Module 1.01 Flashcards
(25 cards)
Motility
movements of the alimentary canal that propel partly digested food along the canal
Which 3 arteries supply blood to the GI tract?
Coeliac, inferior mesenteric, superior mesenteric
Coeliac artery
supplies blood to liver, gall bladder, pancreas, stomach and spleen
Inferior mesenteric artery
supplies blood to the terminal portions of the large intestine and rectum.
Superior mesenteric artery
supplies blood to the pancreas, small intestine and most of the large intestine
Why is mucosal blood flow required?
- Maintains the viability of the mucosa
- Provides the precursors for secretory products
- Delivers hormones to their target cells
- Removes absorbed digestion products, toxins and drugs from the mucosa.
2 types of digestion
Physical + chemical
Physical digestion
mechanical activity of the alimentary canal, breaking down pieces of food into smaller particles.
Food retains its complex chemical structure, but its surface area is increased to expose more sites to enzymic action
Where does physical digestion take place?
- Mouth
- Antrum and pylorus of the stomach
- Small intestine.
Chemical digestion
hydrolysis of the complex food molecules being broken down into their simpler constituents = absorbed.
Where does chemical digestion take place?
Chemical digestion may occur by means of enzyme activity present in the lumen of the alimentary canal or on the luminal-facing membrane of the epithelial cells (enterocytes) in the small intestine.
Carbohydrate digestion
Salivary and pancreatic amylases catalyse the hydrolysis of the interior α-1,4 bonds but do not split either the terminal α-1,4 glycosidic bonds or the α-1,6 glycosidic bonds at the branches of starch and glycogen. The products of amylase action are maltose (a disaccharide), maltotriose (a trisaccharide) and α-limit dextrins (branched oligosaccharides). However, there are no transport systems for the absorption of these carbohydrates in the intestine.
Protein digestion in the stomach
Peptic cells in the stomach secrete pepsinogens.
Acid contents of the stomach activate pepsinogens to pepsins and denature the structure of proteins.
Pepsin is an endopeptidase and specifically hydrolyses peptide bonds, containing an aromatic L-amino acid such as phenylalanine or tyrosine. Pepsin is inactivated by the alkaline pH found in the duodenum.
Protein digestion in the small intestine
trypsin, chymotrypsin and carboxypeptidase are secreted as precursors into the duodenum and activated by enteropeptidase secreted by the duodenal and jejunal mucosa.
Enteropeptidase activates trypsinogen to trypsin.
Once some trypsin is formed, it acts autocatalytically to convert more trypsinogen to trypsin and also activates chymotrypsinogen to chymotrypsin.
Trypsin, chymotrypsin and elastase are endopeptidases and convert protein into polypeptides. Carboxypeptidases release single amino acids from the carboxyl end of polypeptides. The pancreatic proteases active in the lumen of the intestine produce small peptides and some amino acids before being inactivated by autodigestion.
3 lipases
- Lingual
- Gastric
- Pancreatic.
major site of fat digestion
small intestine
Why does fat enter the duodenum slowly?
so emulsification and fat hydrolysis can take place
Role of emulsifying agents
reduce the surface tension of the fat droplets and keep them apart. These actions increase the surface area of the ingested fat
Fat digestion
Fat enters the duodenum.
Fat is emulsified into droplets by bile salts, with their hydrophobic side dissolving in the fat and their hydrophilic side acing outwards in the water of the intestinal fluid.
This process is aided by lecithin and cholesterol, which are also found in bile.
An additional pancreatic enzyme, colipase, anchors pancreatic lipase to the fat/water interface and activates it. This allows an interaction between the water-soluble pancreatic lipase and the ingested fat. Pancreatic lipase (optimum pH 8.0) preferentially hydrolyses the bonds between glycerol and the fatty acid residues at positions 1 and 3 to produce 2-monoacylglycerols (monoglycerides) and free fatty acids.
A small quantity of the 2-monoacylglycerols is hydrolysed to glycerol and a free fatty acid.
The presence of fat digestion products, bile salts, cholesterol and phospholipids causes the emulsion to break up into smaller particles, known as mixed micelles. The bile salts form an outer coat, with the fatty digestion products in the centre.
The micelles are small enough to diffuse between the microvilli of the enterocytes.
4 layers of GI tract
- Mucosa
- Submucosa
- Muscle
- Serosa
What does the mucosa consist of ?
- A lining layer of epithelial cells that have secretory and absorptive functions
- The lamina propria, a connective tissue support for the mucosa, together with blood vessels and the glandular ducts in some regions
- The muscularis mucosae, a thin layer of smooth muscle cells that moves the epithelium, producing mixing activity, and leads to folding of the mucosal layer
what does the submucosa contain?
connective tissues, blood vessels and lymphatics together with nerves innervating the structures within the mucosa.
what does the external muscle consist of?
the inner layer in which the long axis of the muscle fibres is arranged in a circular manner around the canal (circular muscle) and an outer layer with the muscle fibres lying along the length of the canal (longitudinal layer). Contraction of the circular and longitudinal muscle layers narrows the lumen and shortens the canal, respectively.
These muscles are responsible for mixing the food with the digestive secretions and for propelling the contents along the digestive tract. The first third of the oesophagus and the anal sphincter contain skeletal muscle
what does the serosa consist of?
connective tissues covered with a squamous epithelium.
