in stool water

Stimulant - increased peristalsis

Prokinetic - increased peristalsis

24
Q

How is calcium absorbed via passive transport?

A

The passive absorption of Ca2+ throughout the small intestine occurs via the paracellular pathway, which is not under the control of VDRs (vitamin D receptor)

25
Q

How is calcium absorbed via Active transport?

A

Active, trans-cellular uptake of Ca2+ occurs only in the epithelial cells of the duodenum and is under the control of VDR

26
Q

What does the VDR do to allow for calcium absorption?

A
  1. The uptake of Ca2+ across the apical membrane via Ca2+ channels
  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 the Ca2+ from the cell into the interstitial fluid
27
Q

What is the active form of Vitamin D called and what does it do?

A

D-1,25-dihydroxy-vitamin D which stimulates the 3 steps in active calcium absorption.

28
Q

What happens with vitamin D deficiency and what is the treatment?

A

Rickets
leads to hypocalcaemia

Treatment:
increasing dietary intake of calcium, phosphates and vitamin D.
Exposure to ultraviolet (sunshine),
fish oils are good sources of vitamin D.

29
Q

What causes an increase in luminal pH?

A

Luminal secretion of HCO3 by pancreatic, biliary and duodenal tissues.

30
Q

Where does sodium uptake occur by the Na-H exchanger?

A

Mainly in the jejenum and duodenum.

31
Q

What is the primary Na absorption method in the fasted state (between meals)?

A

Electroneutral parallel Na-H and Cl-HCO3 exchange.

32
Q

What regulates Electroneutral parallel Na-H and Cl-HCO3 exchange?

A

cAMP and cGMP as well as intracellular Ca.

Increase in these molecules causes a reduction in NaCl absorption.

33
Q

How do enterotoxins induce secretory diarrhoeas?

A

Elevating cAMP levels and therefore inhibiting NaCl absorption.

34
Q

What are the clinical symptoms of congenital chloridorrhea?

A

affected children have diarrhoea with high stool [Cl-].

Patients are also alkalotic (increased plasma HCO3 since secretion is reduced)

35
Q

What is the treatment for congential chloridorrhea?

A

Proton pump inhibitors.

36
Q

What stimulates the release of angiotensin and aldosterone?

A

Dehydration and a drop in the effective circulating volume.

37
Q

How does angiotensin work to increase Na absorption?

A

In the small intestine, it enhances electroneural NaCl absorption by upregulating apical membrane Na-H exhcange.

38
Q

How does aldosterone work to increase Na absorption?

A

Stimulates Na absorption in the colon through ENaC

39
Q

How does the immune system induce diarrhoea?

A

histamine and prostaglandins released from mast cells and macrophages induce diarrhoea through cAMP.

IBD can also cause diarrhoea.

Digestive and Renal Sciences (26 decks)

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