Digestive Hormones and Regulation

Digestive Hormones and Regulation

• Types of Digestive Hormones: Polypeptides are the most likely type.

Major Digestive Hormones

• Gastrin: Released by G cells in the stomach and duodenum, stimulated by food in the stomach or duodenum. Stimulates gastric release of hydrochloric acid and pepsinogen.

• Secretin: Secreted by the duodenum. Released in response to low pH (chyme entering the duodenum). Affects the pancreas (bicarbonate release, acid inhibition), and slows peristalsis.

• CCK: Secreted by cells in the duodenum and jejunum in response to lipids and their catabolites. Stimulates gallbladder contraction, pancreas, slowing gastric emptying, and the satiety center.

Other Digestive Hormones

• Somatostatin: Released by cells in gastric glands and the pancreas (and some hypothalamic cells). Functions to inhibit secretions of the stomach, duodenum, and pancreas, reducing nutrient absorption.

• VIP: Produces vasodilation in GI tract blood vessels during digestion in response to parasympathetic input.

Nutrient Absorption and Blood Flow

• Increased blood flow to the GI tract: Ensures adequate oxygen to working cells and nutrient absorption, facilitating plasma precursor for mucus secretions.

Regulation by Nervous Systems

• CNS and Autonomic Outflow: GI secretions are regulated via the CNS (autonomic outflow). Parasympathetic activity is generally more prominent than sympathetic during digestion.

• Enteric Nervous System (ENS): Moderates GI secretions and directly influences GI cells, extending along the length of the digestive tract composed of the Myenteric and Submucosal plexuses.

• Myenteric Plexus: Regulates smooth muscle contraction, causing peristalsis and sphincter control.

• Submucosal Plexus: Regulates secretions and blood flow throughout the digestive tract.

• Sensory Inputs: The plexuses detect mechanical (stretch, tension), thermal, osmotic, and chemical stimuli, transmitted to the CNS along afferent vagal fibers.

• Parasympathetic Nerves: The vagus nerve (and IX, VII) innervate most of the digestive tract. IX and VII innervate salivary glands, with sacral nerves innervating the colon.

• Sympathetic Innervation: Splanchnic nerves conduct sympathetic innervation to most of the GI tract, while superior cervical ganglion nerves innervate salivary glands.

• Digestive Reflexes: Digestive reflexes involve sensory input relayed to the CNS, which triggers motor responses.

Secretion from Various Digestive Organs

• Mouth: Saliva is the major secretion.

• Esophagus: Mucous (bicarbonate rich) lubricates and protects against acid erosion.

• Stomach: Secretes gastrin into the blood; hydrochloric acid, pepsinogen, mucous, gastroferrin, and intrinsic factor into the digestive tract.

• Small Intestine: Secretes secretin and CCK into the blood; mucous, lysozyme, defensins into the lumen.

• Colon: Secretes mucous rich in mucin.

• Pancreas: Secretes insulin, glucagon, somatostatin into the blood, pancreatic enzymes (lipase, amylase, ribonucleases) and inactive enzymes (trypsinogen, chymotrypsinogen, procarboxypeptidases A & B, pro-elastase) into the lumen, and bicarbonate-rich fluid (up to 2 liters/day).

• Liver: Synthesizes plasma proteins, VLDLs, and HDLs, releasing them into the blood, and secretes bile into the lumen. Bile contains bile salts/acids for lipid emulsification, and bilirubin from heme breakdown.