Absorption of ions and water

Question 1

Name the 5 cells of the small intestine wall

Answer 1

Absorptive cell
Goblet cell
Enteric endocrine cell
Stem cell
Paneth cell

Question 2

What are villous cells mostly responsible for?

Answer 2

Absorption of nutrients and electrolytes

Question 3

What are crypt cells responsible for?

Answer 3

Secretion

Question 4

What mediates secretion in the colon?

Answer 4

Gland cells

Question 5

What do all mucosa have?

Answer 5

A stem cell compartment

Question 6

What increases the surface area of the small intestine?

Answer 6

Macroscopic folds of Kerckring
Villi
Microvilli

Question 7

What is transcellular movement?

Answer 7

The movement of solute across two membranes in series.

Active transport

Question 8

What is paracellular movement?

Answer 8

The movement of solute passively between adjacent epithelial cells via tight junctions

Question 9

What is the absorption of water dependent on?

Answer 9

The absorption of ions, Na+ and Cl-

Question 10

What is the importance of Na+ and Cl-

Answer 10

Transport of Na+ and Cl- causes the cell to be hypertonic and water moves in via osmosis

Question 11

Where is Na+ absorbed?

Answer 11

Along the entire length of the intestine

Question 12

Name the 4 methods of Na+ absorption

Answer 12

Na-Glucose transporter
Na-H exchanger
Parallel Na-H and Cl-HCO3 exchanger
Epithelial Na+ channels

Question 13

Describe how the Na-Glucose transporter works

Answer 13

Na+ exported through basal membrane by Na+K+ATPase
Na+ crosses apical membrane down an electrochemical gradient
Provides energy for moving sugars and neutral amino acids into the cells against concentration gradients
Common postprandially

Question 14

What is the meaning of postprandial?

Answer 14

After eating

Question 15

Describe how the Na-H exchanger works

Answer 15

Na+ uptake is coupled with H+ export

The energy comes from the Na+ gradient created by Na+K+ATPase pump on the basal membrane

Question 16

Where is the Na-H exchanger most commonly used?

Answer 16

In the jejunum and to a lesser extent in the duodenum

Question 17

Describe how the parallel Na-H and Cl-HCO3 exchanger works

Answer 17

Carbonic anhydrase coverts carbonic acid it to a bicarbonate ion and a H+
These are exported from the cell in the exchange for Na+ and Cl-
This regulated by cAMP and cGMP and Ca2+
An increase of any of these reduces absorption
Common in fasted state

Question 18

Explain how the Na+ epithelial channels work

Answer 18

Channels are highly efficient at transporting Na+

Question 19

How do mineralocorticoids affect Na+ absorption?

Answer 19

Increase it
Increase opening of apical Na+ channels
Insertion of preformed Na+ channels into apical membrane
Increased synthesis of Na+ channels and Na+K+ATPase

Question 20

Where does Cl- absorption occur?

Answer 20

Throughout the small and large intestine

Question 21

Name the three methods of Cl- absorption

Answer 21

Passive Cl- absorption
Parallel Na-H and Cl-HCO3 exchanger
Cl-HCO3 exchanger

Question 22

Describe passive Cl- absorption

Answer 22

A passive process driven by the electrochemical gradient for Cl-
Driving force is derived from Na-Glucose transport and Na+ channels

Question 23

What is congenital chloridorrhea?

Answer 23

The congenital absence of an apical Cl-HCO3 exchanger

Affected children have diarrhoea with high Cl- concentration and are alkalotic due to reduced HCO3 secretion

Question 24

What do absorptagogues and secretagogues promote?

Answer 24

Absorptagogues promote absorption

Secretagogues promote secretion

Question 25

Name two absorptagogues

Answer 25

Angiotensin and aldosterone

Question 26

What stimulates the release of absorptagogues?

Answer 26

Dehydration | A drop in effective circulating volume stimulates the renin-angiotensin-aldosterone axis

Question 27

What does angiotensin stimulate?

Answer 27

Electroneutral NaCl absorption by upregulating apical membrane Na-H exchange

Question 28

What does aldosterone stimulate?

Answer 28

Stimulates Na+ absorption in the colon through the epithelial Na+ channels

Question 29

What are the four categories of secretagogues?

Answer 29

Bacterial enterotoxins Hormones and neurotransmitters Products of the immune system Laxatives

Question 30

How do bacterial enterotoxins induce secretory diarrhoea?

Answer 30

First induces intracellular concentration of cAMP to increase which stimulates Cl- and K+ secretion to increase whilst inhibiting NaCl absorption

Question 31

Give an example of hormone mediated diarrhoea

Answer 31

Verner-Morrison syndrome | Pancreatic tumours produce vasoactive intestinal peptide which via cAMP induces diarrhoea

Question 32

Describe immune mediated diarrhoea

Answer 32

Prostaglandins and histamine released from mast cells and macrophages induce diarrhoea via cAMP

Question 33

Describe how anti-diarrhoeals work

Answer 33

Act on myenteric plexus of large intestine | Reduce smooth muscle motility increasing the amount of time water stays in the lumen increasing absorption

Question 34

Name the 6 kinds of laxative

Answer 34

``` Bulk laxatives Stool softeners Lubricant agents Osmotic laxatives Stimulant laxatives Prokinetic agents ```

Question 35

How is calcium absorbed?

Answer 35

Passively and actively

Question 36

Describe the passive transport of calcium

Answer 36

Through the paracellular pathway, not dependent on VDR

Question 37

Describe the active transport of calcium

Answer 37

Transcellular uptake in the epithelial cells of the duodenum, dependent of VDR

Question 38

Describe the three stages of active Ca2+ absorption

Answer 38

Via Ca2+ channels Cytostolic Ca2+ binds to calbindin which acts as a buffer A Ca2+ pump and a Na-Ca exchanger on the basal membrane export Ca2+ The active form of Vitamin D1 stimulates this

Question 39

What will vitamin D deficiency lead to?

Answer 39

Rickets, leading to hypocalcaemia