Digestive System

Question 1

Different parts of the digestive tract

Answer 1

Mouth: entry point for digestion - chewing and enzymatic action of saliva (tongue, cheeks, roof)
Oesophagus: connects digestive tract to laryngopharynx to stomach (2 sphincters upper and lower)
Stomach: muscular J-shaped sac (fundus, body, pyloric antrum) mechanical + chemical digestion
Small intestine: 3 major parts duodenum, jejunum, ileum (has plicae circularis, villi and microvilli) most digestion/absorption here
Large intestine: caecum, colon (ascending, transverse, descending, sigmoid) rectum and anus - absorbs water + forms faeces

Question 2

Sphincters regulating movement

Answer 2

Upper oesophageal sphincter: controls movement of food from pharynx to oesophagus
Lower oesophageal sphincter: prevents acid reflux - controls entry of food from oesophagus to stomach
Pyloric sphincter: passage of partially digested food from stomach to duodenum
Ileocecal valve: controls flow of digested material from ileum to caecum of large intestine
Anal sphincters (internal/external): expulsion of faeces from rectum outside

Question 2

Microscopic structure of digestive tract

Answer 2

Mucosa: innermost layer; contains epithelial cells - secretion, absorption, protection
Submucosa: Dense connective tissue containing blood vessels - provides nourishment/supports mucosa
Muscularis externa: 2 layers of smooth muscle - peristalsis and segmentation
Serosa (Adventitia): outermost layer, protective layer (serosa - abdomen, oesophagus/rectum - adventitia)

Question 2

Accessory organs and locations

Answer 2

Salivary glands: secrete saliva into mouth (parotid/submandibular glands)
Liver: produces bile to digest fats
Pancreas: secretes digestive enzymes and bicarbonate into duodenum

Question 2

Five major systems that control the digestive system and example

Answer 2

Enteric nervous system: afferent (sensory) neurones detect stimuli + interneurones (info) and efferent (motor) neurones produce contraction
Autonomic nervous system: parasympathetic nervous system - rest and digest, sympathetic - fight or flight
Somatosensory system: afferent (sensory) components of long reflexes mediated by neurones of this system (sensations - nociception)
Endocrine system: hormones released by cells mediate effects of cells in other organs
Paracrine system: chemical communication - paracrine control

Question 3

Physiology responsible for peristalsis and segmentation
Functional significance?

Answer 3

Peristalsis: involuntary wave-like muscle contractions
Physiology: circular muscles contract behind food bolus whilst longitudinal muscles ahead contract to shorten and widen tract
Significance: propels food from oesophagus to stomach

Segmentation: rhythmic contraction of circular muscles
Physiology: alternating contractions of circular muscles to mix/break down food
Significance: enhance digestion/absorption by mixing food w/ digestive juices

Question 3

What is deglutination?
Steps and consequences of dysphagia

Answer 3

Swallowing
1. Elevation of the soft palate (prevents food going back into nose)
2. Elevation of the larynx (prevents food falling into airways)
3. Cessation of breathing (deglutition apnoea)
4. Relaxation of upper oesophageal sphincter
5. Peristaltic contractions
6. Relaxation of the lower oesophageal sphincter

Dysphagia: compromised deglutition - life-threatening is food enters/blocks airways (Parkinson’s)

Question 3

Major blood vessels supplying/draining the digestive system

Answer 3

Arteries
Celiac trunk: stomach, liver, spleen, upper duodenum
Superior mesenteric artery: supplies small intestine and first half large intestine
Inferior mesenteric artery: second half large intestine

Venous
Hepatic portal vein: collects blood from digestive organs and transports it to liver for detox/metabolism
Superior mesenteric vein: drains small intestine and part of large intestine
Inferior mesenteric vein: drains distal large intestine
Splenic vein: drains spleen, pancreas, stomach -> eventually merges superior mesenteric vein/hepatic portal vein

Question 3

Constituents of saliva and how saliva secretion is regulated

Answer 3

Salivary amylase: enzyme that initiates breakdown of carb
Mucin: glycoprotein - lubrication
Proline-rich proteins: tooth enamel production/antibacterial actions

