Flashcards in Digestive System Deck (36):
Describe the pathway taken by food in the body
Mouth, oesophagus, duodenum, jejunum, ilium, cecum (large intestine), ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anus.
What is the mesentery?
A fold of the peritoneum which attaches the stomach, small intestine, pancreas, spleen, and other organs to the posterior wall of the abdomen.
What are the four layers of the gut wall?
• the mucosa (innermost)
• the submucosa -subtending layer of connective tissue containing mucus-secreting glands.
• the external muscle layers (muscularis externae).
• the serosa (outermost).
What is the submucosa?
Submucosa (a layer of connective tissue bearing glands, arteries, veins and nerves).
What does the muscularis externa consist of?
2 layers of smooth muscle
What is the function of the muscularis externa?
The muscularis externa creates successive peristaltic waves to move luminal contents along the gut.
What are the major functions of the GI tract?
• to provide a port of entry for food into the body
• to mechanically disrupt the food
• to temporarily store the food
• to chemically digest the food
• to kill pathogens in the food
• to move the food along the tract
• to absorb nutrients from the resultant solution
• to eliminate residual waste material
What are the main properties of saliva?
• Starts digestion (amylase and lipase)
• Bacteriostatic (contains Immunoglobulin A antibody (IgA))
• High calcium (protects teeth)
• Assists swallowing
• Protects the mouth
How is food digested in the mouth?
1. physical - by action of teeth, tongue, and muscles of mastication
2. chemical - by action of salivary amylase and lipase
Does the oesophagus undergo voluntary or involuntary control?
Upper end oesophagus – voluntary control (some striated skeletal muscle)
Lower end oesophagus – involuntary control (solely smooth muscle)
What is the oesophageal mucosa made up of?
• Epithelium – stratified squamous non-keratinized (withstands abrasion).
• Lamina propria – loose connective tissue bearing blood & lymph vessels, some smooth muscle cells & many cells of immune system.
• Muscularis mucosae – thin layer of smooth muscle cells.
Describe the structure of the muscularis externa of the oesophageal mucosa
Smooth muscle layers (inner – circular; outer – longitudinal) which move food by peristalsis.
What is the adventitia?
Thin outermost layer of connective tissue of the oesophagus (no peritoneal enfolding of this portion of GI tract).
How does the stomach digest food?
1. physical - by churning (3 muscle layers and mucosal rugae/ridges)Of
2. chemical - by acid (HCl) and enzymes (e.g. pepsin)
What are the functions of the stomach?
• acts as a necessary food store (we can eat faster than digest)
• wall relaxes so pressure doesn’t rise (called receptive relaxation)
• contracts rhythmically to mix and disrupt
• secretes acid and proteolytic enzymes to break down tissues and disinfect
• protects its epithelium by secreting mucus
• produces hypertonic chyme by combined action of acid, enzymes and agitation
• delivers incompletely digested chyme slowly, and in a controlled way, to the duodenum
Describe the layers of the stomach?
- Mucosa - secretes acid, digestive enzymes and the hormone gastrin
- Muscularis mucosae
- Muscularis externa - 3 layers of smooth muscle in stomach (oblique, circular and longitudnal)
- Rugae - folds of gastric mucosa forming longitudinal
ridges in empty stomach
Why does the stomach have abundant surface mucous cells?
- Their secreted mucus is resistant to pepsin (enzymatic) degradation.
- The mucus is released in response to distension, stomach contents, and acid secretion from the gastric glands.
- Alcohol or aspirin can damage the mucous cells but they are quickly replaced by mitosis in deeper cells in the neck of the gastric pit.
- The secreted mucus contains HCO ions which neutralise the effect of H+ ions and thereby protect the stomach lining.
Describe the structure of a gastric gland
1. Gastric pit - lined by mucus secreting cells, very like goblet cells.
2. Isthmus - the region in which stem cells divide to populate the gland by upward or downward migration
Which cells are abundant in gastric glands?
