Flashcards in Physiology of digestion and absorption Deck (90)
What is the major site for digestion and absorption?
The small intestine
How big is the small intestine?
Approx 6m long and 3.5 cm wide
Duodenum: approx 25cm
jejunum: approx 2.5 m
ileum: approx 3m
What does the small intestine receive, and where does it receive it from?
Chyme from the stomach
Pancreatic juices from the pancreas
Bile from the gallbladder
Which hormones are secreted into the bloodstream by the small intestine?
Name the hormone, the part of the intestine it is secreted by, and the type of cell which secretes it.
Gastrin- from G cells in the duodenum
Cholecystokinin (CCK) from I cells in the duodenum and jejunum.
Secretin- from S cells of the duodenum
Motilin- from M cells of the duodenum and jejunum
Glucose-dependent insulinotropic peptide- from K cells in the duodenum and jejunum
Glucagon like peptide 1- L cells in the small and large intestine.
What do the hormones secreted by the small intestine all act on?
G protein coupled receptors.
What is succus entericus?
How much is secreted per day?
About 2 litres
Describe the control mechanisms of the secretion of intestinal juice
Parasympathetic nerve activity
Sympathetic nerve activity
What does intestinal juice contain?
Mucous from goblet cells- for protection and lubrication
Aqueous salt (mostly from the crypts)- for enzymatic digestion
No digestive enzymes
What are the two processes which occur in the mixing and propulsion of chyme?
Segmentation (mixing) and peristalsis (propulsion)
Describe what happens in the mixing of chyme.
This is segmentation.
Pacemaker cells in the small intestine cause a BER which is continuous. Distension by entering chyme causes the BER to reach threshold, which activates segmentation.
Segmentation in the empy ileum is triggered by gastrin from the stomach- this is the gastro-ileal reflex.
Segmentation is alternating contraction and relaxation of segments of circular smooth muscle. The chopping which results moves the chyme back and forth.
The duodenum has frequent segmentation contractions, and the ileum has fewer.
The movement is slow (take about 3-5 hours) to allow absorption to occur.
Describe what happens in the propulsion of chyme.
This is peristalsis.
Two activities which occur in the interdigestive, or fasting state.
1. A few localised contractions
2. The migrating motor complex (MMC)
Occurs between meals every 90-120 minutes
It is a strong peristaltic contraction passing the length of the small intestine- from the stomach to the ileo-caecal valve.
It clears the small intestine of undigested debris, mucous and sloughed epithelial cells between meals (housekeeper function).
What increases and decreases the strength of segmentation?
Parasympathetic stimulation enhances the strength of segmentation and sympathetic stimulation decreases the strength of segmentation.
What inhibits and triggers the migrating motor complex?
It is inhibited by feeding, vagal activity, gastrin and CCK.
It is triggered by motilin.
What are the pancreatic endocrine and exocrine secretions?
What is the collective name for the pancreatic exocrine secretions which go to the duodenum?
Endocrine: Insulin and glucagon
Exocrine: digestive enzymes secreted by acinar cells, aqueous sodium bicarbonate secreted by duct cells. These are secreted to the duodenum collectively as pancreatic juice.
How are the enzymes stored in the pancreas?
They are stored in acinar cells, in zymogen (proenzyme) granules.
Name the proteases secreted by the acinar cells.
What types of enzymes are secreted by acinar cells?
Proteases, amylases and lipases
What amylase is secreted by the pancreas?
What lipase is secreted by the pancreas?
Are the amylases and lipases secreted by the pancreas proenzymes or active enzymes?
They are active enzymes.
How and when are the proteases secreted from the pancreas activated?
What are they activated to?
They are activated in the duodenum, by the action of enterokinase from mucosal cells.
Trypsinogen becomes trypsin, which can then go on to also activate the other inactive proteases.
Chymotrypsinogen becomes chymotrypsin
Procarboxypeptidase becomes carboxypeptidase (A and B).
What is the function of the bicarbonate released from the duct cells in the pancreas into the duodenum?
It neutralises the chyme from the stomach.
This provides the optimum pH for pancreatic enzyme function, and protects the mucosa from erosion by acid.
Describe how fluid is secreted from duct cells in the pancreas
Carbonic acid is formed from carbon dioxide and water, under the action of carbonic anhydrase.
Carbonic acid then dissociates into bicarbonate and a proton.
The proton can be exported through the basal lateral membrane via: a sodium/hydrogen exchanger, or a potassium/hydrogen ATPase.
On the apical surface of the cell, bicarbonate exits via a chloride/bicarbonate antiporter.
The chloride is recycled via the CFTR chloride channel.
Therefore, in patients with CF with a mutated CFTR channel, they have reduced fluid secretion since they have less chloride exiting the cell.
What are the three phases of pancreatic secretion?
Cephalic, gastric and intestinal.
Describe the cephalic phase of pancreatic secretion.
This is mediated by vagal stimulation of mainly the acinar cells.
This contributes to 20% of the total pancreatic secretion.
Describe the gastric phase of pancreatic secretion.
Gastric distension evokes a vagovagal reflex, resulting in parasympathetic stimulation of acinar and duct cells. The stomach signals to the brainstem, which then signals to the pancres.
This makes up 5-10% of the total secretion.
Describe the intestinal phase of pancreatic secretion.
The two stimuli here are the present of acid in the duodenum and the presence of fat in the duodenum.
The presence of acid in the duodenum causes an increase in Secretin release from S cells, which is carried by the blood to the pancreatic duct cells and causes an increased secretion of aqueous sodium bicarbonate into the duodenum. This then neutralises the acid in the duodenum.
Fat and protein in the duodenum evokes an increased release of CCK from I cells in the duodenum. This is carried in the blood to the pancreatic acinar cells and causes increased release of digestive enzymes into the duodenum. These digest the protein and fat in the duodenum.
What does the biliary system comprise of?
The liver, the gallbladder and associated ducts
Where is bile synthesised?
By hepatocytes in the liver