Small intestine and pancreas 2 Flashcards
What are the components of the small intestine?
Duodenum, jejunum, ileum
Describe the arrangement of the components of the small intestine
- Duodenum from pylorus of stomach (descending then ascending portion)
- Transition between duodenum and jejunum at duodenojejunal flexure (sits under rest of intestine)
- Jejunum makes up most of SI
- Ileum last part of SI
- Ileocaecal junction very tight
- In middle of abdomen and slightly to the right
What is the function of the jejunum?
Major site of absorption
Describe the duodenum
- Brunner’s glands sit in submucosa
- Compound tubular submucosal glands
- Produce alkaliine secretion with bicarbonate
- Protects duodenal epithelium from incoming stomach acid
- Provide alkaline condition for intestinal enzymes to be active
- Lubricate the intestinal wall from rough ingesta
Describe the ileum
- Caudal to jejunum
- Attached to caecum by ileocaecal fold
- Contains blood vessels from caecum and colon
- Enters ascending colon at level L1-L2 through distinct anatomic ileocaecal sphincter
- Mostly for reabsorption of bile acids to be recycled
- Contains Peyer’s patches
What is the clinical signifiance of the ileocaecal sphincter?
- Is a narrowing
- Common site for obstruction
Where does the pancreas sit?
In the duodenal flexure
Give a basic overview of the Peyer’s patches of the ileum
- GALT
- Contain B cells and M cells
- Take up foreign materials e.g. bacteria, viruses
What is the major arterial supply to the small intestine?
- Caudal mesenteric and cranial mesenteric
- Many branches
- Mesenteric supply
- Ileum also has antemesenteric supply from ileal branch of ileocolic artery
What is the major venous drainage from the small intestine
- Portal vein (to liver) and cranial mesenteric
Describe the innervation of the small intestine
- Parasympathetic pathways
- Vagus nerve
- General viscero-effector neurons in medulla oblongata
- Fibres run through vagus -> dorsal vagal trunk -> coeliac and mesenteric ganglia -> along blood vesels -> gut
- Sacral part of PSNS
- Pelvic nerves form retroperitoneal plexus which supplies descending colon and rectum
- Mechanical and chemo-receptors
- Also from sympathetic ganglia (solar pelxus, caudal mesenteric ganglia)
Describe the blood supply to the pancreas
- Coeliac and cranial mesenteric (splenic, hepatic, superior mesenteric branches)
- Right lobe: cranial pancreatoduodenal artery (branch of hepatic)
- Left lobe: caudal pancreatoduodenal artery (branch of cranial mesenteric)
Describe the venous drainage of the pancreas
Portal vein
Describe the innervation of the pancreas
- Vagus - neural control
- Sympathetic and parasympathetic control
- PSNS: dorsal vagal trunk
- SNS: solar plexus (splanchnic nerves)
Describe the microscopic structure of the small intestine
- Lumen
- Mucosa
- Submucosa
- Muscularis
- Serosa
Describe the lumen of the small intestine
- Contains chyme
- Food, bacteria, toxins, viruses, secretions, foreign bodies
Describe the mucosa of the small intestine
- Single epithelial layer
- Semipermeable membrane, secretion absorption, protection
- Mucus covered in mucin, glycocalyx and IgA
Describe the submucosa of the small intestine
- Lamina propria
- Structural support
- Blood vessels
- Lymphatics
- Nerves
Describe the muscularis of the small intestine
- Smooth msucle circular and longitudinal) for motility
- Mixes, stimmulated by distention, aim is to increase mucosal contact and delay passage
- Slow aboral movement of chyme (peristalsis)
- Interdigestive migrating motlity complex (cleaning in unfed periods)
Describe the serosa of the small intestine
Strong, protective, supportive outer layer
What happens to the rate of passage of chyme in hypermotility?
Delayed passage
What happens to the rate of passage of chyme in hypomotility?
- Rapid passage
- Little resistance
What is meant by ileus?
