Physiology of the small intestine Flashcards
What does the small intestine secrete?
Na+, Cl- and HCO3- (water follows by osmosis)
Mucus (alkaline)
Hormones
What secretes mucus into the small intestine?
Brunner’s glands in the duodenal mucosa
What hormones does the small intestine secrete?
Secretin
Cholecystokinin
Motilin
Vasoactive intestinal peptide (VIP)
Gastric inhibitory peptide (GIP)
What does chyme trigger the release of in the small intestine?
Hormones
Alkali (to correct acidity)
Water (helps correct acidity)
Mucous (alkali)
Digestive enzymes
What is the hormone motilin responsible for?
Stimulates migrating motor complexes via the enteric NS and ANS for rapid peristaltic movement.
Acts between periods of digestion to clear undigestible products from the GI tract
What is the hormone vasoactive intestinal peptide (VIP) responsible for?
Increasing blood flow to GI tract
What is gastric inhibitory peptide (GIP) responsible for?
Inhibits gastric secretion, stimulates insulin secretion
What is cholecystokinin responsible for?
Stimulation of gall bladder contraction
Stimulation of pancreatic enzyme secretion from exocrine pancreas.
What is secretin responsible for?
Stimulation of alkali release from exocrine pancreas
Stimulation of bile production in the liver
What is alkali?
Isotonic solution rich in HCO3-. Neutralises duodenal contents.
What digestive enzymes are secreted from the exocrine pancreas?
- Proteolytic enzymes (peptidases):
- Trypsin
- Chymotrypsin
- Carboxypeptidase
(proteolytic enzymes- cleave proteins and peptides) -Secreted in inactive forms
- Pancreatic amylase (breakdown of starch)
- Lipases (breakdown of fats)
What neutralises chyme in the small intestine?
Alkali from exocrine pancreas
Water secreted from intestinal mucosa
Alkaline bile
Mucus from duodenal mucosa
How are the inactive pancreatic enzyme precursors activated?
The pancreas releases inactive enzymes including trypsinogen.
Trypsinogen is converted into trypsin by membrane-bound enterokinase enzyme.
Trypsin activates inactive enzymes which are released into the intestinal lumen
Describe the mechanism of pancreatic HCO3- secretion
CO2 and H2O are converted by carbonic anhydrase to HCO3 and H+.
HCO3- is released into the duct lumen
H+ is pumped out of the duct cell into the extracellular fluid.
(ions moved across opposite membranes than in gastric parietal cells)
How are fats (triglycerides) digested in the duodenum and proximal jejunum?
Fats (triglycerides) emulsified by bile salts and phospholipids into emulsion droplets.
Digested by pancreatic lipase to monoglycerides and fatty acids.
Held in micelles, combined with bile salts and phospholipids.
Micelles diffuse into an ‘unstirred layer’ next to the surface of epithelial cells.
Monoglycerides and fatty acids diffuse into cell membrane.
Inside the cell monoglycerides and fatty acids are reassembled into triglycerides and packaged into chylomicrons
Exported across the basolateral membrane and leave the intestinal villus via the lacteals of its lymph system.
What controls pancreatic secretion?
Enzyme secretions are stimulated by CCK and acetylcholine (from parasympathetic post-ganglionic vagus nerve fibres)
Alkali secretion is stimulated by secretin and further potentiated by CCK and Ach (would not stimulate secretion on their own)
Which enzymes are responsible for digestion of which substances in the duodenum and proximal jejunum?
- Pancreatic lipase digests triglycerides (fats)
- Pancreatic amylase digests starch
- Brush border enzymes digest disaccharides (eg maltose, sucrose and lactose)
- Trypsin and Chymotrypsin digest proteins into peptide fragments.
- Peptide fragments are digested by carboxypeptidase (from the pancreas) and aminopeptidase (luminal surface of epithelial cells)
Describe the process of digestion of carbohydrates and absorption of monosaccharides
- Starch is broken down into maltose and glucose by alpha amylase. Lactose and sucrose taken in from diet.
