Exam II - GI System

Digestion, absorption of nutrients and elimination of waste

the three functions of the GI system.

sublingual, submandibular, parotid, parasympathetic

Digestions begin in the mouth, via secretions from (these three glands) under stimulation from the _______ nervous system.

Oral phase

phase of swallowing that is initiated when the tongue forces a bolus of food back towards the pharynx. Stimulation of somatosensory receptors found in the pharynx then activated the involuntary swallowing phase.

Pharyngeal phase

phase of swallowing where food is propelled from the pharynx to the esophagus. The soft palate is pulled upwards, the epiglottis covers the larynx, the esophageal sphincter relaxes, and peristaltic contraction moves the food through the open sphincter. Breathing cannot occurring during this phase because the larynx is sealed shut.

Esophageal phase

phase of swallowing where food is propelled from the esophagus to the stomach. The contents pass through the upper esophageal sphincter via peristalsis at the level of the diaphragm (T10) which then closes to prevent acid reflux.

Stomach

functions to mix ingested food with acidic fluids that partially sterilize food and denature proteins to reduce their biological activity.

Chyme

the very acidic mixture that leaves the stomach at the pyloric sphincter after about 2 hours of digestion.

Parietal cells

cells of the stomach that secrete hydrochloric acid and intrinsic factor.

H2 receptors

histamine receptors which bind histamine released from enterochromaffin like cells of the gastric mucosa. Binding of histamine to these receptors stimulates the release of hydrogen ions by parietal cells.

H2 antagonists

antacids that block histamine receptors preventing the release of hydrochloric acid by parietal cells.

Vagus nerve (CN 10)

the stomach receives stimulatory autonomic innervation from this cranial nerve which stimulates the release of hydrochloric acid from parietal cells.

Intrinsic factor

enzyme secreted by parietal cells that is required for the absorption of water soluble vitamin B12. Failure to absorb B12 prevents proper hemoglobin formation and leads to pernicious anemia.

G cells

cells found in the pyloric antrum of the stomach that secrete gastrin, which functions to stimulate the secretion of hydrochloric acid by parietal cells. These cells are also found in the duodenum and pancreas.

Gastrin

substance secreted by G cells of the stomach that functions to stimulate the secretion of hydrochloric acid by parietal cells, as well as aiding in gastric motility.

Cholecystokinin B receptors

receptors on parietal cells where gastrin binds. This causes the insertion of K+/H+ ATPase pumps into the apical membrane of parietal cells causing an increase in hydrogen ions to be released into the stomach cavity against its concentration gradient.

K+/H+ ATPase

To maintain a chyme pH of about 1 requires two ______ pumps are required. This is why the production of stomach acid is a very energy intensive process.

Proton pump inhibitors

antacids that block proton pumps found on the membrane of parietal cells preventing the secretion of hydrochloric acid.

Chief cells

cells of the stomach that secrete pepsinogen.

Pepsinogen

substance secreted by chief cells of the stomach that is further converted to pepsin when the stomach pH drops due to hydrochloric acid secretion by parietal cells. It functions in the breakdown of proteins (protease) and lipids (lipase).

NSAIDS

drugs that inhibit the production of mucus by foveolar cells of the stomach causing an increased risk of gastric irritation. A proton pump inhibitor can be co-administered to counteract this negative side effect.

Foveolar cells

cells of the stomach that produce mucus. Are targeted by NSAIDs, resulting in decreased mucus and stomach discomfort or even ulcer formation.

Incretin

gastrointestinal hormones that promote the secretion of insulin by pancreatic beta cells. The reason why oral glucose stimulates a faster insulin response than IV glucose. These hormones can be stimulated to increase insulin production in those patients having type 2 diabetes. Ie) GIP and GLP1

Glucagon like peptide 1 (GLP 1)

incretin hormone which stimulates insulin secretion, but also slows gastric emptying leading to a sense of satiety. Additionally, it inhibits glucagon secretion preventing further glucose formation.

DPP4

a soluble or membrane bound enzyme that degrades a wide array of peptides, but also destroys incretins. Therefore, the inhibition of this enzyme can be used to increase insulin production in those having type 2 diabetes.

Duodenum

the first part of the small intestine

First part

part of the duodenum most vulnerable to ulceration due to the fact that chyme has not yet been neutralized by secretions of the exocrine pancreas released at the ampulla of Vater.

Ampulla of Vater

common opening that allows for secretions from the liver, gallbladder, and exocrine pancreas to enter the duodenum. Here chyme is neutralized, its pH rising.

Sphincter of Oddi

sphincter which controls the opening and closing of the ampulla of Vater found at the duodenum.

S cells

cells of the duodenum that release secretin in response to acid entering the first part of the duodenum.

Secretin

peptide released by s cells in response to acid entering the duodenum. It functions to open the sphincter of Oddi, as well as to promote secretion of bicarbonate by the pancreas in order to neutralize chyme.

I cells

cells of the duodenal and jejunal mucosa that release cholecystokinin into the bloodstream in response to fat containing chyme entering the first part of the duodenum.

Cholecystokinin (CCK)

peptide that is released by I cells into the bloodstream in response to fat containing chyme entering the first part of the duodenum. It functions to trigger gallbladder contraction and the release of bile produced by the liver into the second part of the duodenum. It also stimulates the release of the pancreatic enzyme packet.