Basal release - keep mouth moist/assist with speech
Stimulated release: elevation of saliva - sight, smell or taste
Sensory neurones - relay info to salivatory nuclei in brainstem
Parasympathetic tone - increase in flow of watery saliva, rich in amylase and mucin
Sympathetic tone - enhance salivary amylase concentration in saliva

Question 4

Stages involved in emesis and functional significance of this reflex

Answer 4

Vomiting
1. Contraction of the pyloric sphincter (stomach to mouth)
2. Contraction of the diaphragm and abdominal muscles (pressure increase)
3. Relaxation of the lower and upper oesophageal sphincters (movement up from stomach)
4. The larynx is raised and breathing is inhibited (positions epiglottis)
5. The soft palate is raised

Question 4

Roles the mouth plays in digestive system function

Answer 4

Mechanical digestion: chewing breaks down food
Chemical digestion: saliva contains enzymes like amylase (carb breakdown)
Formation of bolus: chewed food + saliva

Question 4

How does the three layers of smooth muscle in the stomach coordinate together to mix foodstuff?

Answer 4

Fundus -> body -> pyloric antrum

Wave like movement propelled to pyloric sphincter (closes) to be recycled back

Question 5

Different cell types found in the walls of the small intestine and functional significance

Answer 5

Brush border - microvilli has enzymes

Goblet cells - secrete mucous
Paneth cells - secrete lysozyme - antibacterial enzyme
Intestinal glands - secrete watery mucus to protect from acidic chyme
Stem cell - reconstitute epithelium
S cells - secrete secretin - low pH - products of protein/fat digestion
I cells - secrete cholecystokinin - fats/protein digestion

Question 5

Major types of cell found in gastric gland and functional significance?

Answer 5

Mucous neck cells: entrance - secrete thick, sticky mucous -> mucosal barrier
Stem cells: undifferentiated steam cells - capacity to renew gastric epithelium
Parietal cells: responsible for HCl and intrinsic factor (absorption of B12 by small intestine)
Chief cells: Proteins - pepsinogens - converted to pepsins by HCl - protein digestion
Enteroendocrine cells: base of gastric gland - secrete hormones into bloodstream -> G cells and secrete gastrin

Question 6

Physiological processes involved in the control of gastric motility

Answer 6

Neural and hormonal mechanisms
1. Cephalic phase: thought, sight, smell and taste = increase parasympathetic tone - prepares stomach
2. Gastric phase: presence of food detected by mechanosensitive neurones and G-cells (gastric glands) to secrete hormone gastrin
3. Intestinal phase: inhibitory effect mediated by acidic pH - release of secretin and cholecystokinin from enteroendocrine cells in duodenum

Question 6

End result of physiological process of the stomach

Answer 6

End result - complex mixture of food, salvia and gastric juice - chyme

Question 7

Processes responsible for the digestion of proteins

Answer 7

Begins in stomach under influence of pepsins (activated by HCl)
assisted by brush border enzymes, pepsins, trypsin and chymotrypsin - cleave whole proteins/larger polypeptides

carboxypeptidase/brush border - smaller enzymes

Question 7

How long does food spend in the stomach and types of substances absorbed

Answer 7

Highly variable
Liquid lunch - ~30mins
Pizza - up to 4hrs

Question 7

Major constituents of pancreatic juice and functional significance

Answer 7

Proteolytic enzymes:
Pancreatic amylase
Lipolytic enzymes: lipases

Question 8

Processes responsible for the digestion of neutral fats

Answer 8

Neutral fats - triglycerides
Unsaturated or saturated

Pancreatic lipases - digest exposed surface of triglycerides -> yield free fatty acids and monoglycerides

Surrounded by shell of bile salts and lecithin - structures known as micelles and made available for absorption

Question 8

Physiological processes involved in the control of pancreatic juice secretion

Answer 8

Cephalic phase:
Gastric phase:
Intestinal phase:

Question 8

Major constituents of bile and functional significance

Answer 8

Bicarbonate
Bile salts: cholic acid and chenodeoxycholic acid - synthesised by liver from cholesterol
Bile pigment: bilirubin - haeme component from haemoglobin (RBC death)

Bile major route for excretion of cholesterol
Lecithin - phospholipid helps fat digestion

Question 8

Processes responsible for the absorption of carbs

Answer 8

2 step process
glucose + galactose transported against conc. gradient by active transport w/ Na+

