GI Physiology

How is folate absorbed?

Absorbed in the jejunum and ileum

How is B12 absorbed?

Absorbed in the terminal ileum along with bile acids, Requires intrinsic factor.

What is the role of gastric acid. What cells is it secreted form and how is it regulated?

Gastric acid is also secrete by parietal cells in the stomach. It decreases stomach pH Gastric acid secretion is stimulated by histamine, ACh, and gastrin and inhibited by somatostatin, GIP, prostaglandin, and secretin A GASTRINOMA is a gastrin-secreting tumor that causes high levels of acid secretions in the stomach, causing ulcers that are refractory to medical therapy.

What products are contained in pancreatic secretions?

-Alpha-amylase - starch digestion, secreted in active form -Lipase - Fat digestion -Proteases - protein digestion. (trypsin, chymotrypsin, elastase, carboxypeptidase). Secreted is proenzymes known as zymogens. - Trypsinogen - converted to trypsin --> activation of there proenzymes and cleaving additional trypsinogen molecules into trypsin. Converted to trypsin by enterokinase/enteropeptidase, a brush border enzyme on the duodenal and jejunal mucosa

What is the role of Bicarbonate? From what cell is it secreted and how is it regulated?

Bicracbonate ions are secreted by gastric and intestinal mucosal cells. It neutralizes stomach acid. Bicarbonate ion release is stimulated by pancreatic and biliary secretion with secretin. Bicarbonate ions are trapped in mucous that covers the gastric epithelium.

What is Bile? What is it composed of?

Bile is composed of bile salts (bile acids conjugated to glycine and taurine, making them water soluble), phospholipids, cholesterol, bilirubin, water, and ions. Cholesterol 7α-hydroxylase catalyzes rate-limiting step of bile synthesis. Bile acts to digest and aid in absorption of lipids and fat-soluble vitamins, Cholesterol excretion (body's only means of eliminating cholesterol) , and Antimicrobial activity (via membrane disruption)

What is bilirubin and how is it metabolized?

Bilirubin is a product of heme metabolism. Bilirubin is removed from the blood by the liver, conjugated with glucuronate, and excreted in bile. Conjugated bilirubin is water soluble. Unconjugated bilirubin is not water soluble.

Where are Brunner glands located? What is their purpose?

Brunner glands are located in the Duodenum. They secrete an alkaline mucus. Peptic ulcer disease causes hypertrophy of Brunner glands.

What is the action of Cholecystokinin (CCK), what cells is it secreted from? How is it regulated

CCK is a peptide hormone secreted by I cells in the duodenum and jejunum. Activation of CCK receptors leads to increased pancreatic secretions, increased gallbladder contraction, decreased gastric emptying, and increased relaxation of the sphincter of Oddi. CCK release is stimulated by increased Fatty acid and amino acids

What is the action of Glucose-dependent insulinotropic peptide, or Gastric inhibitory peptide (GIP)? What cells is it secreted from, and how is it regulated?

GIP is released from K cells in the duodenum and jejunum. GDIP has both exocrine and endocrine activity. Exocrine : decreases gastric H+ secretion. Endocrine: Increased Insulin release GDIP release is stimulated by increased fatty acids, amino acids, and oral glucose.

What is the action of Gastrin? What cells is it secreted from, and how is it regulated?

Gastrin is released by G cells in the antrum of the stomach. Activation of gastrin receptors causes increased gastric H+ secretion, increased growth of gastric mucosa, and increases gastric motility. Gastrin increased acid secretion primarily through its effects on enterochromaffin-like cells leading to histamine release rather than direct action on parietal cells. Gastrin secretion is significantly increased in Zollinger-Ellison syndrome. Chronic PPI use increases gastrin secretion. Phenylalanine and tryptophan are potent stimulators. Gastrin secretion is STIMULATED by Increased stomach dissension, alkalinization, amino acids, peptides, vagal stimulation. INHIBITED by stomach pH < 1.5

How is iron absorbed?

Iron is absorbed in the Fe2+ state in the duodenum.

What is the action of Motilin? What cells is it secreted from, and how is it regulated?

Motilin is secreted by the small intestine. Motilin produces migrating motor complexes (MMCs) (waves of activity that cause peristalsis in the fasting state) Motrin release is stimulated by fasting conditions. Motrin-receptor agonists (erythromycin) are used to stimulate peristalsis.

What is the action of Nitric oxide? What cells is it secreted from, and how is it regulated?

Nitric oxide is released from myenteric nerves and acts on smooth muscle layer of intestinal mucosa causing smooth muscle relaxation, including lower esophageal sphincter. Loss of NO secretion is implicated in increased LES tone of achalasia.

How are carbohydrates absorbed in the GI tract?

Only monosaccharides are absorbed by enterocytes. Glucose and galactose are taken up by SGLT1 (sodium dependent transporter) Fructose is taken up by facilitated diffusion by GLUT-5. All are transported to the blood by GLUT-2 D-xylose absorption test: distinguishes GI mucosal damage from other causes of malabsorption

What is the role of pepsin? From what cells is it secreted and how is it regulated?

Pepsin is secreted by chief cells in the stomach. Pepsin acts by breaking down proteins. Pepsin release is stimulated by vagal stimulation, and local acid. Inactive pepsinogen is activated by H+

What is the role of Peyer's patches?

Peyer's patches are unencapsulated lymphoid tissue found in the lamina propria and submucosa of the ileum. Peyer's Patches contain M cells that sample and present antigens to immune cells. B cells located in Peyer's patches can differentiate into IgA-secreting plasma cells. IgA can be transported into the intestinal lumen to deal with intraluminal antigens. IgA = Intra Gut Antibody

What is the action of Secretin? What cells is it secreted from, and how is it regulated?

Secretin is secreted by S cells in the duodenum. Activation of secretin receptors causes increase in pancreatic bicarbonate ion secretion, decreased gastric acid secretion, and increased bile secretion. Bicarbonate ions are needed to neutralize the acidic contents that empty into the duodenum so that pancreatic enzymes can work. Secretin is stimulated by an increase in acid and fatty acids in lumen of duodenum

What is the action of Somatostatin? What cells is it secreted from, and how is it regulated?

Somatostatin is secreted form Delta cells (from pancreatic islets) Somatostatin causes a decrease in gastric acid and pepsinogen secretions, decreased pancreatic and small intestine fluid secretions, decreased gallbladder contraction, and decreased insulin and glucagon release. Somatostatin can be considered as an anti-growth hormone. It inhibits digestion and absorption of substances needed for growth. Somatostatin release is stimulated by increased acidity and inhibited by vagal stimulation.

What is the action of Vasoactive intestinal polypeptide (VIP)? What cells is it secreted from , and how is it regulated?

VIP is release from parasympathetic ganglia in sphincters, gallbladder and small intestine. VIP acts by increasing intestinal water absorption, and electrolyte secretion. It also increases relaxation of intestinal smooth muscle and sphincters. VIP is stimulated by dissension of the bowels and vagal stimulation. VIP is inhibited by adrenergic input. VIPoma—non-α, non-β islet cell pancreatic tumor that secretes VIP. Copious Watery Diarrhea, Hypokalemia, and Achlorhydria (WDHA syndrome).

What is the role of intrinsic factor? What cells is it secreted from and how is it regulated?

intrinsic factor is secreted by parietal cells in the stomach. IF binds vitamin B12 and facilitates uptake in the terminal ileum. Autoimmune destruction of parietal cells leads to chronic gastritis and pernicious anemia