Digestive system part # 2 Flashcards
(8 cards)
Esophagus -
Stratified squamous, subject to abrasion. A muscular tube that extends from laryngopharynx to the esophageal hiatus which pierces the diagphram.
Activities during eating and swallowing : no secretion of enzymes or absorption, secretion of mucus for ease of deglutition, 2 sphincters that are normally closed, open, propulsion via peritalisis which involves alternating contraction and relaxation of circular and longitudinal muscle present, the muscularis externa transitions from skeletal to a mixture of skeletal and smooth muscle.
2 Sphincters - (both composed of smooth muscle) the upper esophageal spincter is open for the pharnygeal stage of deglutition and the gastroesophageal ( aka cardiac or lower esophageal) spincter is open for the esophageal stage of deglutition.
GERD - gastroesophageal reflux disease, first symptom is “heartburn”. The gastroesophageal spincter dosen’t close properly and acid will move up into the esophagus. A Inflamation of the spincter or hiatal hernias ( stomach protruding above the diaphragm) can be the cause.
Stomach -
Located inferior to the diagram, and is large enough to occupy the left hypocondriac, epigastric, and umbilical regions.
Substructures in the stomach: Cardia - first reigon located just below the heart. Fundus - rounded holding chamber on the left. Body - large central region. Pyloric part - funnel shaped region that had a broad top known as the pyloric antrum and a narrow pyloric canal which ends with the pylorus. Pyloric sphincter - ring of smooth muscle that controls the movement of digested food into the duodenum. Rugae - “wrinkles”, folds that form to allow for expansion of the stomach. Lesser curvature - small “c” shape that forms with the medial curve of the stomach. Greater curvature - larger, more lateral curve that forms.
Histology : Epithelium - simple columnar cells that form gastric pits that contain gastric glands. Secretory cells found in the gastric glands : mucous neck cells - secrete acidic mucus. Chief (zymogenic) cells - secrete pepsinogen and gastric lipase. Parietal cells - secrete HCL and intrinsic factors which is needed for B12 absorption. Enteroendocrine cells - secrete the hormones : gastrin (G cells) and somatostatin (GHIH). Muscularis externa of stomach has 3 layers - longitudinal, circular, and oblique.
Mechanical and Chemical Digestion in the Stomach -
Peristalsis results in waves that mix the food with the gastric juices. Chyme is the thick mixture that results. Salivary amylase and lingual lipase continue to hydrolyeze expecially in the fundus where little mixing occurs.
HCL - hydrochloric acid, denatures proteins, activates pepsinogen and stimulates gastrin.
Pepsin - a protease ( a type of enzyme where protein is its substrate. Its most active at pH 2. Once activated, it converts pepsinogen to pepsin.
Gastric lipase - breaks down short and medium chain fatty acids (lipids)
Gastrin - (hormone) stimulates the secretion of gastric juice, specifically HCL and pepsinogen secretions.
Reasons why the stomach doesn’t digest itself -
Pepsinogen is present is a inactive zymogenic form, there is alkaline layer that counteracts the acid produced, the alkaline layer is also rich in mucus 1-3 mm, food in the stomach dilutes the acid.
Gastric ulcers - lesions in the stomach previously thought, by excessive acid production, low mucus and stress. Bacterium, helicobaerpylori, has clearly been implicated.
Regulation of Gastric Secretion and Motility -
Both neural and hormonal controls play a role in the regulating secretions and contractions.
3 phases - Cephalic phase ( short - a few minutes) - stimulatory events: stomach is getting ready, initiated in the head by the thought, smell or sight of food. Cerebral cortex and feeding center in the hypothalamus signal the medulla oblongata. the medulla oblongata sends signals to preganglionic nreve fibers of Vagus nerve which are in turn transmitted to postganalionic nerve fibers of the submucosal plexus (all parasympathetic stimulation) the submucosal plexus stimulates the production of gastric juice. Inhibitory events include: a decrease in parasympathetic activity, stress,depression or lack of appetite.
Gastric phase: 3 to 4 hours (stomach working) stimulatory events, presence of food in the stomach results in activation of stretch receptors and chemoreceptors that detect an increase in pH. These changes signal the submucosal plexus to activate smooth muscle contractions which result in waves of peristalsis and continued secretions. Food (especially caffeine and protein) in the stomach stimulates the release of gastrin (hormonal control). Inhibitory events: a decrease in pH, less strech in the stomach and sympathetic stimulation (neural control).
Intestinal phase:4 -6 more hours (stomach emptying), chemoreceptors are activated by the presence of fatty acids (low pH) and carbohydrates in the duodenum. Stimulatory events - intestinal (enteric) gastrin released by duodenum act similar to gastrin stimulating gastric secretion and motility.
