Physiology of absorption in the small intestine

Question 1

Where is the brunner’s gland found?

Answer 1

Found in the first few centimeters of the duodenum, between the pylorus of the stomach and the papilla of vater

Question 2

What is the function of the brunner’s gland?

Answer 2

It produces alkaline mucus which protects the duodenum from the acidic gastric juices and it neutralizes the acid

Question 3

What stimulates the brunner’s gland to secrete alkaline mucus?

Answer 3

1) Tactile or irritating stimuli

2) Vagal stimulation

3) GI hormones (especially secretin)

Question 4

What inhibits the secretion of mucus from brunner’s glands?

Answer 4

Sympathetic stimulation (which is the reason of ulcer in excitable people)

Question 5

What is the crypt of liberkuhn?

Answer 5

• They line the entire surface of the small intestine, found between the intestinal villi
• The crypts and villi are covered by the epithelium (with two types of cells):

1) Goblet cells

2) Enterocytes: Secretes large quantities of water and electrolytes on the other hand on the surface of adjacent villi it reabsorbes water, electrolytes and end products of digestion

Question 6

Where are the crypts of lieberkuhn found?

Answer 6

The are invaginations between adjacent villi

Question 7

What are the type of cells that covers the crypts and villi?

Answer 7

Epithelium with two types of cells (goblet cells & enterocytes)

Question 8

What is the function of the enterocytes that lines the villi and crypts?

Answer 8

• It has two sides one facing the blood (named as the basolateral side) and one which faces the lumen (named as the apical/brush border)

1) Secretion

• The ones which lies the crypts secretes large quantities of water and electrolytes

2) Absorption

• The ones which lines the villi, they reabsorb water, electrolytes and end-products of digestion

Question 9

What are the intestinal secretions?

Answer 9

• AKA succus entericus (1800ml/day)
• These secretions represents pure ECF with an alkaline pH (7.5-8)
• These secretions helps in the absorption (they will run like a current into the villus, allowing the nutrients to be absorbed)

Question 10

How are the intestinal secretions produced?

Answer 10

1) Active secretion of chloride ions by the enterocytes of the crypts

2) Active secretion of bicarbonate ions

3) Electrical drag of the positively charged sodium ions

4) Osmotic movement of water

Question 11

What is the composition of the succus entericus?

Answer 11

1) Chloride

2) Bicarbonate

3) Sodium

4) Water

Question 12

What is the mechanism of intestinal secretions?

Answer 12

• The apical membrane of the crypts contains Cl- channels (which opens only due to certain hormones or neurotransmitters binding to receptors on the basolateral membrane)
• the basolateral membrane contains a NA+, K+ AND 2Cl- cotransporter and a Na+/K+ channel (Na+ out and K+ in in exchange with ATP)

1) Hormones or neurotransmitters (like Vasoactive intestinal polypeptide) stimulates the receptor on the basolateral side

2) The receptor will then produce cAMP which will open the chloride channels on the brush border to secrete chloride

• Chloride are gotten into the cell by the sodium/potassium/2 Cl cotransporter

3) Sodium ions are then secreted parafollicularly and water follow it

Question 13

What are the digestive enzymes found in the small intestine?

Answer 13

• The enzymes are present in the enterocytes of the villi (and that is why they can absorb stuff)

1) Peptidases

2) Sucrase

3) Maltase

4) Isomaltase

5) Lactase

6) Intestinal lipase

Question 14

What is the function of the enzyme peptidases?

Answer 14

It splits small peptides into amino acids

Question 15

What is the function of the enzymes intestinal lipase?

Answer 15

It splits neutral fats into glycerol and fatty acids

Question 16

What is the function of the enzymes (sucrase, maltase, isomaltase, and lactase)?

Answer 16

They split disaccharides into monosaccharides

Question 17

Where are the stem cells of the enterocytes found?

Answer 17

In the crypts

• The life span of an enterocyte is 5 days

Question 18

What is the normal daily intake/output of water in the GIT?

Answer 18

1) Ingested: 2000ml

2) Endogenous secretions: 7000 ml

3) Total input: 9000 ml

• 90% Of this fluid is reabsorbed
• 100ml is lost in the stool

Question 19

How is water absorbed?

Answer 19

• Water is transported through the intestine entirely by diffusion (osmosis driven by the electrochemical gradients established by active transporters)
• When the chyme is diluted enough water is reabsorbed, water is absorbed through the intestinal mucosa into the blood via osmosis
• If the chyme wasn’t diluted and instead it was hyperosmotic water will travel from the blood into the lumen

Question 20

What are the different ways by which sodium is absorbed?