Fructose down conc. gradient - facilitated diffusion

All 3 monosaccharides -> basolateral membrane by facilitated diffusion and diffuse into capillaries of lamina propria

Question 8

Processes responsible for the digestion of carbs

Answer 8

Galactose, glucose and fructose

Cellulose unable to break down in digestive system

Combined actions of salivary amylase, pancreatic amylases and brush border enzymes

dietary polysaccharides and disaccharides are quickly broken down into monosaccharides that are small enough to be absorbed

Question 9

Processes responsible for the absorption of proteins

Answer 9

use co-transport w/ Na+ down conc. gradient

move across by facilitated diffusion and into capillaries

Question 10

Processes responsible for the absorption of neutral fats

Answer 10

micelles - move through simple diffusion

cytoplasm - FA and monoglycerides -> triglycerides in smooth endoplasmic reticulum - covered in protein -> chylomicrons

packed into vesicles by golgi apparatus and transported by exocytosis

exocytosed chylomicrons are too large to move into capillaries (don’t go into cardiovascular system). Diffuse into lymphatic vessels (lacteals) and broken down by lipoprotein lipase to FA and monoglycerides (released)

Question 11

Different mechanisms by which fat soluble and water soluble vitamins are absorbed

Answer 11

fat soluble - generally associated w/ dietary fats and incorporated into micelles - move across apical/basolateral membranes

water soluble - (b vitamins and vitamin C) cross apical membrane by simple or facilitated diffusion - some require active transport

b12 has to bind to intrinsic factor (parietal cells of gastric glands) before absorbed

Question 12

Physiological processes involved in defecation

Question 12

How osmosis is responsible for the absorption of water by the small intestine

Answer 12

absorption of water is a passive process - driven by osmotic pressure

as osmolarity decreases water will move out of lumen across epithelium by osmosis. absorption of water is linked to absorption of nutrients

Question 12

Why does the rate at which chyme moves along the large intestine important?

Answer 12

rate at which chyme moves through is crucial for digestion and absorption

chyme moves too quick - not enough time for water and electrolytes to be absorbed = diarrhoea

chyme moves too slow - excessive water absorbed = constipation

Question 12

How electrolytes (Na+, K+ and Ca2+) are absorbed

Answer 12

Na+ = sodium-potassium exchange pump
K+ = simple diffusion
Ca2+ = calcium channels - active transport (no. Ca2+ pumps regulated by vit D levels)

Question 13

Difference between haustral contractions and mass movements? Functional significance

Answer 13

haustral contractions - most common movement type in large intestine which propels contents of one haustra to the next - regulated by mechanosensitive neurones of enteric nervous system

mass movements - accelerates movement of contents towards rectum - usually occurs after a meal. control of these involve long reflex by activation of sensory neurones in stomach by distensions –> enhanced parasympathetic tone

mass movements attenuated by opiate analgesics (morphine/codeine) but enhanced by presence of undigested dietary fibre

Question 14

What is flatus? Who is responsible for production?

Answer 14

flatus - digestion consequence 500ml of methane and hydrogen sulphide

responsible - bacteria that digest cellulose and some lipids to release small fatty acids

Question 14

Why do young children and people with spinal cord injuries suffer for incontinence of faeces?

Answer 14

young children - spinal cord pathways that control external anal sphincter have not developed so they have incontinence of faeces

spinal cord injuries - disruption of nerve pathways that control bowel movements

Question 15

What is bile?

Answer 15

Produced by liver and concentrated in gallbladder
contains: bicarbonate, bile salts and pile pigments

Bicarbonate neutralises acidic chyme from stomach
Bile salts - cholic acid and chenodeoxycholic acid (from cholesterol) - forms micelles and important for digestion of fats
Bile pigments - bilirubin (haeme component)
Lecithin - helps process of fat digestion

Question 16

Control of bile secretion?

Answer 16

Fasting state - sphincter of Oddi - closed redirects bile to cystic ducts so it concentrates

monoglycerides and fatty acids in chyme stimulates cholecystokinin from I cells –> causes contractions in gallbladder and relaxation of sphincter of Oddi - bile flow into small intestine

low pH of chyme in duodenum - release of secretin from S cells - inhibit gastric motility and stimulates pancreatic juice secretion. Secretin causes secretion of bicarbonate and water and increases flow rate of bile.