Alimentary transport 2: Across the GI tract Flashcards
Describe the microstructure of the small intestine, how does this relate to the function of the small intestine?
- Epithelial folds - increase SA for absorption
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Villi - absorption
- Also crypt cells - involved in secretion
- Microvilli - makes up large surface area of gut, increased SA for absorption
- Intestinal crypts - Acts as protective barrier, absorbs nutrients from lumen
- Muscularis mucosae - Has perpetual motor function that keeps mucosa in flux
- Glands in the submucosa - secretes mucus, buffers acidic chyme, prevents damage to walls
- Tunica submucosa - Provides connective support, delivers blood vessels, nerves, lymphatics
- Tunnica muscularis externa - layer of 2 muscles, outer = thinner, longitudinal, moves food across GI tract, Inner = thicker, circular, responsible for intestinal secretions
- Lymphatic nodules in the submucosa - Provide homeostasis in GI tract by filtering fluids, blood cells, plasma proteins
- Myenteric nerve plexus - Influences motor control, regulates GI motility
- Tunica serosa (peritoneum) - Provides protective covering against friction in the abdominal cavity
- Intestinal villi - Increase SA so more nutrients from lumen can be absorbed into circulatory system
Describe the gross structure of the small intestine
- Duodenum
- Jejenum
- Ileum
Describe the microstructure of the large intestine, how does this relate to the function of the large intestine?
- No villi
- Intestinal crypts
- Muscularis mucosae
- Tunica submucosa
- Tunica muscularis externa
- Myenteric nerve plexus
- Tunica serosa (peritoneum)
Describe the gross structure of the large intestine
- Colon (ascending, transverse, descending, sigmoid)
- Cecum
- Rectum, continuous with anus
- Alimentary canal
What is transcellular and paracellular transport?
- Enterocytes -> Cells of intestinal lining
- Transcelluar transport - through enterocytes between lumen and blood
- Paracellular transport - between erythrocytes via tight junctions, passive and regulated, mainly ions and selective
What is the role of the brush border?
Where is the brush border located?
- At the microvilli of the small intestine
- Brush border enzymes - breakdown carbs + proteins
Where are carbohydrates digested and how are they digested?
- Digestion of carbohydrates occurs in the intestinal lumen and at the brush border
- Carbs can only be absorbed as monosaccharides
- Complex carbs are reduced to disaccharides by amylases - This is pancreatic amylase which is secreted by the pancreas into the small intestine
- Specific brush border enzymes convert disaccharides to monosaccharides (e.g. glucose, galactose)
What are the sites of protein digestion and how are proteins digested?
Proteins arrive in small intestine as polypeptides which are denatured by gastric acid and broken down by pepsin ins stomach (pepsinogen = inactive form which becomes pepsin in acidic conditions)
3 sites of digestions:
- Intestinal lumen - Proteins converted to ogliopeptides and amino acids by pepsins and pancreatic proteases
- Brush border - Ogliopeptides converted into dipeptides, tripeptides and amino acids by brush border peptidases
- Enterocytes - amino acids, dipeptides and tripeptides absorbed by enterocyte where they’re broken down into amino acids by cytoplasmic peptidases or pass into blood as they are
How are fats digested?
- Mouth - salivary lipase digests a small amount of fats into triglycerides
- Pancreatic lipase - water soluble
- Triglycerides are not water soluble so are dissolved in aqueous phase before digestion and emulsified- This is chyme (emulsification of large fat particles in water)
- Triglycerides are emulsified by bile salts (made in liver) from gallbladder
- Bile salts and pancreatic juices enter sphincter of Oddi (smooth muscle that surrounds the end portion of the common bile duct and pancreatic duct) and mix with acidic chyme
- Bile emulsification = form emulsion droplets
- Colipases (cofactor of pancreatic lipases) - allow lipase to bind to droplets as they’re amphiphatic (molecule that has both hydrophobic (non-polar) and hydrophilic (polar) regions). This means that colipase attaches to the lipid on the fat globule, and the other tail acts on the lipase, allowing lipase to act on the fat globule
- Pancreatic lipase binds to colipase to break down emulsion droplets = micelles form where bile salts bind to products of lipid breakdown (monoglycerides and fatty acids) = small droplet
- Micelles fuse with enterocyte of lumen
How are glucose and galactose transported across the small intestine?
- Rapidly absorbed by a secondary active transport process
- The lumen side is called the apical side, so the opposite side containing the enterocytes is the basolateral side
- The basolateral side contains Na+/K ATPase pumps that cause Na+ to be actively transported out of the enterocytes, and therefore there’s a low concentration of Na+ on that side, and a high concentration of Na+ is in the lumen.
- For Na+ to transport into the enterocytes from the lumen, it must do so via the SGLT-1 transporter.
- Na+/glucose co-transporter (SGLT 1) - When Na+ travels into the enterocytes, it must bring glucose along with it.
- Glucose is reabsorbed by a GLUT-2 carrier from the enterocytes into blood
- Fructose can be passively transported via a GLUT-5 carrier
What are the 2 mechanisms that absorb proteins from the GI tract?
- Amino acids transported on a sodium coupled carrier system (like glucose, but separate carriers for amino acids)
- Some di and tripeptides are transported on a carrier system using inwardly directed H+ gradient
What are micelles?
- Temporary, water soluble compounds formed during the fat digestion and absorption process.
- They enable the lipid digestion products to be transported to the small intestinal surface for absorption
How is fat transport across the small intestine different from carbs + proteins?
After micelles fuse with enterocytes:
- Short chain free fatty acids enter portal vein and combine with bile in liver to form micelle through action of lipase
- OR triglycerides formed from fatty acids and monoglycerides, then packaged with cholesterol, phospholipids and proteins into chylomicrons which are transported into lacteal and then into lymph
Absorption of large number of chylomicrons = lymph draining from small intestine is milky
Bile salts go back to liver, recycled
Micelles → enterocytes → chylomicrons → lacteals → blood
Chylomicrons (in blood) → wall of capillaries → fatty acids → liver and fat cells
Chylomicrons = largest lipoprotein
Define steatorrhoea
Reduced fat digestion and absorption causing the presence of abnormal amount of fat in the stool
What are the causes and features of steatorrhoea?
Causes:
- Gallstones
- Pancreatitis
- Crohn’s disease
- Liver disease
Cause: ↓ secretion of pancreatic lipase
Reduced fat-soluble vitamins (vits A,D,E & K are trapped by the unabsorbed fats)
All lead to fat mal-absorption
Causes stool to become oily, foul-smelling, bulky, pale