Pancreatic peptide hormone

hormone secreted by F cells in the Islets of Langerhans that can inhibit the actions of cholecystokinin in the exocrine pancreas.

Bile

conjugated and excreted by products of red blood cell breakdown produced by the liver that assists in the digestion of fats.

Jejunum

the second part of the small intestine

Ileum

the third part of the small intestine

Cecum

the first part of the large intestine

Appendix

organ found at the cecum of the large intestine. Its function is still not known, but can be used in reconstructing the wall of the bladder or ureter in surgery.

Parasympathetic innervation

branch of innervation of that stimulates GI motility and secretion

Vagus nerve (CN 10)

nerve that provides parasympathetic innervation to the upper gastrointestinal tract up until the left colic flexure.

Splanchnic nerves

nerves that provide sympathetic innervation via sacral outflow to the lower gastrointestinal tract past the left colic flexure.

Sympathetic innervation

branch of innervation that inhibits GI motility and secretion

Celiac, superior mesenteric, inferior mesenteric, and hypogastric

four sympathetic ganglia that innervate the GI system.

Enteric nervous system

intrinsic innervation of the gastrointestinal tract which can direct all functions, even in the absence of extrinsic innervation.

Valsalva maneuver

exhaling against a closed airway (ie during coughing, defecation, or heavy lifting) causes an increase in intrathoracic pressure and increases venous pressure in and above the heart. This can lead to a rupture in blood vessels around the macula releasing blood into the space between the ILM and the posterior vitreous face (pre-retinal hemorrhage).

monosaccharides

Only ______ (glucose, galactose, fructose) can be absorbed by the intestinal epithelial cells. Therefore, all ingested carbohydrates must be broken down.

Amylase

enzyme that digests starches into disaccharides

SGLT1

secondary active symport that moves glucose and galactose from the intestinal lumen into the cell. The sugar is taken in (against its electrochemical gradient) along with sodium (along its electrochemical gradient established by Na K ATPase) in the same direction.

Fructose

monosaccharide that is transported across the apical and basolateral membranes of intestinal cells via facilitated diffusion. At the apical surface it is transported by GLUT 5, and at the basal surface it is transported by GLUT 2.

GLUT 5

fructose specific uniport that transports fructose via facilitated diffusion from the intestinal lumen into the intestinal epithelial cell cytoplasm.

GLUT 2

uniport that transports all monosaccharides from the intestinal cell cytoplasm into the blood stream via facilitated diffusion.

Pepsin

Digestion of proteins begins with the activity of this enzyme found in the stomach. Proteins are coagulated by the acidity of the stomach which inactivates their intrinsic activity.

Enterokinase

enzyme that converts trypsinogen into its active form, trypsin

Trypsin

enzyme found in the brush border of the small intestinal wall that cleaves pro-enzymes of peptidases and enterokinases which go one to cleave proteins into final produces of tripeptides and dipeptides which can be absorbed.

acidity

The symports found in small intestinal cell membranes used in the transport of proteins depend on the protein's....

Lingual and gastric lipases

initiate lipid digestion by hydrolyzing the ingested fats.

Pancreatic lipase, cholesterol ester hydrolase, phospholipase A2

three lipases found in the small intestine that function to further break down lipids

Bile salts

are secreted into the lumen of the small intestine by the gallbladder. It functions to surround and emulsify dietary lipids into micelle structures. This creates further surface area for lipid breakdown via lipases and also allows for diffusion across the epithelial cell membrane.

Monoglycerides, fatty acid, cholesterol

three final products of lipid digestion which can go on to be absorbed

Chylomicron

complex formed when lipids are absorbed and packaged within apolipoproteins. These structures leave the intestinal cells via exocytosis, are taken up by central lacteals into the lymphatic system, and are transported to the thoracic duct for emptying into the bloodstream.

Lacteal

lymphatic capillaries found within the small intestine that absorb lipid chylomicrons.

Thoracic duct

point in the lymphatic system where lipid chylomicrons are emptied into the bloodstream.

Left jugular vein

vein where lipid chylomicrons are emptied into the bloodstream

Fat soluble vitamins

vitamins that are absorbed like lipids

Water soluble vitamins

vitamins that are absorbed like peptides and monosaccharides (via sodium co-transporters

Vitamin D

is required for the absorption of calcium.

Iron

is absorbed either as free molecules or as bound to hemoglobin or myoglobin. It is shipped to the bone marrow where it aids in red blood cell production.

60%

percent of dry mass of feces that is made up of bacteria

MALT

specialized immune mechanism found in all mucosal surfaces of the body that functions in surface tolerance, memory, and communication. Exposure to an antigen on one surface of the GI tract will confer future tolerance to that particular antigen in ALL AREAS of all other mucosal surfaces of the body (including conjunctiva)

Peyer's patches

packets of immunocompetent cells found below mucosal surfaces, including the small intestine. They trap foreign particles and trigger immune responses.

Microfold cells (M cells)

cells found in the epithelium overlying Peyer's patches. They sample antigens directly from the intestinal lumen and transport them to APCs, dendritic cells, and macrophages found within Peyer's patches. Can be exploited by microorganisms for entry into the intestinal epithelium.