Gastrointestinal

Gastrointestinal Tract

Relating to the stomach and the intestines.

Digestive Tract

The gastrointestinal tract and its accessory organs.

Functions of the GI Tract

Motility, digestion, absorption, and secretion.

Motility

Refers to the movement of the GI tract, serving two purposes: moving the contents from the mouth to the anus, and mixing the contents to facilitate digestion and absorption.

Digestion

The process of breaking down large particles of food and high-molecular weight substances into small molecules.

Absorption

Movement of the products of digestion across the intestinal epithelium into the body.

Secretion

Release of substances into the lumen of the GI tract which facilitate digestion, absorption, and motility.

GI Accessory Organs

Salivary glands, liver, gallbladder, and pancreas

Intestinal Epithelium

Barrier of cells that nutrients must cross to be absorbed into the body.

Circular and Longitudinal Muscle

Layers of smooth muscle that allow motile contractions of the intestine.

Myenteric Plexus

Lies between the circular and longitudinal muscle layers; rich nerve supply which controls multiple aspects of motility and secretion.

Endocrine Cells

Secrete hormones into the blood that regulate digestion and appetite.

Exocrine Cells

Secrete substances into the lumen that aid digestion.

Mucous Cells

Function as ducts from accessory organs.

Sphincters

Rings of muscle creating a constriction point that regulates flow through the tract and prevents backflow.

Upper esophageal, lower esophageal, pyloric, ileocecal, internal anal, and external anal

Sphincters in the GI Tract

Function of the Mouth

Functions in the mechanical breakdown of food using the teeth and facial muscles, prepares the food bolus for swallowing by moistening with saliva, and begins digestion using salivary amylase.

Swallowing

A reflexive response under control of the autonomic nervous system which is initiated by the voluntary collection of food into the pharynx by the tongue. This triggers involuntary contraction of the pharyngeal muscles to push the bolus into the esophagus where it is driven own by peristaltic contractions.

Function of the Stomach

Stores ingested material and continues digestion started in the mouth. Regulates emptying into the small intestine.

Rugae

Folded surface on the inside of the stomach that facilitates mechanical digestion.

Chemical Digestion in the Stomach

Hydrochloric acid denatures proteins to make them more soluble as well as cleaving pepsinogen into pepsin which breaks down proteins.

Chyme

Ingested food that is leaving the stomach into the small intestine.

Subsections of the Small Intestine

Duodenum, jejenum, and ileum

Small Intestine

Primary site for digestion and absorption; digestion is aided by the hydrolytic breakdown of dietary macromolecules and facilitates absorption into the body.

Large Intestine

Functions to store and concentrate undigested material prior to its excretion.

Subsections of the Large Intestine

Cecum/appendix, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum

Colon

Functions in the absorption of ions and water, and facilitates bacterial metabolism.

Salivary Glands

Parotid, sublingual, and submandibular; empty water and mucus to lubricate the food bolus and amylase to begin digestion into the oral cavity.

Liver

Secretes bile salts to facilitate fat digestion, bicarbonate to neutralize acidic chyme from the stomach, and organic waste products that need to be excreted.

Gallbladder

Stores and concentrates bile and releases it during feeding.

Exocrine Pancreas

Part of the digestive system; acinar cells secrete bicarbonate and digestive enzymes into the small intestine via the pancreatic duct.

Endocrine Pancreas

Part of the endocrine system; islets of Langerhans secrete hormones into the blood stream.

Pancreatic Amylase

Enzyme secreted by the pancreas which breaks down carbohydrates.

Trysin and Chemotrypsin

Enzymes secreted by the pancreas which break down proteins.

Pancreatic Lipase

Enzyme secreted by the pancreas which breaks down fats.

Peristalsis

A reflexive response triggered by the luminal contents stretching the intestinal wall; waves of contraction that pass along the gastrointestinal tract to move the contents downstream.

Steps of Peristalsis Initiation

Local stretch causes the release of serotonin that activates the myenteric plexus; upstream neurons release factors that cause smooth muscle contraction (acetylcholine); downstream neurons release factors that cause smooth muscle relaxation (nitric oxide).

Segmentation

Local contraction of muscle to separate the intestine into pockets and mix the contents.

Basic Electrical Rhythm (BER)

Oscillations in smooth muscle membrane potential which cause the rhythmic contractions of segmentation.

BER Modulators

Acetylcholine increases activity and stimulates contraction, epinephrine decreases activity and depresses contraction.

Migrating Myoelectric Complex

Sweeping wave of contraction that occurs after segmentation stops. Has 3 phases: quiescent (no activity), irregular, and burst (sweeping wave).

Lower Esophageal Sphincter

Tonically active but relaxes upon swallowing to allow entrance of the food bolus into the stomach. Contracts in response to acetylcholine, relaxes in response to NO and VIP.

upper part of the stomach.

Peristaltic waves initiate at the…

Extrinsic Factors Affecting Gastric Emptying

Stomach and intestinal contents, acidity, distension, and hypertonicity.

delay

High fat and protein meals ____ gastric emptying

inhibits

Exposure of the duodenum to acidity ____ gastric emptying.

increases

Distension of the stomach _____ peristaltic contractions.

Aerophagia

Air is unavoidably swallowed during eating and drinking, and the air is regurgitated.

Emesis

Vomitting; involuntary, forceful expulsion of the stomach contents via the mouth which is typically preceded by salivation and nausea.

