E1. GI Tract Physiology Flashcards
what are the two main divisions of the GI system?
Gastrointestinal tract and accessory glands
Describe gastrointestinal tract
several organs joined in series to form a passageway for conduction of food and digestion products
Describe accessory glands
that secrete various fluids and enzymes into the lumen of the tract for the digestive process
ONE NOTE
What are the four main functions of GI system?
-Ingestion and Propulsion
-Digestion
-Absoprion
-Elimination
Describe the four main functions of GI system
-Ingestion and Propulsion: food intake via eating and drinking, mixes and moves the content along the alimentary tract.
-Digestion: mechanical breakdown of food (mastication) and chemical digestion into
small molecules by digestive enzymes.
-Absorption: digested food products are transported in the bloodstream. Fluids
containing enzymes and other substances are transported into the lumen of the GT Tract via a process called secretion.
-Elimination: excretion of food substances not digested and/or absorbed.
Function of oral cavity?
-Sensory analysis of food material before swallowing.
-Mechanical processing via the action of the teeth, tongue, and palatal surfaces.
-Lubrication by mixing food material with mucus and salivary gland secretion.
-Limited digestion of carbohydrates and lipids.
Describe tongue
-Mechanical processing by compression, abrasion, and distortion;
-Manipulation to assist in chewing and prepare material for swallowing; -Sensory analysis by touch, temperature, and taste receptors;
-Secretion of mucins and lingual lipase.
Describe teeth
-Mechanical breakdown of food materials;
-This process also saturates the salivary secretions and enzymes within the
food material for better digestion.
Describe Pharynx
-Passageway of food material to the oesophagus;
-Respiratory function for air movement into the lung;
-During swallowing, closure of the nasopharynx and larynx occurs to
maintain the proper direction of food;
-From the pharynx, food material goes to the oesophagus.
Describe the three pairs of salivary glands
-Parotid salivary glands: produce serous secretions
containing a large amount of salivary amylase, which
breaks down carbohydrate complexes.
-Sublingual salivary glands: produce a mucous secretion
that serves as both a buffer and lubricant.
-Submandibular salivary glands: secrete a mixture of
buffers, glycoproteins called mucins, and salivary amylase.
what does the 1.0 to 1.5 litres of saliva do each day:
-Lubricate the mouth to prevent friction between the
mucosa of the oral cavity and the food material;
-Moisten the food material for easy swallowing process;
-Initiation of lipid and carbohydrate complex digestion.
-Promotes oral hygiene (contains bicarbonate – neutralises acids in food to reduce dental cavities).
Production under
-Parasympathetic control.
Describe the formation of saliva
-Acinar cells: secrete amylase and electrolytes with similar tonicity to plasma (primary saliva).
-Duct cells: modify secretion (actively reabsorb Na+ (and Cl) ions and secrete K+ and HCO3- ions) (secondary saliva).
-The saliva becomes hypotonic as Na+ and Cl− reabsorption is greater than K+ and HCO3− secretion, and the ducts are relatively impermeable to water.
ONE NOTE
Describe control of salivary secretion via sympathetic innervation
-Sympathetic Innervation via alpha- and beta- adreno receptors.
-Decreased production of saliva by acinar cells.
-Increased protein secretion.
-Decreased blood flow to the salivary glands
Describe control of salivary secretion via parasympathetic innervation
-Parasympathetic Innervation via M3 muscarinic receptors.
-Increased secretion of saliva by acinar cells.
-Increased HCO3– secretion.
-Increased blood flow to the salivary glands.
-Contraction of
myoepithelium to increase the rate of expulsion of saliva.
-Increased parasympathetic stimulation results in an increased flow of saliva that is waterier in composition.
Describe the structure of the oesophagus
-Fibromuscular tube, approximately 25cm in length, that transports food from the pharynx to the stomach.
-The upper one-third is predominantly skeletal muscle.
-The middle one-third is a mixture of both the skeletal muscle and smooth muscle.
-The lower one-third is mainly smooth muscle.
ONE NOTE
Describe the upper oesophageal sphincter
The upper oesophageal sphincter controls the movement of food from the pharynx into the oesophagus. Rhythmic waves of peristalsis, which begin in the upper oesophagus, propel the bolus of food toward the stomach.
Describe the lower oesophageal sphincter
The lower oesophageal sphincter relaxes to let food pass into the stomach, and then contracts to prevent stomach acids from backing up into the oesophagus. Surrounding this sphincter is the muscular diaphragm, which helps close off the sphincter when no food is being swallowed.
