GI Physiology

Question 1

Describe motility of the mouth

Answer 1

mastication

• skeletal muscle
• under voluntary control

Question 2

Describe mouth secretions and the functions of saliva

Answer 2

salivary glands secrete saliva (99% is water)
functions:
• Lubrication for swallowing (and speech)
• Antibacterial – (lysozyme, thiocyanate)
• Amylase (carbohydrate digestion)

Question 3

Describe the digestion that occurs in the mouth

Answer 3

Salivary Amylase – Begins carbohydrate (polysaccharide) digestion: Starch (plants) and Glycogen (animals)

• Breaks alpha 1,4 linkage: Hydrolyses into mixture of maltose and glucose
• No effect on other disaccharides (lactose, sucrose) or monosaccharides (glucose, galactose, fructose)
• Cellulose cannot be broken down in humans due to lack of cellulase to break beta 1,4 linkage)

Question 4

Describe absorption in the mouth

Answer 4

no absorption

Question 5

Describe the 2 types of saliva reflexes which enhance saliva secretion

Answer 5

1. Simple / Unconditioned Salivary reflex:
   Chemo and pressure receptors in oral cavity, info sent from afferent nerves fibres to salivary centre in medulla
2. Acquired / Conditioned Salivary reflex
   Thinking about food etc.

Question 6

describe motility of the oesophagus

Answer 6

• propels bolus from pharynx to stomach
• Swallowing initiated voluntarily (pharyngeal phase 1 sec), then completed involuntarily (oesophageal phase 5-9 secs)
• Primary Peristaltic wave: innervated by vagus nerve (parasympathetic), controlled by swallowing centre in medulla
• Secondary Peristaltic wave: if food stuck – pressure receptors in oesophagus – more forceful contractions

Question 7

describe secretion of the oesophagus

Answer 7

mucous for lubrication and protection

Question 8

describe digestion and absorption of the oesophagus

Answer 8

no digestion and absorption

Question 9

Describe motility of the stomach

Answer 9

• Storage through receptive relaxation (fundus dilates when food passes down the OE) – BER for the body
• Mixing food and gastric secretions into chyme – strong antral peristalsis propels chyme against closed pyloric sphincter

Question 10

Describe secretion of the stomach

Answer 10

• Vagal stimulation (Ach) causes G cells to release gastrin
• vagal stimulation and gastrin causes parietal cells to release HCl
• gastrin acts on enterochromaffin cells to release histamine which acts on parietal cells to produce HCl
• Vagal stimulation and Gastrin Acts on Chief cells to promote secretion of Pepsinogen which is converted to Pepsin by presence of acid
• Increased acid causes D cells to release somatostatin which inhibits Gastrin -> inhibits acid secretion
• parietal cells also secrete intrinsic factor which combines with B12 to facilitate absorption in ileum

Question 11

what are the functions of HCl

Answer 11

• Pepsinogen to Pepsin
• Connective tissue and muscle fibre break down
• Denatures Protein
• Barrier Immunity

Question 12

describe protein digestion in the stomach

Answer 12

Pepsin initiates protein digestion by breaking down splitting amino acid linkages to yield polypeptides, peptide fragments which are further broken down by pancreatic secretions

Question 13

Absorption of which two things occurs in the stomach

Answer 13

alcohol

aspirin

Question 14

Describe the triggers of the cephalic and gastric phase of gastric secretion (stimulation)

Answer 14

• Cephalic Phase – smelling, tasting chewing food = increased Ach release Vagus Nerve (parasympathetic)
• Gastric phase – Presence of proteins in stomach, distention (enlargement)- food entering stomach

Question 15

Describe the factors that inhibit gastric secretion (inhibitory)

Answer 15

• Removal of proteins and distention as stomach empties
• Accumulation of acid – D cells release Somatostatin (inhibits Parietal, G and ECL cells)
• Intestinal phase – Enterogastric reflex, enterogastrones released (CCK and secretin). Inhibit parietal and chief cells as well as smooth muscle contractions

Question 16

Describe the motility of the small intestine

Answer 16

1. Segmentation: mixes chyme with digestive enzymes and slowly propels food through the digestive tract. Oscillating (moving back and forth in a rhythm), ring like contractions of circular smooth muscle.
   Controlled by BER, influenced by Gastrin and extrinsic nervous system
2. Migrating Motility Complex: following absorption these between-meal short peristaltic contractions “sweep up” meal remnants as well as mucosal debris and bacteria, moves it towards colon. Takes 100-150 minutes to migrate from stomach. Regulated by hormone Motilin

Question 17

What is succus entericus and where is it found?

