Digestive System 3 Flashcards
What type of organ is the pancreas?
Dual-function
How is the pancreas an exocrine gland?
It manufactures the precursors of digestive enzymes and secreting then as alkaline “pancreatic juice” via a duct system leading to the duodenum; enzyme precursors are converted to their active form once they arrive in the duodenum. Most food substances are digested by pancreatic enzymes (proteins, carbohydrates, lipids and nucleic acids)
How is the pancreas an endocrine gland?
Islets of langerhans make up 1% of the pancreas and secrete hormones into the bloodstream. The hormones (insulin and glucagon) regulate blood glucose levels
What is the trunk of the pancreas?
Main pancreatic duct
What is the small branch of the pancreas?
Interlobular ducts
What are the twigs of the pancreas?
Intercalated ducts
What are the leaves of the pancreas?
Acini (made up of serous type secretory cells)
Dimensions of the small intestine?
3 diameter, a little over 3m long
What occurs in the small intestine?
Most digestion and absorption- at its upper end it receives exocrine secretions from the liver (ie bile, stored in the gallbladder) and pancreas (pancreatic juice, containing digestive enzyme precursors)
Three regions of the small intestine?
Duodenum (25cm long, c shaped tube, not suspended by mesentery, receives biliary and pancreatic ducts)
Jejenum (1m)
Ileum (2m)
Differences of the small intestine to the four layers of the gut?
- The mucosa is specialised to greatly increase the surface area available for secretion and absorption
- The submucosa just downstream of the pyloric spinchter contains mucous glands (glands of brunner)
Orders of increasing surface area in the small intestine?
Gross convolutions
Plicae
Villi
Microvilli
What are plicae?
Circular folds 1cm high- each plica is covered with mucosa and has a core of submucosa
What is the villi?
Fingers 1mm high. Covering is epithelium, core is lamina propria
What are microvilli?
Fingers 1micrometre high. They form a “brush border” on the surface of individual absorptive cells. Each microvillus is covered with cell membrane and filled with cytoplasm
What do columnar absorptive cells (enterocytes) do?
Absorb the small molecules resulting from digestion
What do goblet cells do?
Secrete mucus
What do enteroendocrine cells do?
Secrete hormone secretin (and others) into capillaries of the lamina propria
What do undifferentiated cells do?
They’re stem cells dividing to generate new epithelium
What do paneth cells do?
Secrete bacterial enzyme lysozyme and are phagocytic
What are venules in the submucosa tributaries of, and what travels through them?
Hepatic portal vein- transports carbohydrates (absorbed as monosaccharides), proteins (absorbed as amino acids), water and electrolytes
What travels through lymphatic vessels of the small intestine?
Absorbed lipids
What is the lifespan of the epithelium of the small intestine like?
Short-lived. Cell divisions occurs deep in the glands, then the entire sheet (except paneth cells) move slowly up the gland walls and up the sides of the villi. Old cells are shed from the villous tip. The entire journey lasts 5 days
What is the lymph vessel within each villus like?
Lacteal
What happens to the lymph in the small intestine?
It’s “milked” along the lacteal by contraction of smooth muscle fibres in the lamina propria which shorten the villus- these fibres arise from the muscularis mucosae
What stimulates the release of secretin from enteroendocrine cells?
The arrival of acid chyme
What does secretin in the bloodstream do?
Stimulates the release of pancreatic juices (cholecystokinin similar)
What is also secreted by the enteroendocrine cells of the small intestine and stomach and what does it do?
Gastrin- stimulates gastric gland to secrete acid and enzymes
Describe the small intestine?
A soft muscular tube about 1.5m long with 17 parts
Functions of the large intestine?
- Absorption of salts and water (1 litre/day compared with 8litre/day absorbed in the small intestine)
- Conversion of chyme into faeces. Bacteria ferment remaining carbohydrates
- Bacteria produce some vitamins (B & k) which are absorbed
- Secretion of mucous to lubricate faeces
- Defecation
Parts of the large intestine?
- Vermiform appendix
- Cecum
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
- Rectum/anal canal/anus
What controls intermittent flow of chyme from ileum into caecum?
Ileocecal valve
What is the caecum like?
A dilated pouch (much larger in herbivores), where bacteria (not enzymes) are responsible for digestion. Humans produce no enzymes capable of digesting the cellulose of plants
What is the appendix?
A vestigial worm-like appendage of the caecum
What do faeces contain?
Bacteria (30%), undigested dietary fibre (30%), cells shed from the intestinal lining and mucus. Their colour is from pigments derived from bacterial decomposition of bilirubin
What is the mucosa of the large intestine like?
No villi but many intestinal glands (also called crypts of lieberkuhn)
Surface epithelial cells are enterocytes (absorptive) similar to small intestine
Intestinal glands contain mostly goblet (mucous), no paneth cells
Cell renewal same as small intestine (entirely renewed every 4-5 days)
Many clusters of lymphocytes (lymphoid nodules) in the lamina propria, reflecting large bacterial content of colon
What’s the external muscle of the large intestine like?
Outer longitudinal muscle is thickened in three strips. The strips (teniae coli) contract to pull the intestinal tube into sac like pockets (haustra coli). Haustra change shape and position
What do columnar absorptive cells (enterocytes) do in the large intestine?
Absorb water
What do goblet cells do in the large intestine?
Secrete mucus to lubricate the passage of faeces
What do undifferentiated cells do in the large intestine?
Stem cells which divide to generate new epithelium
What do white blood cells do in the large intestine?
Mostly lymphocytes which provide defence against bacteria invading from the lumen of the colon
Describe the rectum?
Makes up final 20cm of the gut tube (last 2cm are the anal canal), the canal is closed by two sphincters
What are the two sphincters of the rectum like?
Internal= involuntary smooth muscle Outer= voluntary skeletal muscle
When is there an urge to defecate?
When the rectum fills to 25% capacity
What does stretching of the rectal wall do?
Initiates a reflex contraction of teniae coli in the descending colon and rectum. Shortening of this part if the gut tube further increases pressure in the rectum. The internal anal sphincter relaxes (opens) as part of the reflex
What happens in regards to time and the external sphincter?
If convenient, it’s relaxed voluntarily, if not; maintained in contact reacted state and defecation reflex subsides (children take time to learn to control external spinchter)
