gastrointestinal tract

lab practical

basics of GI

• Peritoneum: serous membrane lining the abdominopelvic cavity

  • Two layers

    • Parietal peritoneum: lines the internal surface of the abdominopelvic wall

    • Peritoneal cavity: space between the parietal and visceral peritoneum

      • Contains peritoneal fluid: water, electrolytes, substances similar to interstitial fluid

    • Visceral peritoneum: covers the intraperitoneal organs

• Kidneys are retroperitoneal → behind the peritoneal cavity

mesentery

• Mesentery: serous membranes attached to the abdominal organs to hold them in place

  • Fat tissue → covers organs

  • Greater omentum: 4 layered mesothelium made up of simple squamous epithelium

    • Inferior to stomach

    • Go down towards the bladder

  • Lesser omentum: connects the lesser curvature of the stomach and the proximal end of the duodenum to the liver and the diaphragm

    • Superior to stomach

gastrointestinal tract cross section

• Epithelial lining of the mucosa exposed to the lumen is stratified squamous

  • Same for any lining of the internal tubes (GI tract, anus, esophagus, etc)

• The GI tract has its own nervous system

• Myenteric plexus: controls main GI movements

• Meissner’s plexus: inner plexus controlling GI secretions and local blood flow

• Things move because there are muscles contracting

• Slow waves: low electrical potential that occurs 24/7

  • Do not require an AP to occur

  • Generated and propagated by Interstitial cells of Cajal → spread to surrounding smooth muscle cells and control motility

• Spike waves: initiated by APs in the smooth muscle 

  • Promote motility → movement of contents through the GI tract

saliva

• Saliva glands

  • Parotid gland

  • Sublingual gland

  • Submandibular gland

• Contents of saliva

  • Lysozyme

  • Ptyalin or salivary amylase → breaks down carbs

  • Bicarbonate ions → buffer neutralizing acids

  • Immunoglobulins → prevent bacterial infections

  • Saliva protects the outer mucosa

  • Mucus protects the digestive tract from physical irritation

  • Lingual lipase helps begin lipid digestion

  • Increased salivation can be initiated by the facial nerve and the glossopharyngeal nerve

  • Protects epithelial cells from infection, dehydration, and physical or chemical injury

esophagus

oropharynx

• Oropharynx: starts from the mouth

  • Stratified squamous epithelium → non-keratinizing

    • Rests on a lamina propria

    • Contains a thick layer of longitudinally oriented elastic fibers

  • Lacks muscularis mucosae and submucosa

peristalsis

moves bolus toward the stomach

propulsion

moving food forward as in peristalsis

mixing

segmental contractions (non-propulsive)

No net movement forward

slow waves

interstitial cells of Cajal of the myenteric plexus forms specialized pacemaker cells that promote rhythmic contractions of smooth muscle throughout the gastrointestinal tract

motility

contraction of smooth muscle along the G.I. tract and moving it contents along with it

spike waves

true action potentials

stomach

• Food first enters the fundus

  • Muscles relax → allow for stretching of the stomach

  • “Vagovagal reflex” results from the stretching of the stomach

    • Signal travels from stomach to the brainstem → reduces the tone in the muscular wall of the stomach body

      • Accommodates greater amount of food 

• Food in the stomach mixes with stomach secretions → what flows down the gut is chyme

  • Stomach emptying: pushing of food into the duodenum

• Duodenum: breaks down food

  • Pyloric sphincter holds food in the stomach until duodenum is clear

  • Both the gastroesophageal and the pyloric sphincter control the movement of the food in and out of the stomach

hydrochloric acid production

1. CO2 and Cl- diffuse from the blood into the stomach cell

2. CO2 combines with H2O to form H2CO3

3. H2CO3 dissociates into bicarbonate (HCO3-) and H+

4. ATP pump is necessary to pump the H+ and Cl- into the duct since the concentration of HCl- is about a million times more concentrated in the duct than in the cytosol of the cell

5. H- combines with Cl- in duct of gastric gland to form HCl-

6. Now you have stomach acid

7. HCO3- goes to the respiratory system to expel CO2

gastrin

source: stomach

stimulus: stretches walls of stomach

response: increase secretion of HCL (hydrochloric acid)

cholecystokinin

source: small intestine

stimulus: fatty acids and pepties

response:

• stimulates pancreatic enzyme, pancreatic bicarbonate secretion,

• gallbladder contraction

• growth of exocrine pancreas

• inhibits gastric emptying

ghrelin

source: stomach

stimulus: empty stomach

response:

• stimulates appetite

• increases food intake

• promotes fat storage

intrinsic factor

source: parietal cells of stomach

response: necessary for B12 absorption

secretin

source: duodendum

stimulus: acidity of chyme

response:

• decreases gastric secretion

• stimulates pancreas secretion of bicarbonate ions

pepsin

source: stomach

stimulus: presence of pepsinogen

response:

• HCL and pepsin converts pepsinogen → pepsin

  • (pepsin is a protein)

GIP

source: duodenum

stimulus: fat and glucose levels

response:

• stimulates insulin release

• inhibits gastric acid secretion

motilin

source: duodenum

stimulus: fatty acids

response: stimulates gastric and intestinal motility

pancreatic amylase

source: pancreas

stimulus: digestion in small intestine

response: breakdown of carbohydrates

nuclease

source: pancreas

stimulus: digestion in small intestine

response: breakdown of nucleic acids

trypsin

source: pancreas

stimulus: digestion in small intestine

response: breakdown of proteins

peptidase

source: pancreas

stimulus: digestion in small intestine

response: breakdown of amino acids

gastric lipase

source: pancreas

stimulus: digestion in small intestine

response: breakdown of lipids

liver

• Food does not go through the liver

• Can regenerate itself

• Functions

  • Manufactures bile

    • Stored by the gallbladder

    • Is a detergent

    • Opens up a molecule → breaks down bonds that are in the way

      • So that enzymes can reach/bind to the active site of the molecule

  • Protein synthesis

  • Immune system

  • Metabolism

  • Detoxification of chemicals and bacteria

  • Production of cholesterol

  • Storage of micronutrients

  • Blood sugar balance

  • Produces blood proteins

  • Filters blood 

duodenum

• Epithelial tissue faces the lumen ALWAYS → here its columnar cells

• Layers in order

  • Simple columnar with microvilli lining the mucosa

  • Lamina propria (CT)

  • Muscularis mucosa (submucosa) → with Brunner's glands

    • Secretes mucus

      • Coats the duodenal epithelium to protect it from stomach acids

    • Empty into the intestinal glands

intestinal villi

• Increase surface area (SA)

• Contain goblet cells: secrete mucus

• On top of each villus is microvilli: one of the brush borders

• Increase surface area to absorb nutrients

• Have a blood supply to receive nutrients into the bloodstream

jejunum

ileum of small intestine

• crypts (i.e, mucosal glands) of the ileum of the small intestine lie within a lamina propria

  • rich in lymphocytes, eosinophils, and plasma cells

• Peyer’s patches: lymph nodules found in the ileum of the small intestine

  • Part of the immune system

  • Have different immune cells: B and T lymphocytes

• Not a lot of nutrient absorption occurs here

• Main function in immune system → instead protection from microorganisms

large intestine

• Feces consists of water, undigested food (cellulose), microorganisms, sloughed-off epithelial cells

• Good bacteria

  • Break down carbs

  • Involved in immune response

  • Produce vitamin K → absorbed

• Mucus provides protection