Gastrointestinal System Flashcards
What is digestion?
Breaking down macromolecules (nutrients) into forms that can be transported across the epithelium.
What is absorption?
The transport of nutrients, water, ions, and vitamins across the epithelium.
What do you need in order to accomplish digestion and absorption?
- Secretion (release of enzymes into gut lumen)
- Motility (keep the gut contents moving)
What are some additional considerations in order to maintain proper GI function?
- The ability to digest macromolecules but not itself.
- The ability to allow entry of digested nutrients but not pathogens.
- The ability to maintain balance between water input/output.
Protection from pathogens is mediated by:
- Epithelial barrier
- Mucus
- Digestive enzymes
- Acid
- Gut-Associated Lymphoid Tissue (GALT)
What is the purpose of GALT?
To react to pathogens but not to “foreign” proteins associated with food.
What are some sources of fluid input (into the GI tract)?
- Food/drink
- Saliva
- Bile
- Gastric secretions
- Pancreatic secretions
- Intestinal secretions
What are the 4 parts/sections of the stomach?
- Fundus
- Body
- Antrum
- Pylorus
What are the 3 parts/sections of the small intestine?
- Duodenum
- Jejunum
- Ileum
What are the layers of the mucosal surface (from top to bottom)?
- Mucosa
- Submucosa
- Smooth muscle layers
- Serosa
What are the 3 layers of the mucosa (from top to bottom)?
- Epithelium
- Lamina propria
- Muscularis mucosa
Which layer is a part of the submucosa?
Meissner’s (submucosal) plexus
What are the 3 layers part of the smooth muscle layers?
- Circular muscle
- Auerbach’s (myenteric) plexus
- Longitudinal muscle
How does the small intestine’s microanatomy differ from the stomach’s?
The small intestine does not have epithelium. Instead, it has villi and crypts.
It also has Peyer’s patch (absent in stomach), and does not have lamina propria (present in stomach).
What are the 2 major patterns of contraction?
1) Peristalsis (moving food from mouth to anus)
2) Segmental contractions: Mixing/Churning (maximizing exposure to digestive enzymes and epithelium)
What is most gut muscle connected by?
Gap junctions.
Which regions of the gut contract tonically (minutes to hours)?
- Smooth muscle sphincters
- Anterior part of stomach
Which regions of the gut contract phasically (few seconds)?
- Posterior stomach
- Small intestine
What happens in the GI tract between meals? What happens during/after meals?
Between: Migrating motor complexes sweep slowly down tract
During/after: Peristaltic and segmental contractions
Where are single-unit smooth muscle cells most commonly found?
- Walls of GI and urinary tract
- Blood vessels
When do action potentials fire?
When slow wave potentials exceed threshold.
What are the force and duration of muscle contraction directly related to?
The amplitude and frequency of action potentials.
In the gut smooth muscle, what does it mean when the degree of contraction is graded according to the amount of Ca that enters?
Longer wave -> More time for Ca to enter -> Larger contraction
In the gut smooth muscle, what is the amplitude and duration of contraction influenced by?
- Neurotransmitters (autonomic input)
- Hormones
- Paracrine factors
How does the slow wave frequency vary in different regions of the GI tract?
More frequent in duodenum vs. stomach.
What are the interstitial cells of Cajal?
Pacemaker cells between smooth muscle layers that set the slow wave potential (which leads to contraction).
How are water and ions secreted into the gut?
- Via membrane transporters
- Following an osmotic gradient
- Passing between cells (paracellular)
True or False: The gut contains different channels/transporters than the kidney in order to secrete water and ions.
False. Still contain ENaC, Na/K-ATPase, NKCC, etc.
How does the pH of the lumen differ from the pH of parietal cells?
Lumen: pH = 1
Parietal cell: pH = 7.2
As H+ is secreted from the apical side, what happens to bicarb?
It is absorbed into the blood (basolateral side).
What is the purpose of secreting bicarb into the duodenum?
To neutralize acid arriving from the stomach.
What are acinar cells?
From the pancreas, and secrete enzymes.
What do the epithelial cells lining the pancreatic duct secrete?
Bicarb solution.
What is the process of bicarb secretion?