- Maltose, lactose and sucrose (disaccharides) broken down by brush border enzymes to give glucose, galactose and fructose (monosaccharides).
- Transported into epithelial cell:
- Glucose and galactose by SLGT1 (using concentration gradient formed by Na+/K+ ATPase on the lateral membrane of the cell)
- Fructose by GLUT5
- Monosaccharides exported across basolateral membrane via GLUT2 into blood.
Describe the digestion of proteins and absorption of amino acids and oligopeptides
Proteins are broken down by pepsins in the stomach and carboxypeptidases, trypsin, chymotrypsin and HCO3- in the small intestine to form polypeptides.
Polypeptides are broken down by carboxypeptidases, trypsin, chymotrypsin and HCO3- to form tri- and dipeptides
Broken down by brush border enzymes (aminopeptidases and dipeptidases) into amino acids.
Amino acids transported into epithelial cell by H+ and Na+ amino acid co transporters
How is calcium absorbed across the intestinal epithelium?
Down its concentration gradient via a calcium channel
How is calcium moved out of the epithalial cell into the blood?
Moved against its concentration gradient via 2 transporters:
- Plasma membrane Calcium ATPase
- Na+/Ca+ exchanger
How is iron transported across the intestinal epithelium?
Haem iron:
- Absorbed by receptor-mediated endocytosis
- Digested in the enterocyte to release Fe3+
- Reduced to Fe2+
- Binds to ferroportin 1 in basolateral membrane for export
- Some held in ferritin stores and lost by shedding of epithelial cells
Free iron:
- Fe3+ reduced to Fe2+ by duodenal cytochrome B ferric reductase in brush border
- Fe2+ crosses brish border via divalent metal transporter 1
- Binds to ferroportin for export or ferritin for storage
How is iron absorption regulated?
Excess iron is taken up into storage bound to ferritin.
Lost in body in faeces due to high turnover of cells in intestinal epithelium.
Absorption regulated according to need:
-Iron depletion upregulates ferroportin: more iron transported into blood, less binds to ferritin for storage.
How are vitamins absorbed in the small intestine?
Fat soluble vitamins (A,D,E,K) follows same pathway as triglycerides
Water soluble vitamins absorbed by diffusion, facilitated diffusion or active transport
B12 absorbed by mediated transport system
How is Vitamin B12 absorbed in the small intestine?
Binds to R protein in the stomach
Released from R protein in the duodenum due to pH change
Binds to intrinsic factor in the duodenum
Carried to the terminal ileum where the B12-IF complex is absorbed by receptor-mediated endocytosis
What are the defects in digestion/absorption caused by different GI conditions?
Crohn’s disease: mucosal damage in duodenum and jejunum = fat absorption and lactose hydrolysis defects.
Pancreatitis: Deficient enzyme secretion in pancreas = fat digestion defects
Surgical resection of ileum: impaired vit B12 and bile salt/acid absorption
Lactase deficiency: impaired lactase hydrolysis in small intestine
Describe how lactase deficiency causes lactose intolerance
Lactose is normally broken down by brush border lactase enzyme into glucose and galactase.
Lactase insufficiency causes lactose to be broken down by bacteria in the large intestine causing fermentation which releases gases, organic acids and osmotically active molecules = ++ water by osmosis, bloating and increasing motility.
What are the symptoms of malabsorption?
Weight loss
Abdominal distension
Diarrhoea
Steatorrhoea
Pernicious anaemia
Hypochromic anaemia
Describe the role of the GI tract in acid-base balance
Under normal conditions, only a small amount of alkali is lost in stool.
Excess vomiting can cause large losses of acids from the body resulting in metabolic alkalosis
Excessive fluid loss through diarrhoea can cause large losses of NaHCO3- causing metabolic acidosis
Which cells in the pancreas secrete pancreatic digestive enzymes?
Acinar cells
Which cells of the exocrine pancreas secrete alkali?
Duct cells
