What, essentially is the GI tract?
What are the 5 general things that take place in it?
- It is essentially just a long tube.
- Motility, digestion, secretion, absorption, excretion.
What happens at the Mouth and pharynx, and at the Salivary glands?
Salt and water are secreted to facilitate chewing, and swallowing.
Mucus and amylase are secreted to moisten and lubricate food and to digest polysaccharides.
What happens at the esophagus?
Mucus is secreted which lubricates the food ball as it moves down the esophagus through peristaltic waves
What is secreted in the stomach?
HCL is secreted to make food soluble and to kill microbes.
Pepsin is released to digest protein
Mucus is released to lubricate
What is secreted at the pancreas?
- The pancreas aids chemical digestion by producing an alkaline solution rich in bicarbonate as well as several enzyme.
- The bicarbonate neutralizes the acidity of chyme and acts as a bugger.
- Among the pancreatic enzymes are trypsin and chymotrypsin, proteases secreted into the duodenum in active forms - activated when safely in the duodenum.
What is secreted at the liver?
- The liver produces bile, a mixture of substances that contains bile salts which act as detergents (emulsifiers) that aid in digestion and absorption of lipids.
- Bile is stored and concentrated in the gallbladder.
- The liver also breaks down toxins that enter the body and helps balance nutrient utilization.
What is secreted at the small intestine?
- The epithelial lining of the duodenum is the source of several digestive enzyme. Some are secreted into the lumen of the duodenum, whereas others are bound to the surface epithelial cells.
- While enzymatic hydrolysis processds, peristalsis moves the mixture of chyme and digestive juices along the small intestine.
- Most digestion is completed in the duodenum.
- The remaining regions of the small intestine, called the jejunum and the ileum, function mainly in the absorption of nutrients and water.
What is secreted in the large intestine?
Mucus for lubrication
Average water exchange for mouth, esophagus, stomach, small intestine, colon, rectum, per day.
- Mouth: 2L
- Esophagus: 1.5 L
- Stomach: 2L
- Small intestine: 8.5L
- Colon: 0.4-1L
- Rectum: 0.1L
In the enteric nervous system what is the myenteric plexus
Auerbach's plexus (or myenteric plexus) provides motor innervation to both layers of the tunica muscularis.
In the enteric nervous system what is the submucosal plexus?
The submucosal plexus (or Meissner's plexus) has only parasympathetic fibers and provides secretomotor innervation to the mucosa nearest the lumen of the gut.
How do the parasympathetic and sympathetic nervous systems affect the GI tract?
- Parasympathetic: increases GI motility
- Sympathetic: Decreases GI motility
What do the nervous pathways affecting the GI tract achieve?
What do the GI reflexes respond to?
They allow co-ordination and control of the enviroment. They control mechanisms by luminal volume and composition.
- The GI reflexes respond to:
- 1) Volume - e.g. distension of wall
- 2) Osmolarity - e.g. gastric chyme
- 3) Acidity - e.g. duodenal chyme,
- 4) Chyme products - i.e concentration of digestion products.
How is control achieved via nervous pathways?
- Extrinsic (outside) nerves - Parasympathetic and Sympathetic.
- - Release acetylcholine and noradrenaline.
- Intrinsic (inside) nerves - more important.
- - Embedded in the walls of the esophagus, stomach, small intestine.
Comment on long and short reflex pathways in the GI tract.
- The enteric nervous system has short reflex pathways - i.e. independent of CNS
- - Neural activity of one plexus influences activity of the other.
The autonomic nervous system
involves long reflex pathways, allowing CNS to influence motility and secretory activity.
Comment on the endocrine control mechanisms in the GI tract.
The GI tract produces its own regulators: They stimulate digestive juices and cause organ movement - has over 20 different regulatory peptides.
Hormones (Endocrine): are released into the circulation - Eg. Gastrin family (gastrin, CCK) and Secretin family (Secretin, Gastric inhibitory peptide)
Local hormones (paracrine): released into interstital fluid to act locally - e.g. histamine
What do gastrin, histamine, and secretin do?
- Gastrin stimulates parietal and chief cells.
- Histamine stimulates parietal and chief cells.
- Secretin inhibits parietal and chief cells.
Describe the small intestine.
- Over 6m long in humans, the small intestine is the alimentary canal's longest compartment.
- Its name refers to its diameter, compared with that of the large intestine.
- The first 25 cm or so of the small intestine forms the duodenum, a major crossroad in digestion.
- It is here that chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, as well as from gland cells of the intestinal wall itself.
- Hormones released by the stomach and duodenum control the digestive secretions into the alimentary canal.
What are circular folds in the ileum?
The circular folds are large valvular flaps projecting into the lumen of the bowel.
They are composed of reduplications of the mucous membrane, the two layers of the fold being bound together by submucous tissue.
What are lymph follicles in the ileum?
- Peyer's patches (or aggregated lymphoid nodules) are organized lymphoid nodules.
- They are aggregations of lymphoid tissue that are usually found in the lowest portion of thesmall intestine, the ileum, in humans
Describe the villi and microvilli in the small intestine.
- Large folds in the lining have finger-like projections called villi. In turn, each epithelial cell of a villus has on its apical surface many microscopic appendages, or microvilli, that are exposed to the intestinal lumen.
- The many side by side microvilli give the intestinal epithelium a brush-like appearance - reflected in the name brush border. The enormous surface area presented by microvilli is an adaption that greatly increases the total capacity for nutrient absorption.
What are goblet cells?
Goblet cells are glandular simple columnar epithelial cells whose function is to secrete gel forming mucins, which are the major component of mucus
What are enterocytes?
Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells found in the small intestines, colon and appendix.
What are intestinal crypts?
- In histology, an intestinal crypt (also intestinal gland) is a gland found in the epithelial lining of the small intestine and colon.
- The crypts and intestinal villi are covered by epithelium which contains two types of cells: goblet cells (secreting mucus) and enterocytes (absorbing water and electrolytes).
Comment on secretion in the small intestine.
Distension, chyme, vagal activity, and hormones stimulate the cells of crypts to secrete 1-2 litres of fluid per day.
- Distension: Leads to activation of mechanoreceptors
- Acidity of chyme: leads to activation of S-cells - these release the hormone secretin
- Breakdown products in chyme: Leads to activation of I-cells - These release the hormone cholecystokinin.
Intestinal fluid secretion helps neutralise chyme, this is aided by liver and pancreatic secretions
What is Chyme?
Chyme is the semifluid mass of partly digested food expelled by the stomach into the duodenum.
Along with pancreatic enzymes, what three groups of brush border enzymes finalize breakdown?
What are the two major components of the pancreas?
The aqueous and the enzymatic.
- Aqueous: High in HCO3-
- Enzymatic: Enzymes stored in acinal cells with zymogen granules - trypsin, chymotrypsin, elastase, carboxypeptidase, lipase, amylase
What does Cholecystokin do
What are the functional roles of the Liver and Gall bladder?
Uses cholesterol to produce bile salts which are amphipathic and help dissolve fat globules into an emulsion.
Secretes bile - ~94% bile salts that enter intestine are reabsobed via ileum.
Gall bladder acts as a resevoir.
How are the digestion products absorbed in the small intestine?
- Carbohydrates are absorbed in the first 20% of the small intestine.
- - they are transported across enterocyte apical membrane
- - enters via secondary active transport
- Protein: Transported across enterocyte apical membrane
- - 3 groups of transporters for amino acids
- - enters via secondary active transport
- Fat: - Primarily uptake in jejunum
- - Fat globules first from emulsion
- - Diffuses into enterocyte
- - Finally, chylomicrons used for uptake into lacteal.