Lecture 29

Question 1

Describe a cross-section of the gut?

Answer 1

Go from serosa -> circular and longitudinal muscle layers -> submucosa -> lumen. When looking at the cell, the lumen is the external wall and the cell as the ICF and blood as ECF (extracellular fluid).

Question 2

Describe a cross-section of the colon (LI)?

Answer 2

There are goblet cells (PST).

Question 3

Describe what the SI has that the LI does not? (functional difference)

Answer 3

SI have villi, microvilli, crypts of lieberkuhn to increase SA for absorption. The SI will be the size of 2 tennis fields, 600 fold increase in SA.
LI has absence of villi, simply crypts and larger segments to increase SA.
Main functional difference is SI is about absorption of nutrients; LI is absorption of fluids and secretion of ions.

Question 4

What are ways to increase SA of SI?

Answer 4

Surface of SI is amplified (folded) at 3 levels:

1) Folds of kerchring
2) Microvilli and crypts of lieberkuhn.
3) Submicroscopic microvilli.

Question 5

What are ways to increase SA of LI?

Answer 5

Surface of LI is amplified (folded) at 3 levels:

1) Semilunar folds.
2) Crypts, but no villi.
3) Microvilli.

Question 6

How much saliva do we secrete?

Answer 6

1.5L/day.

Question 7

How much gastric secretion do we secrete in the stomach?

Answer 7

2L/day.

Question 8

How much pancreatic secretion do we secrete?

Answer 8

1.5L/day.

Question 9

How much bile do we secrete?

Answer 9

0.5L/day.

Question 10

How much does our SI secrete?

Answer 10

1L/day (HCO3-).

Question 11

How much is reabsorbed by the SI?

Answer 11

6.5L/day.

Question 12

How much fluid is presented to the lumen of SI?

Answer 12

8.5L/day.

Question 13

How much fluid is presented to the colon?

Answer 13

2L/day.

Question 14

How much is reabsorbed by the colon?

Answer 14

1.9L/day.

Question 15

How much is excreted in the faeces?

Answer 15

0.1L/day.

Question 16

What is the length of the SI?

Answer 16

6m.

Question 17

What is the length of the LI?

Answer 17

2.4m.

Question 18

What is the area of apical plasma membrane (m2) in the SI?

Answer 18

200.

Question 19

What is the area of apical plasma membrane (m2) in the LI?

Answer 19

25.

Question 20

What are the characteristics of the SI?

Answer 20

1) Folds.
2) Villi.
3) Crypts or glands.
4) Microvilli.
5) Nutrient absorption.
6) Active Na+ absorption.

Question 21

What are the characteristics of the LI?

Answer 21

1) Folds.
2) Crypts or glands.
3) Microvilli.
4) Active Na+ absorption.
5) Active K+ secretion.

Question 22

Describe intestinal epithelial cells?

Answer 22

Na+/K+/ATPase - take 2 potassium into the cell and 3 sodium out of the cell. It requires ATP. Because there is a concentration gradient of sodium going out, sodium always comes into our body. This will be used by glucose and sodium/glucose.amino acid transporters. Due to gradient into the cell, glucose, and amino acids come into the cell (secondary active transport). SGLT is responsible for it and GLUT. The movement will either move out of the cell (secretion) or into the cell (absorption); transcellular (2 membranes into the cell) or paracellular (using tight junctions).

Question 23

What happens if a drug stops Na+/K+/ATPase?

Answer 23

Digitalisis - blocks the activity of sodium/potassium ATPase.

Question 24

What is a transcellualr pathway?

Answer 24

Solute is moving across 2 membranes: the active membrane (apical) which requires something to move in; and the basolateral membrane.

Question 25

How is water moved across the cell?

Answer 25

Either transcellular or paracellular. So either through osmotic movement or with a solute. Usually paracellular pathway due to low resistance, and primarily in jejunum where there is most fluid absorption.

Question 26

How does sodium absorption occur?

Answer 26

There are Na+/K+/ATPase on the basolateral surface (3 sodium out and 2 potassium in) and a GLUT transporter, and there are SGLT on the apical surface. As sodium moves into the cell, brings amino acid or glucose into the cell. Maintains low Na+ conc, provides force for Sodium to move into the cell.

Question 27

Ways to bring sodium into the cell?

Answer 27

1) Primarily in the jejunum and the ileum, is by cotransport - SGLT.
2) In the jejunum, there are sodium hydrogen exchanges, which exchange 1 sodium to hydrogen. pH dependent.
3) Sodium chloride and hydrogen bicarbonate exchanges. When one sodium is bought into the cell you get one chloride. Then you get hydrogen going out and bicarbonate coming in.
4) In distal colon, apical sodium channels which absorb sodium - aldosterone. increases sodium channels being inserted into the apical membrane.

Question 28

Describe how chloride moves in our body?

Answer 28

Moves depending on where sodium is going. Sodium is net positive, this means you need to electroneutralise - brings chloride into the cell via passive movement.

Question 29

How does potassium move passively into the cell?

Answer 29

Paracellular pathway or passive secretion or could be active secretion (BK channels - apical membrane, primarily in proximal colon). Or by active absorption in exchange of hydrogen ions.

Question 30

What does CFTR do?

Answer 30

Secretion of chloride channels by means of increased cAMP activity.

Question 31

What controls all of the above?

Answer 31

1) Enteric nervous system - ACh - myenteric plexus. Local control of GI movement/activity.
2) Aldosterone - endocrine (hormonal).
3) Paracrine system - 5SG serotonin which increase absorption of sodium/potassium/chloride.

Question 32

How do you diagnose diarrhoea?

Answer 32

1) Increase in the number of bowel movements (3+) or decrease in stool consistency.
2) Increase in stool volume of more than 0.2L/24 hours.

Question 33

What is osmotic diarrhoea?

Answer 33

Large osmotic absorption coming through. e.g. Large osmotic laid, malabsorption syndrome and movement of water from GI cells into the lumen, pancreatic disease and lactose intolerants and coeliac disease.

Question 34

What is secretory diarrhoea?

Answer 34

Increased secretion of chloride (primarily), leads to movement of sodium and affectively water. This can lead to diarrhoea.
e.g. cholera toxin, lead to cAMP activity, leads to ion secretion (Cl-). Can be treated by ORS.

Question 35

What compartment of fluid do people of diarrhoea lose it from?

Answer 35

ECF. There is less plasma in the ECF, so there is less blood volume, this means increased Heart Rate.

Question 36

What does Sandy have?

Answer 36

Increased cAMP, cholera. She has secretory diarrhoea. This is due to increased chloride secretion, so loosing Cl- and water from ECF.

Question 37

How do we treat Sandy?

Answer 37

ORS - Oral rehydration solution. Made up of glucose, bicarbonates and chloride. Cholera has impact only on chloride channels, so you can bring nutrients that you have lost back into cell via giving sodium and glucose. Bicarbonates are given (when you have diarrhoea you become acidotic) to maintain the pH.

Question 38

What is the purpose of ORS?

Answer 38

To correct loss of electrolytes from the body, and restore sodium balance across brush border, aid osmotic absorption in the lumen, maintain electroneutrality.