1) CO2 enters the pancreatic duct cell or duodenal cell, and combines with H2O to produce bicarb and H+.
2) H+ is secreted out the basolateral side in exchange for Na+.
3) Cl- enters the cell via basolateral NKCC transporter and leaves via apical CFTR channel.
4) Bicarb is secreted out via apical Cl-/bicarb exchanger.
5) Cl- in lumen attracts water and Na+.
What does bicarb secretion require (high expression of ______)?
Carbonic anhydrase.
What is the process of NaCl secretion in the small intestine, colon, and salivary glands?
1) Na, K, and Cl enter via NKCC transporter.
2) Cl enters lumen through CFTR channel.
3) Na is reabsorbed
4) Negative Cl in lumen attracts positive Na by paracellular pathway.
5) Water follows.
What do crypt cells secrete?
Isotonic saline that mixes with mucus secreted by goblet cells to lubricate gut contents.
What is Cystic Fibrosis?
A mutation in gene that encodes in CFTR channel.
Leads to defects in Cl- and water transport.
What is the process of Cystic Fibrosis causing pancreatic damage?
1) Cl not transported into ducts.
2) Various effects, including decreased Na and water transport into ducts.
3) Mucus still produced but greatly thickened due to lack of water.
4) Blockage of pancreatic ducts.
5) Exocrine secretions of pancreas not released (bicarb, enzymes).
6) Back pressure/inflammation
7) Damage to pancreas.
Where are enzymes secreted by either exocrine glands or epithelial cells of stomach/small intestine synthesized? Where are the packaged? Where are they stored?
Synthesized by rough ER.
Packaged by Golgi.
Stored in cell.
Why are enzymes sometimes released as zymogens?
To prevent auto-digestion.
What is enzyme secretion regulated by?
Neural, hormonal, and paracrine signals.
What is enzyme secretion usually stimulated by?
Parasympathetic stimulation (via vagus).
What does mucus primarily consist of?
“Mucins”, a mixture of glycoproteins.
What is mucus produced by?
Exocrine cells:
- Serous cells in salivary glands.
- Mucous cells in stomach.
- Goblet cells in intestine.
What are some signals for mucus secretion?
- Parasympathetic stimulation.
- Various neuropeptides (of enteric nervous system).
- Cytokines (from immune cells in response to infection/inflammation).
What is saliva?
Complex hypo-osmotic fluid.
Made up of water, ions, mucus, proteins (enzymes, immunoglobulins).
What is the 2 step process of saliva secretion?
1) Fluid secreted by acinar cells similar to ECF (isotonic saline).
2) As it passes through ducts, epithelial cells take back Na and secrete K, eventually resembling ICF. Water remains in saliva (hypo osmotic).
How is saliva secretion stimulated?
Stimulated: Parasympathetic pathways
Inhibited: Sympathetic pathways
Where is the sphincter of Oddi located?
The duodenum.
What is the organization of bile secretion through the hepatic lobule?
1) Hepatocytes
2) Bile Canaliculi
3) Bile ductules
4) Common hepatic duct & Gall bladder
5) Common bile duct
6) Duodenum
Where do most absorbed nutrients move into?
Capillaries in villi, then into the hepatic portal vein.
What must xenobiotics first pass through before reaching systemic circulation?
The liver.
True or False: Fats also go into the blood vessels.
False. Fats go into the lymphatic system instead of blood.
The hepatic artery and the hepatic vein connect in which structure?
Sinusoids in the liver.
What are the key components of bile?
- Bile salts
- Bile pigments
- Cholesterol
What are some purposes of the liver?
- Glucose and fat metabolism.
- Protein synthesis.
- Hormone synthesis.
- Urea production.
- Detoxification.
- Storage.
What does the liver secrete into the duodenum?
- Bile salts
- Bilirubin
- Water, ions
- Phospholipids
What metabolites does the liver secrete into peripheral tissue?
- Glucose
- Plasma proteins
- Urea
- Vitamin D, somatomedins
What is the organization of blood flow through the hepatic lobule?
1) Hepatic artery & Hepatic portal vein
2) Sinusoids
3) Central vein
4) Hepatic vein
What does it mean when the sinusoids are “very aappy”?
A lot of plasma filtered out -> lymph.
A lot of proteins added in.