Histo Lecture 17
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Histo Lecture 17
GI Secretory Functions
What are glands?
Specialized epithelial cells designed to produce secretory products
What are some types of glands in the GI tract?
Simple surfae glands
complex tubular and acinar glands
Name some stimuli that promote secretion in the GI tract
Presence of food in the GI tract
Distention of the gut wall
Enteric nervous system stimulation
How does the ANS affect secretion?
Parasympathetics - Increase
Sympathetics - Decrease
How does the Enteric Nervous System affect GI secretion?
Controls secretory function at local level without higher CNS input
Sensory nerves of the ENS promote reflexive increase in mucus, digestive enzymes and hormone secretion
What are the three basic stimuli that excite sensory nerves of the ENS?
Mechanical stimulation/presence of food
Chemical irritation in the gut
Distention of the gut wall
Parasmpathetics control what?
Primarily controls secretion from salivary, esophageal, gastric, duodenal, pancreatic and distal colon glands
Sympathetics control what?
Decreased pancreatic secretion, decreased blood flow to GI tract
Increase in salivary secretion
What are the two main types of GI secretions?
Serous - composed of mainly water and electrolytes
Protein (mucus) - composed of enzymes for nutrient digestion and mucus
What are two types of secretions?
Primary secretions - initial secretory product developed by acinar cells
Secondary secretions - Primary secretory product is modified by cells that line the ducts of the glands
What is mucus?
Major secretory product that is secreted in all portions of the GI tract
Composed of water, electrolytes, and glycoproteins
The chemical properties of mucus allows it to serve as a protectant and lubricant within the GI tract
What are two main important properties of mucus?
"Stickiness" allows it to adhere to food particles and to the lining cells of the GI tract
Coats the GI wall and serves as a protectant
Name 3 types of salivary glands
What are some functions of saliva?
Lubricate food and maintain neutral pH
Amylase enzyme to begin digestion of carbohydrates
Protects mucosa of the oral cavity
Aids in taste, speech, and prevention of oral infections
What are the main components of primary secretions?
Water, electrolytes, and ptyalin as an isotonic solution
What is the main function of secondary secretion?
Reabsorption of NaCl from the ducts and addition of K+ and HCO3- into the ducts
What are some stimulators of saliva production?
Tactile stimuli in the oral cavity
Smell of food
Name the gastric glands and what they release
Mucus/HCO3 - mucous neck cells
HCl (acd) - parietal cells
Intrinsic factor - parietal cells
Pepsinogen % lipase - chief cells
Gastrin - G cells
Somatostatin - D cells
What does HCl create a low pH for?
Conversion of pepsinogen to pepsin
Enhanced protein digestion
Preventing bacterial growth
What does pepsinogen do?
When converted to pepsin, aids in protein digestion
What is Intrinsic Factor for?
Necessary for Vitamin B12 absorption in the ileum
What does the hormone Gastrin do?
Stimulates acid secretion
Presence of protein digestion products
Parasympathetic stimulation (vagus nerve)
Histatime does what?
Increase acid secretion
Somatostatin (hormone) is in charge of what?
Inhibits gastrin and acid secretion
Acetylcholine from ENS stimulates what?
what does the parasympathetic nervous system stimulate?
Enteric neuron activity, and secretion of acid, gastrin, histamine, pepsinogen
Gastrin and Histamine increase what?
Somatostatin inhibits what?
Gastrin and H+
What is the Cephalic phase?
Sight, smell, taste or thought of food activates vagal reflex
Stimulates secretion of acid, gastrin and pepsinogen
What is the Gastric phase?
Food entering the stomach stimulates vagus nerve and ENS
Stimulates acid & gastrin secretion
What is the Intestinal phase?
Presence of food ain the duodenum stimulates release of:
Somatostatin, Cholecstokinin, secretin, gastric inhibitory peptide
Small intestine mainly secretes what?
mucus, water, and electrolytes (Cl-, HCO3-)
What else does the intestinal mucosa secrete?
Secretin and CCK
Local mechanisms of the ENS control what?
Large intestine mainly secretes what?
mucus, HCO3-, under local enteric control
What are the Islets of Langerhands in charge of?
Insulin, Somatostatin, Glucagon
Control of blood glucose levels and energy production from carbohydrates
What is the exocrine function of the pancreas?
Secretion fo water, electrolytes
Proteases (trypsin, secreted as trypsinogen)
How do parasympathetics affect pancreatic secretion?
Stimulate exocrine secretion via enteric neurons
How do sympathetics afect pancreatic secretion?
Inhibit exocrine secretion via enteric neurons
Also reduces blood flow
What are some hormonal controls of pancreatic secretion?
Secretin - Stimulate water and bicarbonate release
CCK - Stimulates secretion fo digestive enzymes
Pancreatic polypeptide - inhibits secretion of digestive enzymes (negative feedback)
What is the Cephalic phase of Pancreatic Secretion?
Sight, smell, taste of food stimulates pancreate exocrine secretion
What is the Gastric Phase of Pancreatic Secretion?
Gastric distention stimualtes pancreatic exocrine secretion
What is the Intestinal Phase of Pancreatic Secretion?
Small intestine distention, presence of food products stimulate pancreatic exocrine secretion
CCK and secretin also responsible for secretion of pancreatic digestive enzymes
What is Bile?
Necessary for digestion of lipids
Synthysized in liver, stored in gall bladder
What does bile contain?
Bile acids (salts)
Bile pigments (bilirubin)
Cholesterol & phospholipids
Ions & water
Where do most absorbed fats go through?
Lymphatics to reach the circulation
What does the Gall Bladder do?
Stores bile produced in the liver
Concentrates organic component of bile by removing water and ions
Contracts to eject bile into the duodenum (through the sphincter of Oddi)(CCK stimulates contraction)
What are some facts about the Biliary System?
Bile acids/salts are re-circulated to the liver for re-use
Bile is absorbed in the ileum
Moves through the portal circulation to reach the liver
Bile acids are easily recycled and prevent excess demand for newly synthesized bile
What does the presence of a bolus stimulate?
Gastro colic reflex
H+, gastrin, histamine, mucus, HCO3-, intrinsic factor
Stimulation of pancreatic secretions
Describe Gastric Emptying
Stimulates Duodenal secretions (secretin, CCK)
Stimulates pancreatic secretion (digestive enzymes)
Stimulates bile release (gall bladder)
Inhibits gastric motility and emptying
How often does the gastric and intestinal tract contract?
Every 90 min to "clean out"