GI secretion Flashcards
Role of secretion
facilitate digestion
provide cell protection (production of mucus and neutralization of acid)
Mucus cells
goblet cells, neck mucus cells, and surface mucus cells
throughout the GI tract from mouth to large intetsine
Acinar Cells
watery secretions containing proteins such as digestive enzymes which are emptied into a duct system
Acinous glad principle location
Pancrease, salivary glands
Secretory cells
located in the mucosa of the small intestine (Crypts of Lieberkuhn) - secrete a large volume of extracellular fluid
Mucins
glycoproteins that lubricat the food (and throat) and are made primarily in the submandibular and sublingual salivary glands
Salivary Amylase
breaks down starch to oligosaccharide molecules
terminated when gastric pH <4
Xerostomia
absence of saliva due to lack or block of salivary gland
Types of salivary glands
Parotid
Submandibular
Sublingual
Parotid salivary gland
exclusivley serous (acinous) gland
secretes a watery fluid that is high in salivary amylase (no mucins)
Parasympathetic stimulation of salivary glands
vasodilation ( increases blood flow)
INITIAL tonicity of saliva in the acinus
Isotonic
End tonicity of saliva
Hypotinic
Na, Cl absorbed (to blood)
K and bicarb are sectreted
concentrtaions of K and Bicarb in saliva?
High- saliva is hypotonic to serum (because Na Cl go absorbed)
Parietal cells
located in oxyntic region (fundus and corpus) and secrete HCl and intrinsic factor
Chief Cells
located in oxyntic region secrete pepsinogen
Mucous neck cells
located below the epithelium and secrete soluble mucus
Enteroendocrine cells
hormone secreting cells (somatostatin)
Oxcyntic glands
lots of parietal cells - lots of HCl
Pyloric glands
G-cells- Gastrin
Pepsinogen
becomes pepsin at pH<5 Protease
Intrinsic factor
secreted by parietal cells
Required for B12 absorption
Bicarbonate
produced by epithelial cells
Mechanism of acid secretion by parietal cells
H+/K+ ATPase (proton pump) - Hydrogen is pumped into the lumen against and enormous gradient
K and Cl are cotransported to the lumen and K is recycled
Bicarbonate ions leave the cell
Alkaline tide
Bicarbonate ions leave the cell and enter the blood raising the pH of the venous blood leaving the stomach
What stimulates the release of pepsin from chief cells?
hitamine ACh Gastrin Secretin CCK
what is the only gastric function required for life?
secretion of intrinsic factor by parietal cells
Prostaglandin
increase mucosal blood flow as well as bicrob adn mucus secretion (stimulate cell repair and renewal)
Parietal cell receptors
- Histamine
- Ach
- Gastrin
ACh on parietal cell
From Vagus nerve
stimulates muscarinic receptors on parietal cell or on enterochromafin like cells
if on parietal cell: IP3 to Ca2+ causing release of HCl
If on enterochromafin like cell = release of histamine
Histamine on parietal cell
released from enterochromaffin like cell (ECL) in mucusa. Diffuses into the parietal cells and stimulates H2 receptors (CAMP to HCl release)
How do you block muscarinic receptors
Atropine
How do you block H2 receptors
cimetidine
Gastrin on parietal cells
released from G-cells in the stomach antrum and duodenum
stimulates the release of ACh or histamine
Potentiation (histamine)
Histamine potentiates the effects of ACh and gastrin
Cephalic phase of gastric acid secretion
sight or smell of food as well as chewing and swallowing causes vagal refelxes to parietal and G-cells (ACh and gastrin release to stimulate acid production)
Gastric phase of gasrtic acid secretion
Gastric distention causes both local and vagovagal reflexes to parietal and G-cells (more ACh and gastrin release)
Intestinal phase of gastric acid secretion
protein digestion products in the duodenum lead to stimulation of intestinal G cells and release of gastrin
Somatostatin
released by D cells
Decrease gastrin release by G-cells
Secretin
released by S cells when there is too much acid in the duodenum
inhibit gastrin release and inhibit secretion by parietal cells
effect of CCK and Secretin on pepsinogen secretion
Stimulate chief cells to secrete pepsinogen but inhibit the release of acid
Secretin and pancreatic secretion
aqueous pancreatic juices by causing fluid retention in the ducts
Bicarb and fluid
CCK and pancreatic secretion
stimulates secretion rich in enzymes (Proteases, amylases, lipases)
tonicity of aqueous component of pancreatic juices
isotonic
What cannel is required for pumping bicarbonate into pancreatic juices?
Cl channel
Bicarb in the lumen attracts NaCl (water follows NaCl)
What activates trypsinogen
enteropeptidase
what activates Chymotrypsin
trypsin
what activates carboxypeptidase?
trypsin