When the lower oesophageal sphincter does not completely close, the stomach’s contents can reflux, causing heartburn or gastroesophageal reflux disease (GORD OR GERD).
what are the four major anatomical regions of the stomach?
-Fundus: upper portion, which extends above
the lower oesophageal sphincter;
-Body: middle region, which accounts for the
bulk of the stomach’s volume;
-Antrum: a lower region, which is narrower
and smaller in volume;
-Pylorus: a narrowing where the stomach joins the small intestine.
ONE NOTE
Describe gastric pits
Both the fundus and the body contain Gastric Pits, which are populated by cells that secrete the products found in gastric juice.
Describe the orad region of the stomach
-Proximal;
-Contains the fundus and the proximal
portion of the body;
-Thin walled and easily expands to
accommodate increases in volume following a meal.
Describe the Cauded region of the stomach
-Distal;
-Contains the distal portion of the body
and the antrum;
-Thick walled to generate much stronger
contractions for gastric mixing and emptying.
Describe storage in the stomach
presence of food stretch the stomach and induce a vago- vagal reflex from the stomach to the brain stem and back to the stomach wall, and result in receptive relaxation to accommodate as much as 1.5 L of food.
Describe mixing and digestion in the stomach
food is churned with gastric juices to form a thick, milky material (chyme), which is ejected about every 20 seconds into the duodenum. The peristaltic waves (propulsion) pushes the chyme toward the pyloric sphincter. Because the pyloric sphincter remains closed most of the time, the chyme is forced to move backward—a process called retropulsion. The rate of peristaltic waves is about 3 to 5 minutes.
Gastric factors for the regulation of gastric emptying?
-Gastric food volume: Increased food volume determine increased stretch
in the stomach wall and increased pyloric pump activity.
-Gastrin: released by the antral mucosa induces secretion of acidic
gastric juices and enhances the activity of pyloric pump.
Describe duodenal factors for regulation of gastric emptying
-Inhibitory enterogastric:
-nervous reflexes mediated by: § Enteric nervous system, directly from duodenum to
stomach.
-Extrinsic nerves to sympathetic ganglia.
-Vagus nerves to the brain stem.
Factors able to initiate those inhibitory reflexes are: Duodenal irritation, distension, acidity, osmolality of the chyme, and protein content of the chyme.
-Hormonal control: fat or acidity of chyme induce the release of CCK (cholecystokinin), secretin and GIP
(glucose-dependent insulinotropic peptide).
Gastric Pits contain what secretory cells?
-Surface mucous cells: secrete mucus in an
alkaline fluid (containing bicarbonate).
-Mucous neck cells: secrete mucus.
-Chief cells: secrete pepsinogen, the precursor
for pepsin.
-Parietal cells: secrete HCL and intrinsic factor
(necessary for the absorption of vitamin B12).
-Endocrine cells (e.g. G cells): secrete the hormone gastrin directly into the bloodstream.
ONE NOTE
describe the acidity of stomach
pH of stomach contents can be as low as 2. Acidity necessary for converting pepsinogen into its active form, pepsin, and it helps protecting the body
against certain infections.
Describe how acid is secreted in the stomach
-The acid secreted by the stomach is generated inside parietal cells by the enzyme carbonic anhydrase (CA), converting the reaction of carbon dioxide and water into hydrogen ions and bicarbonate.
-Hydrogen and chloride ions are transported into the lumen while bicarbonate is transported into the interstitial fluid.
-A Proton Pump (H+-K+ ATPase) uses ATP and transports hydrogen ions out of the parietal cell into the lumen of the stomach in exchange for potassium ions, which move from the lumen into the cell
ONE NOTE
What directly or indirectly control parietal cell acid output via…
1.Acetylcholine(astimulatoryneurotransmitter).
2.Histamine(astimulatorylocalhormone).
3.Gastrin(astimulatorypeptidehormone).
4.ProstaglandinsE2andI2(aninhibitorylocalhormones). 5. Somatostatin(aninhibitorypeptidehormone).
what are the three phases of gastric secretion?
cephalic phase, gastric phase, intestinal phase
Describe Cephalic phase
ONE NOTE
Describe gastric phase
ONE NOTE
Describe Intestinal phase
ONE NOTE
Stomach frequently contains HCl and proteolytic enzymes. How does stomach protect itself from self-digestion?
Mechanisms include:
1.Luminalmembranesofgastricmucosalcellsare impermeable to H+ ions. Cells are interconnected with tight junctions. Therefore, no H+ movement between cells.
