Small Intestine Structure and Function Flashcards
Small Intestine Functions
Mix food + digestive enzymes, absorb nutrients and propel undigested molecules towards large intestine
3 segments of small intestine
Duodenum, jejunum and ileum. Not physiologically distinct. Have different absorbent and secretive properties
Layers in the small intestine
Mucosa, submucosa, muscularis and serosa
Structures unique to small intestine physiology
Villi (containing lacteals)
Small Intestine Mucosa Organisation
Kerckring Folds (circular folds in duodenum and jejunum small increase surface area), villi (contains arterioles, venule and lacteal. Intermediate increase surface area) and microvilli (digestive enzymes, largest surface area increase)
Villus Def
Functional unit of small intestine. Lamina propia layer in small intestine contains blood vessels (amino acids + sugars) and lacteals (fat).
Cells in Small Intestine
STEM cells (in crypt), enterocyte (absorption), goblet (mucus production), enteroendocrine (hormone and peptide production), Paneth (immune) and M cells (antigen transport to immune cells)
Brunners Gland Outline
Duodenum only, submucosa linking to mucosa via ducts. Produce alkaline mucus. Neutralises acid, provides optimal digestive enzyme pH, fluid medium from diffusion and wall lubrication
Peyer Patches Outline
Mainly in ileum. Round lymphoids in mucosa and submucosa. Contain B and T lymphocytes. Covered by M cells
Small Intestine M cell Function
Transfer of antigens to immune cells in peyer patches
S Cell Outline
Mucus in duodenum. Secretin produced in response to acid, producing bicarbonate and inhibiting gastric acid
D Cells Outline
Found in mucosa throughout small intestine. Produce somatostatin in response to acid inhibit gastric acid and prevent pepsinogen production
I Cells Outline
Found in mucosa in duodenum/jejenum. Stimulated by lipid produces cholecystokinin. Pancreatic enzyme secretion
Main Function of cells in small intestine mucosa
Neutralise stomach contents and recruiting accessory digestive organs (eg pancreas and liver)
Duodenal Cluster Unit
Organs contributing to substance entering small intestine. Stomach (chyme), duodenum (Brunners gland), biliary tract (bile) and gall bladder (pancreatic juice)
Factors contributing to composition of substance in small intestine
Food volume, nutrient type and amount, enteroendocrine hormones and parasympathetic and sympathetic innervation
Pancreas Endocrine vs Exocrine
Endocrine (islets of Langerman) = metabolism (blood sugar regulation). Exocrine (acinar cells) = digestion (digestive enzymes). Ducts secrete H2O and Bicarbonate
Pancreatic Duct cells mode of action
Acid in duodenal lumen triggers secretin release. Secretin stimulates pancreatic duct cells producing NaHCO3. This decreases acid (self regulating)
Pancreatic Acinar Cells Mode of Action
Fat and protein in duodenal lumen. Increased cholecystokine secretion stimulating acinar cells. Acinar cells release digestive enzymes (eg lipase and pepsin)
Proteases Outline and Examples
Proteases are released as zymogens (inactive precursors). Eg trypsin, chymotrypsin and carboxypeptidase
Trypsin Mode of Action
Secreted as trypsinogen (inactive). Cleaved by enteropeptidase on microvilli. Trypsin cleaves trypsinogen, chymotripsinogen and procarboxypeptidase
Pancreatic Lipase Mode of Action
Digests triglycerides into monoglycerides and 2 fatty acids
Amylase Mode of Action
Hydrolyses starch to maltose
Bile Outline
Synthesised by hepatocytes. Bile acids, bile pigments, cholesterol, phospholipids and bicarbonate. Stored in gall bladder. Facilitates fat digestion and absorption in small intestine
How bile is released in gall bladder
I cells release cholecystokine in response to lipid in lumen. Gallbladder constriction and Oddi sphincter relaxation
How bile acids digest lipid