Triggers of Vomitting

Digestive (gastroenteritis, bowel obstructions, allergies), sensory (motion sickness, viral infections, morning sickness, drug reactions), emetics, social cues, miscellaneous (nauseating sights or smells, anxiety)

Ileocecal Valve

Separates the ileum and colon; increases in colonic pressure squeeze it closed, and increases in ileal pressure push it open.

Gastroileal Reflex

Opening of the ileocecal valve in response to food leaving the stomach.

Mass Action Contractions

Unique to the colon; simultaneous local contraction that moves material through the colon to the rectum.

Internal Anal Sphincter

Under involuntary control and is composed of smooth muscle.

External Anal Sphincter

Under voluntary control and is composed of skeletal muscle.

Mechanoreceptors

Stimulated by changes in volume and effect smooth muscle.

Osmoreceptors

Stimulated by changes in osmolarity and effect exocrine glands.

Chemoreceptors

Stimulated by changes in acidity and nutrient compositions and effect exocrine glands.

Gastrin

Produced in the stomach and triggered by the presence of amino acids/peptides, stimulates acid secretion and motility in the stomach.

Cholecystokinin (CCK)

Produced in the small intestine and triggered by the presence of amino acids and fats, inhibits acid secretion and motility in the stomach, stimulates enzyme secretion from the pancreas, and stimulates contraction of the gallbladder.

Secretin

Produced in the small intestine and triggered by acid, inhibits acid secretion and motility in the stomach and stimulates bicarbonate secretion from the liver and pancreas.

Glucose-Dependent Insulinotropic Peptide (GIP)

Produced in the large intestine and triggered by the presence of glucose and fat, stimulates insulin secretion from the pancreas.

Saliva

Composed of water, electrolytes, mucus, enzymes, and immune modulators.

Cephalic Phase Sympathetic NS Response

Saliva becomes thicker, respiration is increased, norepinephrine predominates.

Cephalic Phase Parasympathetic NS Response

Saliva becomes watery, digestion is increased, acetylcholine predominates.

Gastric Secretions

Composed of mucus, hydrochloric acid, enzymes, and some hormones.

Surface Mucous Cells

Secrete mucus as part of the gastric secretions.

Parietal Cells

Secrete hydrochloric acid as part of the gastric secretions.

Enterochromaffin-Like Cells (ECL Cells)

Secrete histamine as part of the gastric secretions.

Chief Cells

Secrete pepsinogen as part of the gastric secretions.

Regulation of HCl Production

Mediated by cephalic and gastric phase responses; stimulated by gastrin, histamine, and acetylcholine, inhibited by somatostatin.

G-Cells

Secrete gastrin to stimulate production of secretions from the parietal, ECL, and chief cells.

Pancreatic Secretions

Composed of bicarbonate and enzymes.

Biliary Secretions

Composed of water, bile salts (solubilize water-insoluble fats), bilirubin (byproduct of hemoglobin breakdown), and fats (cholesterol, lecithin).

sphincter of Oddi

The gallbladder fills with bile from the liver when the ____ is closed, preventing secretion into the duodenum.

water and salt

The gallbladder absorbs _____ from the dilute bile from the liver to concentrate it.

Intestinal Secretions

Composed of water, electrolytes, mucus, and enzymes

Villi

____ are primarily absorptive.

Crypts

____ are primarily secretory.

osmosis

Secretion of water into the small intestine is primarily driven by _____.

Carbohydrates

Major source of energy; include monosaccharides (glucose, fructose), disaccharides (sucrose, lactose, maltose), and polysaccharides (starch, glycogen).

Proteins

Major source of energy and are a major source of building blocks for numerous biological functions.

Salivary Amylase

Starts the breakdown of amylose, delayed activation by gastric acid, and is a minor contributor to digestion.

Pancreatic Amylase

Soluble protein secreted into the lumen that cleaves polysaccharides into smaller fragments; almost all ingested polysaccharides are broken down by the time they pass into the jejenum.

Brush Border Enzymes

Found on the apical surface of the microvilli and are bound to the brush border of the intestinal epithelium. They are multiple enzymes specific to different disaccharides.

secondary active transport.

Absorption of glucose/galactose is facilitated by…

SLGT-1

Transports one molecule of glucose/galactose for every two sodium ions brought in.

GLUT2

Glucose transporter that brings glucose/galactose/fructose from inside the intestinal epithelium into the interstitial space.

GLUT5

Transports fructose into the intestinal epithelium via facilitated diffusion.

translocate to the apical membrane as well to aid in bringing sugars into the intestinal epithelium.

After the consumption of a sugar-rich meal, GLUT-2 can…

Micelle

Structures that contain the products of fat digestion and bile salts. As the free monomers are absorbed, they may release individual fatty acids to make more available for absorption.

simple diffusion across the cell membrane using the “flip-flop” process.

Uptake of fatty acids occurs by….

Chylomicron

Droplets of triglycerides enclosed by a membrane from the ER which are exocytosed.

Steps of Fat Digestion and Absorption

Emulsification, lipolysis, diffusion, chylomicron formation and secretion.

Endopeptidases

Cleave proteins at their interior peptide bonds, e.g. trypsin, chymotrypsin, elastase.

Exopeptidases

Cleave proteins at their n and c termini

Enterokinase

Tethered to the intestinal epithelium, which cleaves trypsinogen to trypsin.

PepT1

Peptide transporter protein which brings small peptides into the epithelial cells by secondary active transport.

secondary active transport.

Small peptides are brought into the epithelial cells by…

NHE

Apical sodium/hydrogen exchanger which maintains the proton gradient required to bring peptides into the epithelia.