Answer 20

• The intestine must absorb 25-35 gm/day of sodium, and it does so by:

1) Active transport of sodium at the basolateral membrane, and thus sodium will move down the electrochemical membrane (from chyme into the enterocytes)

2) Co-transportation by specific carrier proteins (Na/glucose cotransporters, Na/Amino acid cotransporters, Na-H exchanger)

3) Aldosterone is a hormone that increases the channels for sodium reabsorption, and as we know water follows Na

Question 21

How are chloride ions absorbed?

Answer 21

1) In occurs in the duodenum and jejunum, where Cl- ions diffuses along the electrical gradient

• This is due to the absorption of Na which makes the paracellular spaces electropositive

Question 22

How is chloride absorbed in the duodenum and jejunum (at the brush border)?

Answer 22

By diffusion along the electrical gradient

Question 23

How is chloride ions absorbed in the ileum?

Answer 23

1) Cl-/HCO3- Exchanger

• The chloride ion will then exit through the channels present on the basolateral membrane (excretes bicarbonate “important for protection from the bacteria” and takes chloride)

Question 24

How is the bicarbonate reabsorbed in the duodenum and jejunum?

Answer 24

• A lot of bicarbonate is found in the duodenum and jejunum due to the bile and pancreatic juices
• It occurs by a indirect mechanism where:

1) Inside the cell, Carbonic anhydrase will convert CO2 and Water to form bicarbonate and hydrogen, the bicarbonate produced here will diffuse through the basolateral membrane

2) The hydrogen produced inside the cell from that reaction will be exchanged by the sodium hydrogen channel found in the brush border and into the lumen

3) In the lumen H+ will react with the bicarbonate found in the chyme and forming CO2 and water, the water will stay with the chyme,, and the CO2 will be taken by the blood to be expired

Question 25

What are the molecules that are absorbed in the duodenum and jejunum?

Answer 25

1) Sodium 2) Bicarbonate 3) Amino acids 4) Sugars

Question 26

What happens to the bicarbonate in the ileum?

Answer 26

It gets secreted

Question 27

How is chloride absorbed in the ileum?

Answer 27

- In the ileum (ileum has a net absorption of NaCl) and large intestine 1) Chloride ions are absorbed by exchanging them with HCO3- in the apical membrane - The bicarbonate are secreted in order to neutralize the acid products formed by the bacteria 2) and then the Cl- transporters in the basolateral membrane takes the Chloride ions to the blood

Question 28

What is the net absorption in the ileum and large intestine?

Answer 28

NaCl

Question 29

How is fructose absorbed in the small intestine (duodenum & jejunum)?

Answer 29

- Facilitated diffusion

Question 30

How is glucose absorbed in the small intestine (duodenum & jejunum)?

Answer 30

1) The ATP dependent sodium and potassium pump will get sodium into the blood and K+ into the cell creating a gradient for the entry of sodium - Then driven by the gradient created earlier SGLT1 (A cotransporter) will get Na and glucose/galactose (as monosaccharides only) 2) GLUT5 gets fructose inside (via facilitated diffusion) - They then move to the blood via GLUT2

Question 31

How are the proteins absorbed in the small intestine (duodenum & jejunum)?

Answer 31

- Absorbed as dipeptides, tripeptides, and amino acids (via the Na-cotransport mechanism) - The gradient created by the Na/K channel will make the channel in the brush border transfer amino acids along with Na into the cell, and they are then absorbed into the blood via facilitated diffusion - Di & Tripeptides - In the stomach proteins are broken down by pepsin, which works on acidic environment only (so in the duodenum pepsin wont work anymore), at this point proteins have been converted to di and tri peptidases which can be absorbed by the enterocytes - However di & tripeptides can either go directly to the blood or through the same channel as the amino acids but must be broken if so by the enzyme (peptidase)

Question 32

How are monoglycerides & fatty acids absorbed in the small intestine (duodenum and jejunum)?

Answer 32

- Absorbed after being dissolved in bile micelles (which will then get converted to triglycerides and released as chylomicrons)

Question 33

What is the transporter that transports glucose and galactose from the lumen into the cell?

Answer 33

1) SGLT1 - It only takes them as monosaccharides

Question 34

What is the channel that transports fructose into the cell?

Answer 34

GLUT5 - Via facilitated diffusion

Question 35

What is the channel that transports sugars from the cell to the blood?

Answer 35

GLUT2

Question 36

How is fat absorbed in the small intestine?

Answer 36

- Fat is not soluble in the chyme and thus 1) Bile salts binds to it and takes it to the enterocytes 2) The enterocytes, and once they reach the brush border the bile salts are recycled and the fat enters the cell 3) The fat will then become a triglyceride which are taken by the chylomicrons and then transported by the lacteals (and not the blood)

Question 37

What are the different types of intestinal motility?

Answer 37

- We have two types one propels the food forward, and the other is the mixing contractions which mixes the food with the enzymes and electrolytes and it also segments/chops the chyme 1) Propulsive 2) Mixing

Question 38

What determines the frequency of the mixing contractility of the small intestine?

