Basic Gastrointestinal Physiology

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Sympatheic Control:

Norepinephrine:

• Decreased motility and secretions.

• Increased constriction of sphincters.

Parasympathetic control:

Acetylcholine:

• Increased motility and secretions.

Vasoactive Intestinal Peptide:

• Decreased constriction of sphincters.

Gastrin-Releasing Peptide:

• Increased gastrin.

Gastrointestinal Hormones:

• Inhibitors: secretion, CCK, GIP.

• Stimulators: gastrin.

Gastrointestinal Motility:

• Stretch produces a contractile response.

• Pacemaker activity controls intrinsic motor activity.

Swallowing Reflex:

• Swallowing is controlled by brain stem.

• Upper esophageal skeletal muscle sphincter relaxes.

• Primary peristaltic wave via VIP.

• Lower esophageal smooth muscle sphincter relaxes.

• Proximal stomach relaxes.

• Achalasia = LES fails to relax during swallow.

Stimulation:

• Increased parasympathetic activity via ACh and gastrin.

Inhibition:

• Low pH inhibits gastrin.

• Duodenal overload feedback.

Stomach Emptying:

• Lipids earlier than Carbohydrate earlier than protein earlier than fat.

Migrating Myoelectric Complex (MMC):

• During fasting, a GI propulsive movement that begins in stomach and ends in colon.

• High levels of motilin, a small intestine hormone.

Salivary Secretions:

• Under parasympathetic control.

• Low Na+, Cl-.

• High K+, HCO3-.

• Alpha-amylase (ptyalin) digests Carbohydrate.

• Mucus, glycoprotein.

• Low tonicity.

Gastric Secretions:

• Parietal cells: HCl.

• Intrinsic factor + B12 = both are reabsorbed in the distal ileum.

• Chief cells: convert pepsinogen to pepsin, which digests protein.

Mucous Cells:

• Secrete mucous to protect stomach lining from HCl; simulated by prostaglandins.

Control of Acid Secretion:

• Parietal cells are stimulated by: ACh, histamine, gastrin.

Mechanism of Acid Secretion:

• H+ secreted by H/K-ATPase pump.

• HCl forms within stomach.

• Carbonic anhydrase utilized.

Pancreatic Secretions:

Secreted Active:

Pancreatic amylases:

• Alpha-limit dextrins.

• Maltotriose.

• Maltose.

Pancreatic Lipases:

• Need colipase; bile digestion.

• Cholesterol esterase; cholesterol hydrolyzed.

Secreted Inactive:

• Secreted as zymogens, then activated in small intestine.

• Trypsinogen; forms trypsin by utilization of enterokinase.

• Chymotrypsinogen; forms chymotrypsin by utilization of trypsin.

• Procarboxypeptidase; forms carboxypeptidase by utilization of trypsin.

Pancreatic Secretions Composition:

• Pancreatic secretions are high in HCO3-, low in Cl-; isotonic.

Control of Pancreatic Secretions:

• Secretin (in response to stomach acid): causes pancreas to release fluid high in HCO3-.

• CCK (in response to partially digested materials): causes pancreas to release amylases, lipases, proteases.

Bile Salts and Micelles:

• Bile acids are synthesized in the liver from cholesterol.

• Bile acids are then conjugated with glycine, to make it water-soluble.

Micelles:

• Micelles are formed from concentrated bile salts.

• They have a hydrophilic exterior and a hydrophobic interior.

• Micelles are used in digestion, transport, and absorption of lipid-soluble substances from the small intestine.

Notes on Digestion:

Triglycerides Absorption:

Stomach:

• Fatty acids are decreased in size.

Small Intestine:

• Emulsified by bile, and digested by pancreatic lipases.

Carbohydrate Absorption:

Mouth:

• Salivary alpha-amylase begins digestion.

Small Intestine:

• Pancreatic alpha-lipase carries on.

• Brush border enzymes come into play.

• Alpha-dextrinase: produces free glucose.

• Lactate: lactose converted to glucose + galactose.

• Sucrase: sucrose splits into glucose + fructose.

• Maltase: maltose converted to 2x glucose; maltotriose = 3x glucose.

Protein Absorption:

Stomach:

• Pepsin begins the process.

Small Intestine:

• Trypsin, chymotrypsin, elastase, carboxypeptidase A and B, dipeptidases and aminopeptidases carry on process.

Intestinal Digestions:

Carbohydrate:

• Glucose and galactose actively absorbed via same carrier in luminal membrane.

• Glucose absorbed via facilitated diffusion in basal membrane.

Protein:

• Secondary active transport in luminal membrane.

• Simple diffusion in basal membrane.

Lipids:

• Packed in micelles, they diffuse to brush border of intestine.

• Diffusion = rate-limiting step.

• Diffuse into lipid matrix; chylomicrons formed; leave intestine via lympathetics.

• Water-soluble short-chain fatty acids absorbed via simple diffusion into the blood stream.

• Bile salts actively absorbed in distal ileum.

Electrolytes:

• Secretion: cAMP-dependant pump in small intestine/LI; attacked by cholera toxin.

Duodenum:

• Iron, divalent ions, water-soluble vitamins.

Jejunum:

• Water and electrolytes.

Ileum:

• Water, Na, Cl, K.

• Secretion of HCO3-.

• Distal ileum: bile salts, intrinsic factor, vitamin B12.

Colon:

• Doesn't absorb proteins and carbohydrates.

• Net reabsorption of water and Na.

• Net secretion of K and HCO3-.

• Therefore, diarrhea = metabolic acidosis + hypokalemia.

Additional Readings:

Basic Gastroenterology

1. Basic Gastrointestinal Physiology
2. Digestion FAQ, Defecation reflex, etc.
3. Digestion
4. Notes on Functions of the Liver
5. Notes on Jaundice
6. Types of Jaundice
7. Diagram of Gastric Blood Supply
8. FAQ on Gastric Digestion
9. Usage of the D-xylose Absorption Test

Gastroenterology Videos

1. Video of Abdominal Examination in a Clinical Setting

Related Topics

1. Gastrointestinal Disorders
2. Hepatobiliary and Pancreatic Disorders
3. Histology of the Digestive Tract I: Oral Cavity
4. Histology of the Digestive Tract II: Esophagus through Intestines
5. Histology of the Liver, Pancreas, and Gall Bladder
6. Abdominal Examination for Internal Medicine

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