1. Parietal Cells - secrete H+ ions into the lumen and HCO3- ions into capillaries (move it to surface mucous cells)
2. Chief cells - secrete pepsinogens (converted to pepsins, partly hydrolyse proteins)
3. Enteroendocrine cells - Include G cells (secrete gastrin). Mucosa responds to this by secreting acid
Describe the structure of the duodenum
• 20-25 cm long, C-shaped, proximal portion of small intestine
• Curves around head of pancreas
• Walls contain Brunner’s glands which secrete bicarbonate–rich
mucus, to neutralise acidic chyme.
What is the function of the duodenum?
- Dilution & neutralisation of chyme
• Water drawn in from ECF to render hypertonic chyme, isotonic
• Liver releases bile (generally via the gall bladder).
bile contains: water, alkali and bile salts (emulsify fats)
• Pancreas & liver secrete alkali to neutralise acidic chyme
• Pancreas, liver and intestine secrete specific enzymes which act,
with bile, to complete digestion of chime
How do enzymes from the pancreas and intestine 'complete digestion'?
• Cleave peptides to amino acids
• Cleave polysaccharides to monosaccharides
• Break down and re-form lipids
• Break down nucleic acids
What is required in absorption?
1. Requires lots of energy
2. Requires large surface area
• Gut is folded/villi/micro-villi
• Adequate contact time (control of gut transit)
3. Good blood supply/drainage (latter via hepatic portal vein; all nutrients travel via the liver)
What do each part of the small intestine absorb?
• Duodenum absorbs iron
• Jejunum absorbs most of sugars, aa’s and fatty acids
• Ileum absorbs Vit B12, bile acids and remaining nutrients
How are the bacteria in the large intestine used?
• synthesis of vitamins K, B12, thiamine and riboflavine
• breakdown of 10 to 20 bile acids
• conversion of bilirubin to non-pigmented metabolites - (all readily absorbed)
What lines the intestinal glands of the mucosa in the colon?
- Crypts of Lieberkuhn
- Lined with simple columnar epithelium
What gives the urge to defecate?
- By end of large intestine contents semi-solid
- Contents await expulsion in colon (not rectum)
- At certain times rapidly propelled into rectum
- stretches rectum
- critical mass gives urge to defaecate
- if higher centre control overrides reflex = reverse peristalsis
Why is there a need for control in the GI system?
• It is a delicate balance (why diarrhoea & constipation so common)
• Gut is dealing with large quantities of fluid
- decreased absorption, or increased secretion
can result in:
– life threatening dehydration (depletion of body
fluids, not just what we recently drank/ingested)
– life threatening electrolyte imbalance
How is control within the GI system achieved?
Motility and secretion need precise control
• Achieved by overlapping mechanisms
• Paracrine (especially important in the stomach)
Which parts of the digestive system do we control and which parts are autonomic?
– Somatic (innervating striated muscle)
• Ingestion (mouth and first ⅓ of oesophagus)
• Excretion (last sphincter of anus)
– Autonomic nervous system controls the rest
• Post ganglionic neurones form plexuses (=braid of nerves)
- one between muscle layers of gut wall
- one between submucosa and muscularis externa
Which substances act via the paracrine system in the digestive system?
Substances act locally
• Histamine (controls production of acid in stomach)
• Vasoactive substances (affect blood flow in gut)
What is the role of the endocrine system within the digestive system?
Range of hormones control:
• secretion of stomach acid
• alkali secretion from liver and pancreas
• enzyme secretion
What is secretin?
• 1st hormone top be identified (1902)
• promotes HCO3- (bicarbonate) secretion from duct cells of pancreas
• promotes bile production by the liver
• inhibits secretion of acid by parietal cells of stomach
What is cholecystokinin? (CCK)
• synthesised and secreted by enteroendocrine cells of the duodenum
• promotes release of digestive enzymes from the pancreas
• promotes release of bile from gall bladder (stimulates it to
• is a hunger supressant
What releases gastrin?
- G cells of pyloric antrum of stomach