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Gastric Physiology and Pancreatic Function
Lecture 23 Part 3
Terms in this set (40)
Mechanical digestion in stomach
Several min after food passes into stomach, gentle, rippling, peristaltic movements called
pass over the stomach every 15-25 sec
What does the ingested foods in the stomach get turned into when mixed with secretions of gastric glands?
chyme, acidic soupy liquid
What primary function of the fundus?
serve as storage area
Describe the peristaltic movements in each main section of the stomach.
- few mixing waves are observed at fundus
- more vigorous mixing waves at body
- intensify at pylorus
Describe the process of
As chyme reaches pylorus, each mixing wave forces ~3mL of chyme thru pyloric sphincter into duodenum
- Most of chyme is pushed back up into body of stomach
What type of digestion takes place in the fundus and why?
b/c when food is briefly held in the fundic region for an hr w/o mixing w/ gastric juice, salivary amylase continues to break down carbs
What happens to carb digestion as food in stomach comes into contact w/ gastric juice?
salivary amylase is deactivated from low (acidic) pH of stomach
What enzymes are active in low pH of gastric juice and what macromolecule digestion takes place from each enzyme?
Salivary (lingual) lipase
is activated when it comes into contact with the acidic gastric juice
(secreted by chief cells), they're the 2
that break down triglycerides into monoglycerides & fatty acids
hydrochloric acid = strong acid
- responsible for gastric pH normally being btwn 1.35 - ~3
How is HCl secreted by parietal cell?
Parietal cells secrete H+ and Cl- separately into the stomach lumen
What are the pumps/channels and enzymes involved in the parietal cell during digestion?
- Proton pumps: H+/K+ ATPase
- carbonic anhydrase (CA)
- in apical region of parietal cell
- use ATP to actively transport H+ into lumen while bringing K+ from lumen into cell
K+ that is brought into cell is able to diffuse out of apical surface thru a K+ ion channel thus replenishing supply of K+ in lumen for exchange w/ H+
PPI's (proton pump inhibitors)
decrease acid production by blocking the actions of proton pump
- ex. Prilosec, Nexium, and Prevacid
carbonic anhydrase (CA)
- catalyzes CO2 and H2O into H2CO3 (carbonic acid) within the parietal cell
- Also works in reverse
- The H2CO3 dissociates into H+ and HCO3- (bicarbonate ion) providing the H+ supply for the H+/K+ ATPase
What is the alkaline tide and how is it produced?
- The flow of bicarbonate into the blood leaving the stomach, increasing blood pH and make urine more alkaline
- Bicarbonate (HCO3- ) is exchanged for chloride (Cl-) at the basilar layer of the parietal cell using the Cl-/HCO3- antiporter
What hormones increase HCl secretion?
- Acetylcholine (ACh)- used by PSNS
- Gastrin- secreted by G cells (gastric) and by intestinal mucosal (enteric) cells
- histamine- paracrine substance released by mast cells in lamina propria of stomach
- Binds to H2 receptors on parietal cells
- acts synergistically to increase the effects of ACh and gastrin
decrease acid secretion by blocking the effects of histamine on the H2 receptor
- ex. Tagamet
What hormones and neural inputs decrease HCl secretion?
- Sympathetic stimulation
how does the Cephalic phase regulate HCl secretion?
- Sight, smell, taste, chewing
- Activates parasympathetic nerves to ENS
=> Increases HCl secretion
how does the Gastric phase regulate HCl secretion?
- Distension, increase in peptide content, decrease in stomach lumenal H+ concentration (increase in pH - less acidic)
- Activation of long and short neural reflexes and direct stimulation of gastrin secretion
=> Increases HCl secretion
how does the Intestinal phase regulate HCl secretion?
- Distension, increase duodenal H+ concentration, increased osmolarity due to hypertonic contents, increased nutrient concentrations
- Activation of long and short neural reflexes, secretin secretion, CCK secretion, and other duodenal hormones
=> Decrease HCl secretion
increases HCl secretion
- If ingested proteins entering the small intestine were not broken down enough in stomach, intestine releases "these"
what is the purpose of these interactions during the intestinal phase?
to balance secretory and digestive activity of stomach w/ digestive and absorptive activities of small intestine
What are some of the fucntions of HCl in the stomach?
- destroy ingested pathogens
- main purpose: denaturation of proteins and activation of particular enzymes (pepsin & gastric/lingual lipase)
- cleaves (cuts) certain bonds of proteins which produces smaller peptide fragments
- Only active at a low pH (~2)
- Inactive at higher pH
In what form is Pepsin secreted?
Secreted into the stomach lumen by chief cells in the form of
How is Pepsin converted into its active form?
by coming into contact w/ HCl or other active pepsin enzymes
what happens when the balance btwn hydrochloric acid production, pepsin secretion, and mucosal defenses is disturbed?
lead to erosion of the stomach's epithelial lining
- ex. gastritis, peptic/gastric ulcers
Inflammation caused by anything that breaches mucosal barrier
Peptic or gastric ulcers
- Erosions of stomach wall
- Can perforate => peritonitis; hemorrhage
- Most are caused by Helicobacter pylori bacteria
- Some by NSAIDs, alcohol, and others by cancer
What substances may be absorbed by the stomach in small amounts?
- short chain fatty acids
- certain drugs like aspirin & alcohol
Describe the gastric transit time of each of the different types of macronutrients.
Within 2-4 hrs after eating a meal, the stomach has emptied its contents into duodenum.
- Foods rich in carbohydrate spend the least time.
- High-protein foods remain somewhat longer.
- Emptying is slowest after a fat-laden meal containing large amounts of triglycerides.
What is involved in complete digestion?
pancreatic juice, bile, and intestinal juice in small intestine
Why are the pancreas (and gall bladder) important in digestion within the small intestine?
- secretes enzymes, which digest food in the small intestine,
- and sodium bicarbonate, which buffers the acidic pH of chyme from the stomach
How do pancreas and gall bladder empty their contents into the duodenum?
via hepatopancreatic ampulla and accessory duct (duct of Santorini)
What type of cells make up the majority of the pancreas?
pancreatic acini (glandular clusters)
- participate in exocrine secretion; Secretion of Pancreatic Juice
What are the contents of pancreatic juice?
- Watery bicarbonate rich alkaline solution (~pH 8)- neutralizes chyme entering small intestine from stomach
- Electrolytes (primarily HCO3-)
- Enzymes: Pancreatic amylase, lipase, ribonuclease and deoxyribonuclease (digests nucleic-acids) are secreted in active form but require ions or bile for optimal activity; proteases secreted in inactive form, but become active in lumen of small intestine
What portion of the pancreas functions in endocrine secretion?
pancreatic islets (islets of Langerhans)
- secrete glucagon, insulin, & somatostatin into blood
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