Gastrointestinal Physiology Flashcards
What are the four functions of the GI tract?
- Motility.
- Digestion.
- Absorption.
- Secretion.
What are the two purposes of motility in the GI tract?
- Movement of contents from mouth to anus.
2. Mixing of contents to facilitate digestion and absorption.
What is digestion?
Process of breaking down large particles of food and high-molecular-weight substances into smaller molecules.
What is absorption?
Movement of products of digestion across the intestinal epithelium into the body.
Where does absorption mostly occur?
In the proximal duodenum.
What is secretion?
Release of substances into the lumen of the GI tract, which facilitates digestion, absorption and motility.
What are the accessory organs?
The salivary glands, liver, pancreas, and gallbladder.
What is the epithelium?
The barrier of cells that nutrients must traverse to be absorbed into the body.
What are the functions of the capillaries and lymphatics in the submucosa layer?
Aiding in transport to and from the intestine.
What two layers make up the smooth muscle of the GI tract?
Circular and longitudinal layers.
What is the function of the myenteric plexus?
Control multiple (layer of nerves) aspects of motility and secretion
What is the function of the endocrine cells of the GI lumen?
Secrete hormones into blood that regulate digestion and appetite.
What is the function of exocrine cells in the GI lumen?
secrete substances into the lumen that aid digestion.
What are the six sphincters of the GI tract?
Upper esophageal, lower esophageal, pyloric, ileocecal, internal anal, external anal.
What are the three functions of the mouth?
Mastication (chewing), preparation for swallowing via saliva (moisten food bolus), onset of digestion (salivary amylase).
Describe the act of swallowing in 3 steps:
Oral phase: a voluntary collection of food bolus into the pharynx by the tongue to initiate involuntary reflex.
Pharyngeal phase: involuntary contraction of pharyngeal muscles to push food into esophagus, soft palate elevates to prevent backflow into nasal passage.
Esophageal phase: Bolus travels down the esophagus by peristaltic contraction, epiglottis covers the glottis to prevent flow of bolus into trachea.
What is the function of the stomach?
Storing ingested material, continuation of digestion, regulation of emptying into small intestine.
How does the stomach mechanically dissolve food bolus?
Folded surfaces (rugae) facilitate using peristaltic waves.
How does the stomach chemically digest food bolus?
HCl: denatures proteins and cleaves pepsinogen into pepsin.
Pepsin: enzyme that breaks down proteins.
What is chyme?
Ingested food that leaves the stomach.
What is the antrum?
Layer of stomach that contributes to digestion. Secretes pepsinogen, mucus, and gastrin.
What are the three sections of the small intestine?
Duodenum, jejenum, ileum.
How does the small intestine contribute to absorption?
Microvilli increase surface area to increase contact between intestinal contents and epithelium.
What is the function of the large intestine?
Functions to store and concentrate undigested material prior to its excretion.
What is the cecum?
The ‘pocket’ between the small intestine and the large intetsine. Responsible for digesting cellulose but is too small to do so in humans.
What is the appendix?
Branching appendage of the cecum, thought to be vestigial or haven for gut bacteria.
What are the 4 sections of the colon?
Ascending colon, transverse colon, descending colon, sigmoid colon.
What do the colon sections contribute to (functionality)
Absorption of ions, water, bacterial metabolism.
What is the general function of the accessory organs (GI)?
Secrete substances into the GI tract to aid in digestion.
What are the 3 salivary glans?
Parotid salivary gland, sublingual salivary gland, submandibular salivary gland.
What do the salivary glands secrete?
Water and mucus, and salivary amylase.
Where do the salivary gland secretions empty?
Into the oral cavity.
What does the liver secrete?
Bile salts (fat digestion), bicarbonate (neutralize chyme), organic waste product.
What does the gallbladder do?
Stores bile and concentrates it for release during feeding.
What do the liver and gallbladder have in common?
Both empty into the small intestine via the bile duct.
