Physiology - General GIT Flashcards
What is the primary function of the digestive system?
- to transfer nutrients, H2O, and electrolytes from the food we eat into the body’s internal environment
List the 5 steps involved in nutrient acquisition.
- ingestion –> digestion –> absorption –> distribution –> usage
What are the 4 basic digestive processes?
- motility, secretion, digestion, and absorption
T or F: digestive motility involves smooth muscle and is therefore always autonomic.
- false!
- while autonomic (and smooth muscle) for the majority of the digestive system, motility involving the mouth and anus is voluntary (with skeletal muscle)
Our nutrients are derived from the digestion of which 3 compounds? What is mechanism involved in their digestion?
- carbohydrates, proteins, and fats
- process: enzymatic hydrolysis
What are the accessory digestive organs?
- salivary glands, exocrine pancreas, and the biliary system (liver and gallbladder)
Approximately how long is the GIT?
- about 15 feet
Name the 4 layers (from innermost to outermost) of the GIT.
- (innermost) mucosa –> submucosa –> muscularis externa –> serosa/adventitia (outermost)
What are the three layers of the mucosa?
- mucous membrane (inner epithelial layer): protection and absorption
- lamina propria (middle connective tissue): support and contains the GALT (gut-associated lymphoid tissue)
- muscularis mucosa (outer smooth muscle): motility
The submucosa is a thick layer of __________. What does it contain?
- thick layer of connective tissue
- contains the blood and lymph vessels, and the submucosal (Meissner) nerve plexus right below the muscularis muscosa
What does the muscularis externa contain?
- an inner circular layer of smooth muscle, and outer longitudinal layer of smooth muscle, and the myenteric (Auerbach) nerve plexus in between
What is the serosa? What does it secrete? What is it continuous with?
- an outer connective tissue covering
- secretes a slippery serous fluid (lubrication b/w the digestive organs and surrounding viscera)
- is continuous with the mesentery
Which 4 factors regulate digestive function (motility and secretion)?
- autonomous smooth muscle function, the intrinsic nerve plexuses, the extrinsic nerves, and gastrointestinal hormones
What is autonomous smooth muscle function? What does it create? What type of wave potentials are generated?
- 1 of 4 factors that regulate digestive function
- self-induced electrical activity that creates the GIT’s basic electrical rhythm (BER)
- (stomach: 3 waves/min, duodenum: 12, ileum: 8-9)
- involves the generation of slow-wave potentials
What generates the basic electrical rhythm (BER)? Where is this found?
- the pacemaker cells: interstitial cells of Cajal
- they are found between the inner circular and outer longitudinal layers of smooth muscle in the muscularis externa
What are the intrinsic nerve plexuses? Where are they found? What are they collectively known as?
- 1 of 4 factors that regulate digestive function
- submucosal/Meissner plexus (in the submucosa) used in secretion and the myenteric/Auerbach plexus (in the muscularis externa) used in motility
- collectively known as the enteric nervous system
Which compounds promote contraction of the enteric nervous system? Which promote relaxation?
- ACh: contraction
- NO, vasoactive intestinal peptide: relaxation (note that a loss of NO secretion is implicated in the elevated LES tone seen in achalasia)
- (NA is inhibitory)
What are the extrinsic nerves? Which are stimulatory? Which are inhibitory?
- 1 of 4 factors that regulate digestive function
- these are nerves from the autonomic nervous systems that innervate the digestive organs
- parasympathetic = stimulatory
- sympathetic = inhibitory
What are the gastrointestinal hormones?
- 1 of 4 factors that regulate digestive function
- hormones secreted from endocrine glands that are either stimulatory or inhibitory
Gastrointestinal hormones induce response by binding to what receptor type?
- G-protein coupled receptors
Gastrin
- a gastrointestinal hormone; increases digestion in the stomach
- release is stimulated by protein in the stomach, stomach distenstion, alkalinization, and vagal stimulation
- secreted by G cells in the antrum/PGA (pyloric gland area)
- results in an increase in HCl and pepsinogen secretion, enhances gastric motility, stimulates the gastroileal and gastrocolic reflexes
- inhibited by accumulation of acid in the stomach and by duodenal constituents
Secretin
- a gastrointestinal hormone; lowers acidity of small intestine for protection and proper enzymatic function
- released by S cells in the duodenum in response to acid and fatty acids
- stimulates pancreatic and liver secretion of NaHCO3 and bile, inhibits gastric emptying into duodenum, and inhibits gastrin secretion
CCK
- cholecystokinin; a gastrointestinal hormone; increases digestion in the small bowel
- released by I cells in the duodenum in response to fatty acids (and protein to a lesser extent)
- stimulates pancreatic secretion of pancreatic enzymes, contracts the gallbladder and relaxes sphincter of Oddi (bile secretion), and inhibits gastric motility and secretion to allow time for the current duodenal constituents to be digested and absorbed
GIP
- gastric inhibitory peptide AKA glucose-dependent insulinotrophic peptide; a gastrointestinal hormone
- released by K cells in the duodenum in response to fatty acids and amino acids, and also to an oral glucose load
- stimulates insulin release from the pancreas (also slightly inhibits gastric motility and secretion)
Short-reflex vs. long-reflex
- short-reflexes involve only the intrinsic (enteric) nerve networks
- long-reflexes involve the autonomic CNS (longer pathway = “long-reflex”)
Somatostatin
- a gastrointestinal hormone; inhibitory effects
- released by D cells in the GI mucosa and pancreatic islets in response to increased acidity (decreased via vagal stimulation)
- decreases gastric secretions, pancreatic secretions, gallbladder contractions, and insulin + glucagon release
Where do the extrinsic nerves of the GIT arise from?
- parasympathetic: medulla (esophagus to ascending colon via the vagus nerves) + sacrum S2-S4 (ascending colon onwards via the pelvic nerves)
- sympathetic: spinal cord (T5-L2)
Of the 8.5 liters of inflow per day, how much is absorbed by the GIT?
- 6.5 liters are absorbed by the small intestine
- 1.9 liters are absorbed by the large intestine
- 0.1 liters are excreted
