Digestive System Review

Stomach

• Functions: The stomach is responsible for mechanical and chemical digestion. It temporarily stores food and releases acid (HCl) and enzymes for digestion.

• Regions: Divided into four main parts: 1) Cardia, 2) Fundus, 3) Body, 4) Pylorus. Gastric rugae allow for expansion as it fills.

• Gastric Secretions: Produced by five types of cells:

  • Surface mucous cells: Secrete mucous to protect the stomach lining.

  • Mucous neck cells: Produce neck mucous.

  • Parietal cells: Secrete HCl and intrinsic factor.

  • Chief cells: Produce pepsinogen that activates to pepsin for protein digestion.

  • G cells: Produce gastrin, a hormone that stimulates gastric acid secretion.

• Motility: Gastric mixing turns the bolus into chyme. Gastric emptying is regulated by the pyloric sphincter.

Phases of Stomach Function

1. Voluntary Phase: Initiated when food is swallowed.

2. Pharyngeal Phase: Involuntary closure of the nasal passages begins the swallowing reflex.

3. Esophageal Phase: The bolus is moved down to the stomach via peristalsis.

Breakdown of Nutrients

• Carbohydrates: Begins in the mouth with salivary amylase and continues with pancreatic enzymes in the small intestine, broken down into monosaccharides.

• Proteins: Chemically digested in the stomach by pepsin, and further broken down by pancreatic enzymes in the small intestine.

• Lipids: Lipid digestion is initiated by lingual and gastric lipases. Primary breakdown occurs in the small intestine by pancreatic lipase.

Hormones and Their Functions

• Cholecystokinin (CCK): Released by the small intestine in response to fat and protein; stimulates bile release from the gallbladder and pancreatic enzyme secretion.

• Insulin: Secreted by the pancreas; helps in glucose uptake.

• Glucagon: Also from the pancreas; raises blood glucose levels by promoting gluconeogenesis.

• Gastrin: Secreted by G cells in the stomach; stimulates gastric acid secretion.

Accessory Organs

• Salivary Glands: Produce saliva containing enzymes like salivary amylase to begin digestion in the mouth.

• Pancreas: Produces digestive enzymes and bicarbonate to neutralize stomach acid in the small intestine. Regulates blood sugar with insulin and glucagon.

• Liver: Produces bile for fat digestion and stores nutrients.

• Gallbladder: Concentrates and releases bile into the duodenum for fat emulsification and digestion.

Chart of Structures and Functions of the GI Tract

• GI Tract: Includes oral cavity, pharynx, esophagus, stomach, small intestine, large intestine.

  • Oral cavity: Initial digestion with saliva.

  • Esophagus: Transport food to stomach.

  • Stomach: Digestion and storage.

  • Small intestine: Major site for nutrient absorption.

  • Large intestine: Absorption of water and formation of feces.

Story of the GI Tract

The gastrointestinal tract (GI tract) forms a continuous tube from the mouth to the anus, effectively processing food, absorbing nutrients, and eliminating waste. Each organ plays a specific role in the overall digestive process — from initial ingestion in the oral cavity to enzymatic breakdown in the stomach and small intestine, then absorption of nutrients followed by waste removal in the large intestine.