What is the exocrine function of the pancreas?
Acinar cells secrete into the small intestine via the pancreatic duct (bicarbonate, pancreatic amylase, trypsin, chymotrypsin, pancreatic lipase.)
What are the two general patterns of motility?
Peristalsis and segmentation.
How is peristalsis triggered?
Luminal contents stretching intestinal wall. Initiates circular contraction behind stimulant and relaxation in front of it.
Is peristalsis independent of extrinsic innervation?
Yes, but can be modulated.
Describe the activation and mechanism of peristalsis in 3 steps.
- Local stretch of receptors causes the release of serotinin. Increases 5HT activity activates sensory neurons that activate myenteric plexus.
- Neurons upsteam activate and cause smooth muscle contraction.
- Neurons downstream are activated (inhibited) to cause smooth muscle relaxation.
What factors in the myenteric plexus are responsible for smooth muscle contraction?
Substance P, Acetylcholine.
What factors in the myenteric plexus are responsible for smooth muscle relaxation?
Nitric oxide NO2, vasoactive intestinal polypeptide.
What is the function of segmentation?
Mixing of contents in situ without propelling luminal contents along the intestine.
What are the three steps of segmentation?
- Local contractions separate the intestine into pockets.
- Subsequent contractions divide pockets centrally (offset split contents).
- Rhythmic contractions continue to subdivide pockets and mix contents.
Where does segmentation primarily occur?
Small intestine.
Where does rhythmic contraction originate?
In pacemaker interstitial cells of Cajal; create underlying oscillations known as basic electrical rhythm BER.
What neurotransmitter increases BER activity?
Acetylcholine.
What neurotransmitter decreases BER activity?
Epinephrine.
What occurs after segmentation stops?
Sweeping wave contraction; myoelectric complex MMC.
What is the function of migrating myoelectric complex?
To move undigested material into the large intestine.
Where do MMC contractions start?
In the stomach.
What are the three phases of MMC?
- quiescent period.
- irregular electrical and mechanical activity.
- burst of regular electrical and mechanical activity.
How is MMC controlled?
Circulating levels of motilin, Increase triggers MMC.
When are motilin levels lowest?
During ingestion of a meal.
What are the three major components of the lower esophageal sphincter?
- Internal sphincter.
- External sphincter.
- Clasp and sling fibres.
What is the function of the lower esophageal sphincter?
Relaxes upon swallowing to allow food into the stomach, contracts in response to acetylcholine to prevent reflux of stomach contents into the esophagus.
Receptive relaxation
Stomach relaxes upon entrance of bolus in order to allow increase in stomach volume with marginal increase in pressure.
Describe gastric peristalsis
Wave initiates at upper part of stomach and increases in size as it moves down the stomach, mixing contents and forcing pyloric sphincter closed. Small volume of liquid chyme is forced through sphincter.
What are the 4 extrinsic factors of gastric emptying?
- Stomach and intestinal contents.
- Acidity.
- Distension.
- Hypertonicity.
How does stomach content affect gastric emptying?
Meals rich in protein or fat will delay gastric emptying.
How does acidity affect gastric emptying?
Exposure of duodenum to acidity inhibits gastric emptying via feedback loop.
How does distension affect gastric emptying?
Distension of stomach increases peristatic contracts and emptying. Distension of the duodenum inhibits emptying.
How does hypertonicity affect gastric emptying?
Emptying is fastest when duodenal contents are isotonic. Hypertonic solutions in the duodenum inhibit gastric emptying.
What is Belching?
Air is unavoidably swallowed during eating and drinking and is regurgitated by belching because air in the stomach increases gastric volume. The LES will relax and allow air to escape.
Vomiting
Involuntary expulsion of stomach contents caused by reverse peristalsis. Preceded by salivation to protect esophageal and mouth tissue. Glottis closes to prevent aspiration. Abdominal wall muscles contract to increase abdominal pressure.