Bile acid acts as detergent (solubilises and emulsified lipid). Enables fat droplets to form micelles (hydrophobic component separating lipid from water nuetralises surface tension, oil droplets forming micelles). Pancreatic amylase enters micelles and digest tri/monoglycerides and fatty acids
2 types of small intestine motility
segmentation (circular) and peristalsis (circular and longitudinal)
Relationship between motility and nervous system
parasympathetic = increased motility
Major Migrating Complex Outline
Contractions push indigestible materials towards colon from small intestine. Stimulated by motilin
Enteroendocrine M Cells Outline
In response to high pH motilin is produced. Increases stomach contractions stimulating Major Migration Complex
Jejunum and Ileum Functions
complete digestion and absorption of molecules
Location of amylase
Small intestine lumen
Location of lactase, sucrase and maltase
Brush boarder (small intestine)
Carbohydrate transporters on apical membrane
SGLT1 (glucose + galactose) and GLUT5 (fructose). Facilitated diffusion
Carbohydrate transporters on basolateral membranes
Glut2 (monosaccharides). Facilitated diffusion
Oligopeptides digestion and absorption
Digested by brush boarders (carboxypeptoidase, aminopeptidase and dipeptidase). Amino acids and dipeptidases abslorbed through apical membrane (further digested in membranes where tri/di peptidases broken down to free amino acids). Diffusion of amino acids in blood
Fat digestion
Mainly in duodenum. Triglycerides, fat soluble vits and cholesterol are emulsified and solubilised into micelles by bile acids. Triglycerides in micelles are broken down into monoglycerides and fatty acids by lipase. Micelles diffuse actrioss epithelium and release fatty acids and monoglycerides inside cells
Fat Absorption Outline
Fatty acids and monoglycerides in epithelial cell are stored in endoplasmic recticulum where they resynthesise triglycerides. Triglycerides moved to Golgi apparatus and formed chylomicrons. Chylomicrons excreted on basolateral side and are absorbed by lacteals
Water Absorption Outline
Na/K pumps push Na out of cell onto baslateral side. SGLT 1 (Na/gulucose cotransporter) pumps Na into cell to counterbalance (GLUT2 transporter removes excess glucose and K+ pump removes excess K+). Cl- diffuses through tight junction to basolateral side. Na+ and Cl- combine to form NaCl. Water moves by osmosis from lumen across epithelial (selectively permeable) membrane to area of high solute (NaCl) conc
Ileum Outline
Similar to jejunum but with thinner walls, fewer blood vessels, les prominent circular folds and more peyers patches. Remaining fatty acids, fat soluble nutrients, cholesterol, sugars and amino acids are absorbed.
Site of bile acid reabsorption
Terminal ileum. 90% recirculated back to liver, 10% progress to colon.
Coeliac Disease Outline
Autoimmune response to dietary gluten. Causes atrophy of villi resulting in malabsorption of nutrients. Symptoms: diahorrea, weight loss, fatigue and anaemia. Avoidance of gluten is only solution
Lactose Intolerance Outline
Deficiency of lactase resulting in body inability to matabolise lactose. Lactose then metabolised by bacteria in colon. Results in bloating, cramps and diahorrea. Lactase product (eg lactaid) can be taken with meal to avoid symtoms
Crohn’s Disease Outline
Chroic severe inflammtion in GIT due to genetics/environmental/ immunological factors. Treatment anti-inflammatory drugs or surgery
Irritable Bowel Syndrome outline
Small intestinal bacteria overgrowth (bacteria from colon migrate to small intestine). Results in cramps, bloating and diahorrea. Diagnosed bt aspiration of 10^5 bacteria/ml and halogen breath tests
Drug absorption factors (drug)
molecular weight, particle size, pKa, lipo/hydrophilicity, physical + chamical structures, formulation and concentration
Drug absorption factors (body)
Surface area, vascularity, pH, food vol, gastric emptying rate, disease and secreted substances