- Lack of motility leading to bacterial overgrowth
Describe how the structure of the small intestine is related to its function
- Increased surface area to increase absorption
- Crypts produce immature enterocytes and other gut cells
- Villus structure increases absorptive area
- Mature enterocytes on villi absorb nutrients
- Bigger fenestrations in the capillaries in teh villi allows passage of molecules in and out of blood
- Villus has brush border (greater SA)
- Goblet cells secrete mucin (pH, lubrication)
- Stem cell zone ensures enterocytes continuously replaced at extruction zone_
- Have enteroendocrine cells, Paneth cells, Enterocytes, Brush border
Decribe the function of the enteroendocrine cells of the small intestine
- Produce hormones
- Scattered along epithelium
- Respond to presence of AAs
- Secrete CCK, acts on liver and pancreas
- Secrete secretin, acts on pancreas
Describe the function of the Paneth cells in the small intestine
- Migrate to base of crypts
- Produce anti-microbial defensin peptides
- Act to keep clear of high bacterial count
Describe the function of the enterocytes in the small intestine
- Smooth endoplasmic reticulum allows intracellular transportation of nutrients
- Pinocytosis into enterocyte allows foreign material to enter cell as vesicle
- Fuse with enzyme packed intracellular lysosomes
- Protective function stopping foreign materials entering the body
Describe the function of hte brush border in the small intestine
- Thick mucus layer
- microvilli
- Mature enterocyte brush border enzymes (integral membrane enzymes, peptidases, CHO hydrolases = lactasem maltase, sucrase)
Describe the structure of the pancreas
- 2 lobes
- Left lies in greater omentum
- Right embedded in duodenal flexure
- In cat bile and pancreatic duct fused, separate in dog
- Exocrine: acinar cells
- Endocrine: Islets of Langerhans
Describe the exocrine function of the pancreas
- Acinar cells
- Produce lipase, amylase, nucelases, gelatinases, proteases
- Packaged as zymogens iin secretory vesicles in pacreas acinar cells
- Vesicles contain trypsin inhibitor
- Zymogens enter ducts, go to SI
- Zymogens converted in SI lumen, massive reelase of proteases into lumen
What is the pancreatic enzyme production and secretion stimnulated by?
- Cephalic phase of digestion (autonomic neural and humoral - gastrin, synthesis of enzymes, eating)
- Gastric phase of digestion (events in stomach, neural - gastropancreatic reflex via vagus - and humoral through gastrin)
- Intestinal phase of digestion - events in SI, hormonally mediated e.g. CCK and secretin. Intraluminal ionised calcium stimualtes release
Describe the control of acinar pancreas secretions
- Vagus nerve (neural)
- Gastrin from entero-endocrine cells in stomach
- Acts on acinar cells to produce zymogens
- CCK from enteroendocrine cells in duodenum - sense partly digested proteins and fats, acts on acinar cells to produce zymogens
Describe the cells involved in the endocrine function of the pancreas
- Islets of Langerhans
- Alpha, beta, delta cells
- Alpha: glucagon
- Beta: insulin
- Delta: somatostatin
Describe the effect of detection of acid chyme in the duodenum on the pancreas
- Duodenum entero-endocrine cells sense acidic chyme from stomach
- Secretin release acts on pancreatic ducts
- Epithelial cells in pancreatic ducts produce bicarbonate and water
- Helps flush acinar enzymes into SI lumen
Compare the 2 pathways of absoprtion in the small intestine
- Paracellular and transcellular
- Para: across tight dunctions, small molecules
- Trans: across plasma membrane, large AAs, glucose
Breifly outline the absorption of dietary CHO sugars in the small intestine
- Amylase from pancreases processes diet starch to maltose
- Diet also provides lactose and sucrose
- Too big for absorption still
- Brush border hydrolase enzymes used to produce monosaccharides maltase, lactase, sucrase)
- Produces glucose, galactose and fructose
- Ready to move across SI lumen membrane
Briefly outline the absorption of glucose-galactose in the small intestine
- Sodium dependent glucose transporters located in epithelium of lumen membrane
- Sodium binds in lumen, makes transporter open to glucose/galactose
- Glucose binds and transporterr switches to move pockets inside cell
- Sodium and glucose enter cell
Briefly outline the absorption of peptides in the small intestine
- Diet protein digested by stomach pepsin and pancreas trypsin, chymotrypsin and carboxypeptidases
- Results in oligopeptides (too big for absorption)
- Brish border peptidases used
- Producees small AA groups
- Absorbed via sodium dependent AA transporters
Briefly outline the absorption of lipids in the small intestine
- Diet lipids digested with bile acids and pancreatic lipase
- Fatty acids and TAGs enter GI enteroytes by simple diffusion
- Follow FA transporter mechanism
- Inside enterocytes lipids packaged into chylomicrons
- Enter lymph vessels first, not blood
What is the significance of sodium and chloride movements in the small intestine?
- There is a high sodium concentration outside the cells
- Need to keep sodium in cells at a low level
- Wherever sodium goes, water follows
Briefly outline water absorption in the small intestine
- High osmotic pressure in lower GI lumen and cells
- Sodium pumped into space between cells
- Sodium diffuses into capillary bed
- Water follows sodium into blood
Describe chloride movement in the GI epithelium
- Cl- enters from blood
- cAMP activated
- Cystic fibrosis transmembrane conductance receptor (CFTR) activated
- Trasnports Cl- into lumen
- Na+ follows Cl into lumen
- cAMP action critical in bacterial enterotoxin action
- Stimulate normal mechansism resulting in diarrhoea
What is the gastrocolic reflex?
- A long intestinal reflex
- Food in stomach stimulates colonic contractions
- Reflex medaited via afferents to CNS and efferents in parasympathetic fibres
- Gastrocolic reflex leads to defecation soon after eating to clear way for new food
What is the intestino-intestinal inhibitory reflex?
- Loss of motility in response to sympathetic stimulation
- e.g. Acute distension due to obstruction, trauma or surgery