2.Negativefeedback.pH<2.0inhibitsG-cellsdirectly and via somatostatin release from D cells.
3.Mucousreleasedfromsurfacemucouscells.
4.Rapidrepairofdamagedmucosa(cellslining
stomach turnover – 3 days).
ONE NOTE
What are the main functions of the small intestine?
Mechanical & chemical digestion of nutrients Absorption of nutrients.
small intestine divided in three regions of different length…
-Duodenum (25 cm): the part of intestine exiting from the stomach. Chemical digestion and some absorption.
-Jejunum (2 m): where most absorption occurs.
-Ileum (3 m): the terminal and longest region of the small intestine.
Describe chemical digestion in small intestine
Chemical digestion mostly mediated by enzymes produced by Pancreas, exocrine secretion of digestive enzymes and bicarbonate (to neutralize the stomach acid) via pancreatic duct merging into the common bile duct.
Describe mechanical digestion of small intestine
Motility of the small intestine serves to:
-Mix the chyme with digestive enzymes and pancreatic secretions;
-Expose the nutrients to the intestinal mucosa for absorption;
-Propel the unabsorbed chyme along the small intestine into the large intestine.
-Contractions (migrating myoelectric complexes) occur every 90 minutes to clear the small intestine of residual chyme.
-Parasympathetic stimulation increases contraction of intestinal smooth muscle, and sympathetic activity decreases contraction.
Describe autonomic control of motility of small intestine
-Parasympathetic Nerve Activation: Increased motility
-Sympathetic Nerve Activation: Decreases motility
* Directly (via β-adrenoceptors)
* Indirectly by decreasing ACh release (via α2-adrenoceptors)
-Gastrointestinal activity is also influenced by locally produced chemicals and hormones
ONE NOTE
What are the two patterns of contractions in the small intestine
Segmentation Contraction and Peristaltic Contraction
ONE NOTE
Describe Segmentation Contraction
mix the chyme and expose it to pancreatic enzymes and secretions.
Describe Peristaltic Contraction
propel the chyme along the small intestine toward the large intestine.
Describe how motility is initiated in the small intestine
The food bolus in the intestinal lumen is sensed by enterochromaffin cells of the intestinal mucosa, which release serotonin which binds to receptors on intrinsic primary afferent neurons (IPANs), initiating
the peristaltic reflex in that segment of small intestine.
Chyme passes into the small intestine and is mixed with?
Pancreatic juice;
Bile;
Intestinal juice.
Describe the digestion of all nutrients
-Carbohydrates broken down to monosaccharides.
-Proteins broken down to amino acids.
-Fats are broken down to fatty acids and glycerol.
Describe pancreatic juice and its composition
-Secreted by the exocrine pancreas.
Composition:
-Water.
-Mineral salts.
-Enzymes: amylase, lipase, nucleases.
-Inactive enzyme precursors: trypsinogen and
chymotrypsinogen.
Describe digestion of proteins
polypeptides to tripeptides, dipeptides and amino acids. Trypsinogen and chymotrypsinogen activated by enterokinase into the active trypsin and chymotrypsin.
Describe digestion of carbohydrates
Pancreatic amylase converts all digestible polysaccharides (starches) to disaccharides.
Describe digestion of fats
Lipase converts fats to fatty acids and glycerol.
Secretion of pancreatic juice is stimulated by what?
The secretion of pancreatic juice is stimulated by secretin and CCK, produced by endocrine cells in the walls of the duodenum. The presence in the duodenum of acid chyme from the stomach stimulates the production of these hormones.
Where is bile secreted and stored?
Secreted by the liver, stored in the gall bladder under the stimulation by secretin and CCK.
Composition of bile?
-Water.
-Mineral salts.
-Mucus.
-Bile salts.
-Bile pigments, bilirubin.
-Cholesterol.
Functions of bile?
-Emulsify the fats.
-Eliminate waste products.
-Digest proteins by enzymatic activity.
-Regulate the digestion process.
Describe the absorption of water and electrolytes in the large intestine
-Complete absorption of water, by osmosis.
-Sodium ions absorbed by the action of Na+/K+ pump.
-Mineral salts, vitamins and some drugs are also absorbed.
Describe mass movement in the large intestine
Peristalsis, 1 to 3 times per day, in the ileum forces chyme into the caecum.
Describe microbial activity in the large intestine
Region heavily colonised by certain types of bacteria, which synthesise vitamin K and folic acid.
Describe defecation in the large intestine
Of every 500 ml of food residue that enters the caecum each day, about 150 ml become faeces.
