E1. GI Tract Physiology

Question 1

what are the two main divisions of the GI system?

Answer 1

Gastrointestinal tract and accessory glands

Question 2

Describe gastrointestinal tract

Answer 2

several organs joined in series to form a passageway for conduction of food and digestion products

Question 3

Describe accessory glands

Answer 3

that secrete various fluids and enzymes into the lumen of the tract for the digestive process
ONE NOTE

Question 4

What are the four main functions of GI system?

Answer 4

-Ingestion and Propulsion
-Digestion
-Absoprion
-Elimination

Question 5

Describe the four main functions of GI system

Answer 5

-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.

Question 6

Function of oral cavity?

Answer 6

-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.

Question 7

Describe tongue

Answer 7

-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.

Question 8

Describe teeth

Answer 8

-Mechanical breakdown of food materials;
-This process also saturates the salivary secretions and enzymes within the
food material for better digestion.

Question 9

Describe Pharynx

Answer 9

-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.

Question 10

Describe the three pairs of salivary glands

Answer 10

-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.

Question 11

what does the 1.0 to 1.5 litres of saliva do each day:

Answer 11

-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.

Question 12

Describe the formation of saliva

Answer 12

-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

Question 13

Describe control of salivary secretion via sympathetic innervation

Answer 13

-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

Question 14

Describe control of salivary secretion via parasympathetic innervation

Answer 14

-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.

Question 15

Describe the structure of the oesophagus

Answer 15

-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

Question 16

Describe the upper oesophageal sphincter

Answer 16

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.

Question 17

Describe the lower oesophageal sphincter

Answer 17

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).

Question 18

what are the four major anatomical regions of the stomach?

Answer 18

-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

Question 19

Describe gastric pits

Answer 19

Both the fundus and the body contain Gastric Pits, which are populated by cells that secrete the products found in gastric juice.

Question 20

Describe the orad region of the stomach

Answer 20

-Proximal;
-Contains the fundus and the proximal
portion of the body;
-Thin walled and easily expands to
accommodate increases in volume following a meal.

Question 21

Describe the Cauded region of the stomach

Answer 21

-Distal;
-Contains the distal portion of the body
and the antrum;
-Thick walled to generate much stronger
contractions for gastric mixing and emptying.

Question 22

Describe storage in the stomach

Answer 22

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.

Question 23

Describe mixing and digestion in the stomach

Answer 23

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.

Question 24

Gastric factors for the regulation of gastric emptying?

Answer 24

-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.

Question 25

Describe duodenal factors for regulation of gastric emptying

Answer 25

-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).

Question 26

Gastric Pits contain what secretory cells?

Answer 26

-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

Question 27

describe the acidity of stomach

Answer 27

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.

Question 28

Describe how acid is secreted in the stomach

Answer 28

-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

Question 29

What directly or indirectly control parietal cell acid output via...

Answer 29

1.Acetylcholine(astimulatoryneurotransmitter). 2.Histamine(astimulatorylocalhormone). 3.Gastrin(astimulatorypeptidehormone). 4.ProstaglandinsE2andI2(aninhibitorylocalhormones). 5. Somatostatin(aninhibitorypeptidehormone).

Question 30

what are the three phases of gastric secretion?

Answer 30

cephalic phase, gastric phase, intestinal phase

Question 31

Describe Cephalic phase

Answer 31

ONE NOTE

Question 32

Describe gastric phase

Answer 32

ONE NOTE

Question 33

Describe Intestinal phase

Answer 33

ONE NOTE

Question 34

Stomach frequently contains HCl and proteolytic enzymes. How does stomach protect itself from self-digestion?

Answer 34

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

Question 35

What are the main functions of the small intestine?

Answer 35

Mechanical & chemical digestion of nutrients Absorption of nutrients.

Question 36

small intestine divided in three regions of different length...

Answer 36

-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.

Question 37

Describe chemical digestion in small intestine

Answer 37

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.

Question 38

Describe mechanical digestion of small intestine

Answer 38

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.

Question 39

Describe autonomic control of motility of small intestine

Answer 39

-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

Question 40

What are the two patterns of contractions in the small intestine

Answer 40

Segmentation Contraction and Peristaltic Contraction ONE NOTE

Question 41

Describe Segmentation Contraction

Answer 41

mix the chyme and expose it to pancreatic enzymes and secretions.

Question 42

Describe Peristaltic Contraction

Answer 42

propel the chyme along the small intestine toward the large intestine.

Question 43

Describe how motility is initiated in the small intestine

Answer 43

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.

Question 44

Chyme passes into the small intestine and is mixed with?

Answer 44

Pancreatic juice; Bile; Intestinal juice.

Question 45

Describe the digestion of all nutrients

Answer 45

-Carbohydrates broken down to monosaccharides. -Proteins broken down to amino acids. -Fats are broken down to fatty acids and glycerol.

Question 46

Describe pancreatic juice and its composition

Answer 46

-Secreted by the exocrine pancreas. Composition: -Water. -Mineral salts. -Enzymes: amylase, lipase, nucleases. -Inactive enzyme precursors: trypsinogen and chymotrypsinogen.

Question 47

Describe digestion of proteins

Answer 47

polypeptides to tripeptides, dipeptides and amino acids. Trypsinogen and chymotrypsinogen activated by enterokinase into the active trypsin and chymotrypsin.

Question 48

Describe digestion of carbohydrates

Answer 48

Pancreatic amylase converts all digestible polysaccharides (starches) to disaccharides.

Question 49

Describe digestion of fats

Answer 49

Lipase converts fats to fatty acids and glycerol.

Question 50

Secretion of pancreatic juice is stimulated by what?

Answer 50

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.

Question 51

Where is bile secreted and stored?

Answer 51

Secreted by the liver, stored in the gall bladder under the stimulation by secretin and CCK.

Question 52

Composition of bile?

Answer 52

-Water. -Mineral salts. -Mucus. -Bile salts. -Bile pigments, bilirubin. -Cholesterol.

Question 53

Functions of bile?

Answer 53

-Emulsify the fats. -Eliminate waste products. -Digest proteins by enzymatic activity. -Regulate the digestion process.

Question 54

Describe the absorption of water and electrolytes in the large intestine

Answer 54

-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.

Question 55

Describe mass movement in the large intestine

Answer 55

Peristalsis, 1 to 3 times per day, in the ileum forces chyme into the caecum.

Question 56

Describe microbial activity in the large intestine

Answer 56

Region heavily colonised by certain types of bacteria, which synthesise vitamin K and folic acid.

Question 57

Describe defecation in the large intestine

Answer 57

Of every 500 ml of food residue that enters the caecum each day, about 150 ml become faeces.