Answer 38

The basic electrical rhythm - With the maximum contraction being 12 per minutes in the duodenum and jejunum - While in the ileum it is 8/minute

Question 39

Describe the propulsive contractions (peristalsis)

Answer 39

- At a velocity of 0.5-2cm/sec 1) Excitation occurs behind the bolus (like Ach, Substance P, Neuropeptide Y "these are released into the circular muscles") 2) Inhibition occurs in front of the bolus (vasoactive intestinal peptides, nitric oxide "activated in the circular muscle") It propels the chyme

Question 40

What controls the peristalsis of the small intestine?

Answer 40

Hormonal and nervous signals

Question 41

What are the excitatory signals that are sent behind the bolus?

Answer 41

1) Ach 2) Substance P 3) Neuropeptide Y

Question 42

What are the inhibitory signals sent in-front of the bolus?

Answer 42

1) Vasoactive intestinal peptide 2) Nitric oxide

Question 43

What are the hormones that inhibits the intestinal motility?

Answer 43

1) Secretin 2) Glucagon

Question 44

What are the hormones that enhances the intestinal motility?

Answer 44

1) Gastrin 2) CCK 3) Insulin 4) Motilin 5) Serotonin

Question 45

What is the importance of the ileocecal valve?

Answer 45

- The ileocecal valve protrudes from the ileum into the lumen of the cecum and it closes once the pressure in the cecum increases - The ileocecal sphincter is a thickened circular muscle present behind the valve in the wall of the ileum, which is controlled by reflexes coming from the cecum (like increased pressure, irritants, bacterial infection)

Question 46

What is the gastroileal reflex?

Answer 46

Reflexes coming from the stomach as it is full to relax the ileum

Question 47

From which cells is secretin secreted?

Answer 47

The S-cells of the mucosa of the upper small intestine

Question 48

What stimulates the secretion of secretin?

Answer 48

1) H+ ions (low pH) 2) Fatty acids

Question 49

What are the functions of secretin?

Answer 49

- It mainly helps in neutralizing the acidity of the contents coming from the stomach by: 1) Pepsin secretion 2) Secretion of pancreatic bicarbonate 3) Secretion of biliary bicarbonate 4) Growth of the exocrine pancreas 5) Contraction of the pyloric sphincter 6) Inhibiting the gastric acid secretion 7) Inhibiting the motility (mild effect)

Question 50

What are the cells that secrete cholecystokinin?

Answer 50

The i-cells of the mucosa of the upper small intestine

Question 51

What stimulates the secretion of CCK?

Answer 51

1) Amino Acids 2) Fats

Question 52

What is the function of cholecystokinin?

Answer 52

- It stimulates the gall bladder to release bile acids to emulsify the fat by: 1) Stimulating the contraction of the gall bladder 2) Augmenting the action of secretin (alkaline pancreatic juice) 3) Inhibiting the gastric secretion, motility and emptying

Question 53

What are the intestinal reflexes?

Answer 53

1) Intestinointestinal reflex 2) Gastroileal reflex

Question 54

What is the intestinointestinal reflex?

Answer 54

- Mediated by the enteric neurons and the vagus nerve - Stimulated by the overdistention of one segment of the intestine, will result in the relaxation of the smooth muscle in the rest of the intestine

Question 55

What is the gastroileal reflex?

Answer 55

- Mediated by the vagus nerve - The increased secretory and motor function of the stomach increases the motility of the terminal part of ileum and accelerates the movement of materials trough the ileo-cecal sphincter

Question 56

What is cholera?

Answer 56

- It is a secretory diarrheal disease 1) Subunits of cholera toxins enters into the epithelial cells, triggering the formation of excess cAMP 2) Cl- channels opens up and Cl- flows into the intestinal crypts 3) Na+ ions are also pumped into crypts to go along with chloride 4) Water will then follow by osmosis (increasing the water and electrolyte content of the intestine causing diarrhea)

Question 57

What is the celiac disease?

Answer 57

- It is a genetically predisposed autoimmune response - The toxic effect of gluten (which is found in wheat, oats, etc "not in rice nor corn") stimulates an inappropriate immune response from T-cells - The intestinal epithelial cells will get damaged and the villi are lost, and the mucosa flattens - It is managed by a gluten free diet

Question 58

What is paralytic ileus?

Answer 58

- when the ileum becomes distended by gas and fluid decreasing the tone causing it to be relaxed: 1) When a trauma occurs in the intestine during an abdominal surgery, inhibiting the smooth muscles, activating the opioid receptors which will decrease the intestinal motility 2) Irritation to the peritoneum (which will cause a reflex inhibition of the motility as their is an increased discharge of noradrenergic fibers "sympathetic nervous system" in the splanchnic nerves) - It is relieved by passing a tube through the nose down to the small intestine and aspirating the fluid and gas