17.4 Digestive system Flashcards
GI epithelium
innermost lining of lumen
EPITHELIAL CELLS (same as outer surface of the body and inner surface of respiratory tract)
TIGHT JUNCTIONS = impermeable
skeletal muscle and cardiac muscles are STRIATED (striped)
GI muscle is smooth
GI HAS functional syncytium
GI nerve impulses spread to neighboring cells
It’s own nervous system
two networks of neurons: MYENTERIC PLEXUS and SUBMUCOSAL plexus
MYENTERIC found between circular and longitudinal muscle layers, regulates GUT MOTILITY
SUBMUCOSAL regulates enzyme secretion, gut blood flow, and ion/water balance in the lumen (SPARSE in anus and esophagus)
EXOCRINE ORGANS
release of enzymes from LIVER, GALLBLADDER, and PANCREAS
also GASTRIC GLANDS of stomach are EXOCRINE - secrete pepsinogen (protease zymogen) and acid
MUSCUS-SECRETION from GOBLET cells (all over the GI tract) - produces mucous membrane
SECRETION OF WATER (GALLONS) are secreted and reabsorbed in the small intension or colon
salivary amylase = hydrolyzes starch
also called PTYALIN
saliva also contains LINGUAL LIPASE for fat digestion
NO DIGESTION OF PROTEINS IN MOUTH
Lysozyme = breaks down bacterial cell walls (innate immunity)
epiglottis
an cartilaginous flap that blocks water/food from trachea
lower esophageal sphincter
at end of esophagus, PREVENTS reflux
Stomach
pH = 2
- HCl is secreted by parietal cells, located in the gastric mucosa (“Acidic parents”)
- HCl converts of pepsinogen to pepsin (BREAKS PROTEINS DOWN TO AAs)
- proenzyme (INACTIVE ZYMOGEN - activated by CLEAVAGE/PROTEOLYSIS at a specific site) is pepsinogen, which is released by the CHIEF cells of stomach wall; activated by HCl
- pepsinogen unfolds in low acidity, and cleaves itself in AUTOCATALYTIC fashion (pepsin cleaves pepsinogen, removing 44 amino acids)
- HCl hydrolyzes some polysaccharides
- HCl hydrolyzes polypeptides into FRAGMENTS
chyme
food with gastric secretion
cholecystokinin (stomach emptying)
secreted by epithelial cells of the duodenum, which inhibits the opening of the pyloric sphincter (more food in duodenum) inhibits stomach emptying
Cholecystokinin releases digestive enzymes from pancreas and gallbladder
a hunger suppressant
small intenstine
duodenum, JEJUNUM, ILEUM
10 feet long, 25 feet long when dead
Peyer’s patches
immune system
lymphocytes dotting the villi, confer immunity to gut pathogens
ILEUM
Also known as aggregated lymphoid nodules
Pancreas
digestive enzymes and BICARB exits by PANCREATIC DUCT
shares the SPHINCTER OF ODDI with common bile duct
bile acid sequestrants BIND TO BILE ACIDS and are secreted into the DUODENUM as FECES
drugs which bind acids in the small intestine, causing them to remain in GI lumen and excreted as feces
Duodenal enzymes
- duodenal enterokinase (enteropeptidase) activates the pancreatic zymogen TRYPSINOGEN to TRIPSIN
- brush border enzymes - hydrolyze the smallest carbs and proteins (disaccharides and dipeptides) into monosaccharides and amino acids
Duodenal hormones
- Cholecystokinin (CCK) - response to FATS, prevents stomach emptying, stimulates gallbladder contraction (bile release), decreases gastric motility
- Secretin - response to ACID, releases HCO3- in water, neutralizing the pH from stomach HCl. Duodenal pH must be kept NEUTRAL or slightly basic for pancreastic enzymes to function
- Enterogastrone - decreases stomach emptying
cecum
first part of large intestine
jejunum and ileum
substances not absorbed in duodenum must be absorbed here
LOWER small intestine (ILEUM) absorbs B12 (only when complexed with INTRINSIC FACTOR, a glycoprotein secreted by parietal cells of stomach)
Ileum leads to cecum, divided by the ILEOCECAL VALVE
Colon (LARGE INTESTINE)
3-4 feet long
IMPORTANT for absorbing water and minerals, stores feces
CECUM is first part
appendix is finger-like appendage (lymphatic tissue)
Rectum is last part
Internal anal sphincter
smooth muscle, under autonomic control
external anal sphincter, SKELETAL muscle
SAME AS URINARY SPHINCTERS
Vitamin K
essential for blood clotting
gut bacteria supply vitamin K
Pancreas (EXOCRINE)
- amylase - polysaccharides -> disaccharides
- lipase - hydrolyze triglycerides at micelle
- Nuclease -> hydrolyze DNA and RNA
- Proteases -> hydrolyzing polypeptides into di/tripeptides (released as ZYMOGENS)
- Enzymes activated by trypsin (see below)
enzymes activated by Trypsin
- Chymotrypsinogen (active: chymotrypsin)
- Procarboxypeptidase (carboxypeptidase)
- Procollagenase (collagenase)
THESE BREAK DOWN PROTEINS TO DIPEPTIDES AND TRIPEPTIDES
Pancreas (ENDOCRINE)
ISLETS OF LANGERHANS has 3 types of cells
- Alpha - glucagon - low blood sugar - stimulates liver to hydrolyze glycogen and release glucose, also stimulates adipocytes to release fats in the blood (INCREASE GLUCOSE AND FAT RELEASE) - catabolic
- Beta - secrete INSULIN (removes glucose into storage as glycogen and fat) - anabolic
- delta - secretes SOMATOSTATIN - INHIBITS DIGESTION
raising blood glucose
Glucagon, epinephrine, cortisol
Liver
exocrine: RELEASE BILE (1 liter a day)
bile is synthesized from cholesterol
bile contains bile acids (BILE SALTS in deprotonated form), cholesterol, bilirubin (RBC breakdown)
EMULSIFIES large fat particles in duodenum, creating MICELLES, reducing size and increasing surface area, making them better targets of hydrophilic LIPASES
Bile helps fat particles diffuse across the intestinal mucosal membrane
Bile is secreted into the duodenum or stored in gallbladder.