Answer 17

Succus entericus (intestinal juice) - consists of mucus and aqueous salt, found in small intestine

Question 18

Describe digestion in the small intestine

Answer 18

Brush border on epithelial surface:

• Enterokinase (converts trypsinogen into trypsin)
• Disaccharidases e.g. maltase, sucrase, lactase to complete carbohydrate digestion by hydrolysing into monosaccharides (glucose, galactose and fructose)
• Aminopepitidases – remaining peptides into amino acid constituents

Question 19

describe small intestine structure that helps in digestion

Answer 19

Multiple folds increase surface area, Villi increase surface area another 10-fold, microvilli a further 20 fold. 600 x greater surface area over all

• Bottom of villi – crypts of Leiberkuhn- contains stem cells
• All carbs, fats and proteins ingested are absorbed, nothing wasted
• Only Calcium and Iron absorption are regulated

Question 20

Describe fat absorption in the small intestine

Answer 20

• Monoglycerides and FFA brought to intestinal mucosa by micelle
• They cross lipid bilayer by active diffusion
• Triglycerides aggregate and are coated with lipoprotein to form Chylomicrons
• Chylomicrons cannot cross basement membrane of blood capillaries so enter circulation via lymphatic vessels

Question 21

Describe protein absorption in the small intestine

Answer 21

• Amino acids are absorbed in similar way to glucose via amino acid symporters. They enter epithelial cell by Na2+ and energy dependent active transport
• Glucose, galactose and amino acids all get a “free ride” on energy expended for Sodium transport
• Remaining small peptides enter cell via Na/H symporter and are broken down by intracellular peptidases
• Amino acids directly enter blood stream by facilitated diffusion

Question 22

Describe carbs absorption in the small intestine

Answer 22

• Now in monosaccharide form, glucose and galactose are absorbed into epithelial cell by means of Na2+ and energy dependent active transport (SGLT)
• Fructose enters blood by passive facilitated diffusion via GLUT-5
• Monosaccharides enter interstitial fluid via protein facilitated glucose transporter GLUT-2
• -> Then all enter blood by simple diffusion

Question 23

Describe the other things absorped in the small intestine

Answer 23

• water soluble vitamins absorbed with water whereas fat soluble not absorbed
• Vitamin B12 binds with intrinsic factor and absorbed in ileum
• iron absorbed by duodenal enterocytes
• calcium absorbed by active vitamin D

Question 24

Describe secretion of exocrine pancreas

Answer 24

1) bicarbonate- neutralises chyme pH

2) pancreatic enzymes

Question 25

describe digestion in the liver

Answer 25

- Bile secreted from liver and stored in the gall bladder - Triglycerides are not water soluble and form large fat droplets in chyme and are not digested by pancreatic lipase - Bile salts emulsify large fat droplets into smaller fat droplets. Support from lecithin to create water soluble micelles - -> Facilitates transport of fats to small intestine mucosa for absorption

Question 26

describe control of bile secretion

Answer 26

1. Chemicals - Bile salts themselves (Choleretic). After bile salts participate in Fat digestion and absorption, they are reabsorbed into enterohepatic circulation and stimulate more bile secretion 2. Hormones - Secretin 3. Neural - Vagus nerve (cephalic phase of digestion)

Question 27

describe motility of the large intestine

Answer 27

Primarily for storage. Haustral contractions initiated by autonomous rhythmicity of colonic mucosal cells cause slow shuffle chyme residue forward (once every 30 minutes)

Question 28

describe digestion of the large intestine

Answer 28

no digestion

Question 29

Describe secretion in the large intestine

Answer 29

Alkaline mucous solution – entirely protective

Question 30

Describe absorption in the large intestine

Answer 30

500 mls chyme enters per day. Some salt and water absorption here (not as much as small intestine) Sodium actively absorbed, Chloride follows passively down electrical gradient and water follows by osmosis

Question 31

What controls the frequency of contractions in the smooth muscle of the GI tract

Answer 31

Controlled by interstitial cells of Cajal – BES is the spontaneous depolarisation and repolarisation of these cells.

Question 32

WHat are the nerve plexus that control the GI tract

Answer 32

* Intrinsic nerve plexus – myenteric/submucosal: control all facets of GI tract. Sympathetic and parasympathetic nerve fibres of Vagus nerve. Parasymp predominates in relaxed situations * Extrinsic nerve plexus – modifies intrinsic pathways

Question 33

Describe the autonomic nervous system control of saliva secretion (para/sympathetic))

Answer 33

ANS: - Parasympathetic stimulation produces a continuous flow of watery saliva - Sympathetic stimulation produces low volume, thick saliva which is rich in mucus

Question 34

List the effects that Secretin has

Answer 34

In response to acidic chyme: Inhibits secretion of gastric acid from parietal cells Increases bicarbonate secretion from pancreatic ductal cells Increases bile secretion from the gallbladder

Question 35

What effect does CCK have on the gallbladder?

Answer 35

Increases gallbladder contraction- bile secreted

Question 36

what factors control gastric emptying

Answer 36

Gastric emptying controlled by factors in: - Stomach- Volume of chyme / degree of fluidity (positive effect) - Duodenum- Enterogastrones (Secretin and Cholecystokinin (inhibitory and more powerful effect)