Inhibitory events: stretching of the duodenum and the presence of partially digested food in the chyme(detected by an alkaline pH) results in less gastric motility and prevents movement into the duodenum.
The intestinal phase changes are brought about by the release of 4 hormones by the duodenal enteroendocrine cells of the small intestine:
gastric inhibitory peptide (GIP) ( aka glucose-dependent insulintropic peptide) - inhibits the secretion of HCI during gastric phase. Also insuliotropic because it stimulates the release of insulin when glucose is present.
cholecystokinin (CCK) - inhibits the stomach from emptying by decreasing gastric motility.
secretin - inhibits gastric secreations and gastric motility during gastric phase. Both CCK and secretin have effects on the pancreas and gallbladder.
VIP - vasoactive intestional peptide, stimulates buffer production in the pancreas and inhibits HCI secreation in the stomach.
Overall Regulation of Gastric emptying - the stomach usually takes about 4 hours to empty, six or more hours for a fatty meal. The level of strech (distention) of the stomach affects emptying, more stretch will stimulate more emptying.Increased parasympathetic stimulation will stimulate emptying, increased sympathetic stimulation will inhibit emptying. Both types of gastrin are major regulatory hormones because they increase gastric motility and relax the pyloric sphincter. Enterogastric reflex - neural reflexes that inhibit gastric emptying. Too much stretch in the duodenum (entero-) inhibits the stomach (gastric) from emptying. This reflex includes the release of hormones (GIP, CCK and secretin), the inhibition of local reflexes and the inhibition of vagal (parasympathetic) reflexes.
Pancreas -
located retroperitoneal and tucked into the duodenum. Shaped like a fish and is divided into head,body and tail.
Main pancreatic duct - large major duct that extends across the entire pancreas.
Hepatopancreatic ampulla - pouch that forms when the pancreatic duct and bile duct merge.
Accessory pancreatic duct - small duct that branches from the main pancreatic duct.
Histology of pancreas : Acini - specilized cells of the pancreas that secrete enzymes for digestion, exocrine part of gland. Pancreatic juice is alkaline pH from 7.1 - 8.2 due to presence of sodium bicarbonate. Sodium bicarbonate - buffers the gastric juice acidity, inactivates pepsin, and creates an optimal pH for pancreatic enzymes that are secreted by acini cells.
Pancreatic enzymes: Pancreatic amylase - hydrolyzes starch to smaller glucose chains. Trypsin - a protease that cleaves at a specific type of amino acids. Chymotrypsin - a protease that hydrolyzes from carboxyterminus of protein. Elastase - a protease that cleaves at specific amino acids. Pancreatic lipase - hydrolyzes triglycerides. Ribonuclease and Deoxyribonuclease - aka RN and DN - ase, degrade nucleic acids.
All the proteases are initially a inactive zymogen that forms ( tripsinogen, chymotrypsinogen,procaboxypeptidase,and proelastase).
Enteropeptidase (aka enterokinase) is an enzyme secreted by brush borders of the small intestine which activates trypsin. Substrate for enteropeotidase is trypsinogen (protein). Product is trypsin!
tripsinogen—–enteropeptidase—–trypsin
Trypsin goes on to actvate other zymogens…..
chymotrypsinogen—trypsin—chymotrypsin
procarboxypeptidase—trypsin—–carboxypeptidase
proelatase—-trypsin—–elastase
Regulation of pancreatic secreations: parasympathetic impulse via the vagus nerve, stimulate pancreatic secreations. CCK stimulates release of pancreatic enzymes in bile. secretin stimulates the release of bicarbonate and bile.
Liver -
Second largest organ next to the skin, located in right hypochondraic and epigastric regions. seperated into 2 lobes by the falciform ligament ( a fold in thr peritoneum) lobes further divide into lobules that are seperated from one another by a network of veins with a central vein in the center that feeds into the hepatic vein.
Sinusoids - thin walled capillaries lined with stellate macrophages.
Hepatocytes - specialized epithelial cells that secrete bile into bile canaliculi which empty bile into the bile ducts.
right and left hepatic ducts - the bile ducts on either side of the liver feed into main hepatic ducts which merge to form the common hepatic duct.
Common hepatic and cystic duct (form gallbladder) merge to form bile duct.
Bile -
Yellowish - brown to olive in color, pH 7.6 - 8.6
consists of water, bile salts, bile acids, cholesterol, lecithin and bile pigment that come from the break down of hemoglobin.
Regulation of bile secretion : 1. Vagus nerve stimulates increases bile production. 2. fatty acids and amino acids in duodenum increase secretion of CCK and acidity of chyme stimulates release of secretin. 3. CCK stimulates contraction of gallbladder, relaxes the hepatopancreatic sphincter. 4. secretin increases the flow of bile from the liver.