Gallstone = large crystal formed from bile
Gallbladder
NO SECRETORY ABILITIES
NOT ESSENTIAL
controlled by nervous and endocrine system
CCK (released by duodenal cells) and parasympathetic NS stimulate contraction of gallbladder wall
Liver (2)
receives OXYGENATED blood fo hepatic arteries
receives venous blood draining from stomach and intestines through HEPATIC PORTAL VEIN
HEPATOCYTES absorb nutrients
GLUCONEOGENESIS - when blood glucose is low, breaks glycogen down and GNG
STORES GLUCOSE AS GLYCOGEN (as well as skeletal muscles), only liver can release glucose to bloodstream
glycogenolysis
the product is glucose-6-phosphate, which must be dephosphorylated by GLUCOSE-6-PHOSPHATASE, which then can be released into blood
Liver (3)
waste products of protein catabolism regulated here
NH3 (ammonia) are byproducts of amino acid breakdown during starvation, converted into UREA. this circulates the blood and excreted by kidney in urine
Liver lipid metabolism
chylomicrons degraded by lipases into triglycerides, glycerol, and cholesterol-rich chylomicron remnants
the remnants are taken up by hepatocytes and combined with proteins to make lipoproteins (HDL, LDL, VLDL) which enter the blood and are source of cholesterol and triglycerides
PLASMA PROTEINS (albumin, globulin, fibronogens, CLOTTING factors) are made in liver and secreted into blood
albumin prevents fluid from leaving bloodstream (OTHERWISE, EDEMA)
Liver (detox)
reduces toxicity
smooth ER in hepatocytes contains enzyme pathways to break down drugs and toxins
appetite
ghrelin - GRR I’m HUNGRY
leptin (released by white adipose tissue) - adipostat - maintains stable lipid content, released in response to increased triglyceride levels
Peptide YY - reduces appetite
impact the arcuate nucleus of the hypothalamus
uptake of monosaccharides
actively transported by intestinal epithelial cells
apical SYMPORT transports 1 sugar INTO the cell while allowing sodium to flow IN (the Na/K ATPases are constantly pumping Na+ OUT, keeping INTRACELLULAR SODIUM very LOW)
Facilitated diffusion (Uniport) allows high concentration monosaccharides to flow into nearby capillaries
uptake of peptides
SYMPORT specific to each amino acid uptakes AA and Sodium into INTESTINAL EPITHELIAL cell, and uniporter facilitates movement out into the INTERSTITIUM
AAs end up in liver, where it’s catabolized for energy or synthesis (anabolism)
fat digestion (tri -> mono -> tri -> mono -> tri)
organized into huge hydrophobic droplets
CCK stimulates gallbladder contraction, in which BILE acids emulsify lipids = micelle
pancreatic lipase hydrolyzes triglycerides into monoglycerides and free fatty acids, which are SMELL enough to move into EPITHELIAL cells by simple diffusion (they are SMALL and HYDROPHOBIC).
they are converted back to triglycerides and packages into CHYLOMICRONS (fat + protein) which enter LACTEALS, eventually emptying into thoracic duct, emptying into large vein near heart
The chylomicrons CIRCULATE and removed by adipose and liver tissues which contain LIPOPROTEIN LIPASE, hydrolyzing chylomicron triglycerides into monoglycerides and free fatty acid
these diffuse into adipocytes and liver cells, remade into triglycerides
Vitamins
FAT-SOLUBLE or WATER-SOLUBLE
FS requires bile acids for solubilization and absorption
FS vitamins stored in adipose tissue
Excess WS excreted in urine by kidneys
