Physiology of absorption in the small intestine Flashcards
Where is the brunner’s gland found?
Found in the first few centimeters of the duodenum, between the pylorus of the stomach and the papilla of vater
What is the function of the brunner’s gland?
It produces alkaline mucus which protects the duodenum from the acidic gastric juices and it neutralizes the acid
What stimulates the brunner’s gland to secrete alkaline mucus?
1) Tactile or irritating stimuli
2) Vagal stimulation
3) GI hormones (especially secretin)
What inhibits the secretion of mucus from brunner’s glands?
Sympathetic stimulation (which is the reason of ulcer in excitable people)
What is the crypt of liberkuhn?
- 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
Where are the crypts of lieberkuhn found?
The are invaginations between adjacent villi
What are the type of cells that covers the crypts and villi?
Epithelium with two types of cells (goblet cells & enterocytes)
What is the function of the enterocytes that lines the villi and crypts?
- 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
What are the intestinal secretions?
- 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)
How are the intestinal secretions produced?
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
What is the composition of the succus entericus?
1) Chloride
2) Bicarbonate
3) Sodium
4) Water
What is the mechanism of intestinal secretions?
- 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
What are the digestive enzymes found in the small intestine?
- 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
What is the function of the enzyme peptidases?
It splits small peptides into amino acids
What is the function of the enzymes intestinal lipase?
It splits neutral fats into glycerol and fatty acids
What is the function of the enzymes (sucrase, maltase, isomaltase, and lactase)?
They split disaccharides into monosaccharides
Where are the stem cells of the enterocytes found?
In the crypts
- The life span of an enterocyte is 5 days
What is the normal daily intake/output of water in the GIT?
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
How is water absorbed?
- 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
What are the different ways by which sodium is absorbed?
- 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
How are chloride ions absorbed?
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
How is chloride absorbed in the duodenum and jejunum (at the brush border)?
By diffusion along the electrical gradient
How is chloride ions absorbed in the ileum?
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)
How is the bicarbonate reabsorbed in the duodenum and jejunum?
- 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
What are the molecules that are absorbed in the duodenum and jejunum?
1) Sodium
2) Bicarbonate
3) Amino acids
4) Sugars
What happens to the bicarbonate in the ileum?
It gets secreted
How is chloride absorbed in the ileum?
- 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
What is the net absorption in the ileum and large intestine?
NaCl
How is fructose absorbed in the small intestine (duodenum & jejunum)?
- Facilitated diffusion
How is glucose absorbed in the small intestine (duodenum & jejunum)?
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
How are the proteins absorbed in the small intestine (duodenum & jejunum)?
- 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)
How are monoglycerides & fatty acids absorbed in the small intestine (duodenum and jejunum)?
- Absorbed after being dissolved in bile micelles (which will then get converted to triglycerides and released as chylomicrons)
What is the transporter that transports glucose and galactose from the lumen into the cell?
1) SGLT1
- It only takes them as monosaccharides
What is the channel that transports fructose into the cell?
GLUT5
- Via facilitated diffusion
What is the channel that transports sugars from the cell to the blood?
GLUT2
How is fat absorbed in the small intestine?
- 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)
What are the different types of intestinal motility?
- 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
What determines the frequency of the mixing contractility of the small intestine?
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
Describe the propulsive contractions (peristalsis)
- 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
What controls the peristalsis of the small intestine?
Hormonal and nervous signals
What are the excitatory signals that are sent behind the bolus?
1) Ach
2) Substance P
3) Neuropeptide Y
What are the inhibitory signals sent in-front of the bolus?
1) Vasoactive intestinal peptide
2) Nitric oxide
What are the hormones that inhibits the intestinal motility?
1) Secretin
2) Glucagon
What are the hormones that enhances the intestinal motility?
1) Gastrin
2) CCK
3) Insulin
4) Motilin
5) Serotonin
What is the importance of the ileocecal valve?
- 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)
What is the gastroileal reflex?
Reflexes coming from the stomach as it is full to relax the ileum
From which cells is secretin secreted?
The S-cells of the mucosa of the upper small intestine
What stimulates the secretion of secretin?
1) H+ ions (low pH)
2) Fatty acids
What are the functions of secretin?
- 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)
What are the cells that secrete cholecystokinin?
The i-cells of the mucosa of the upper small intestine
What stimulates the secretion of CCK?
1) Amino Acids
2) Fats
What is the function of cholecystokinin?
- 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
What are the intestinal reflexes?
1) Intestinointestinal reflex
2) Gastroileal reflex
What is the intestinointestinal reflex?
- 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
What is the gastroileal reflex?
- 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
What is cholera?
- 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)
What is the celiac disease?
- 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
What is paralytic ileus?
- 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
