GIT Standard

The Digestive Tract

• The digestive tract is a muscular tube responsible for breaking down food and absorbing nutrients.
• It begins with the mouth and ends at the anus.
• Main components: mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus.
• Accessory organs: salivary glands, liver, pancreas, gallbladder.

Digestive Tract Lining

• The digestive tract is lined with a mucous membrane for protection and secretion.
• This membrane is composed of simple columnar epithelium, which contains specialized cells for absorption and secretion.

Salivary Glands

• There are three pairs of salivary glands: parotid, submandibular, and sublingual.
• Saliva is produced by these glands and contains: water, electrolytes, mucin, and salivary amylase.
• Mucin lubricates food for easier swallowing.
• Salivary amylase begins the digestion of carbohydrates.

Swallowing

• The process of swallowing involves the mouth, pharynx, and esophagus.
• It is initiated voluntarily by collecting food on the tongue and propelling it towards the pharynx.
• After the voluntary stage, the swallowing reflex is triggered, moving food towards the esophagus.

Peristalsis

• This is a wave of smooth muscle contractions that propels food through the digestive tract.

Stomach

• The stomach is a J-shaped organ located in the upper abdomen.
• It has four regions: cardia, fundus, body, and pylorus.
• The lower esophageal sphincter regulates the passage of food into the stomach.
• The stomach churns and mixes food with gastric secretions.

Gastric Secretions

• The stomach secretes gastric juice, which contains hydrochloric acid, pepsin, and mucus.
• Hydrochloric acid activates pepsin, a protein-digesting enzyme.
• Mucus protects the stomach lining.

Phases of Gastric Secretion

• Cephalic phase: stimulated by sight, smell, or thought of food.
• Gastric phase: stimulated by food in the stomach, stretching the stomach walls.
• Intestinal phase: regulated by the small intestine, stimulated by chyme entering the duodenum.

Small Intestine

• The small intestine is the primary site for digestion and absorption of nutrients.
• It is divided into three sections: duodenum, jejunum, and ileum.
• It receives chyme from the stomach, pancreatic juice, and bile.

Large Intestine

• The large intestine absorbs water and electrolytes.
• It consists of the cecum, colon, rectum, and anus.
• The colon is further divided into four sections: ascending colon, transverse colon, descending colon, and sigmoid colon.

Pancreas

• The pancreas is both an endocrine and exocrine gland.
• Exocrine secretions include pancreatic juice, which contains enzymes like lipase, proteolytic enzymes, and amylase.
• These enzymes break down fats, proteins, and carbohydrates, respectively.

Liver

• The liver is a vital organ responsible for many metabolic processes.
• It produces bile, which aids in the digestion of fats.

Bile

• Bile is stored in the gallbladder and released into the small intestine.
• Bile salts emulsify fats, increasing their surface area and making them更容易被消化酶分解。

Hormones that Regulate Digestion

• Cholecystokinin (CCK): Stimulates the release of pancreatic enzymes and bile.
• Secretin: Stimulates the release of pancreatic bicarbonate and bile.
• Gastric inhibitory polypeptide (GIP): Inhibits gastric secretion and stimulates insulin release.
• Vasoactive intestinal peptide (VIP): Stimulates intestinal secretion and relaxes smooth muscle.
• Gastrin: Stimulates gastric acid secretion and motility.

Digestion and Absorption

• Carbohydrate digestion begins in the mouth with salivary amylase and continues in the small intestine with pancreatic amylase.
• Protein digestion begins in the stomach with pepsin and continues in the small intestine with pancreatic proteolytic enzymes.
• Lipid digestion occurs primarily in the small intestine with the aid of pancreatic lipase and bile salts.
• All three macronutrients are broken down into their monomers (monosaccharides, amino acids, fatty acids) for absorption.
• Nutrient absorption occurs primarily in the small intestine.
• Water absorption occurs in both the small and large intestines.
• Vitamin and mineral absorption occurs throughout the digestive tract.

Key Takeaways

• The digestive system is a complex organ system responsible for the breakdown of food and absorption of nutrients.
• It is composed of a muscular tube called the digestive tract and accessory organs, including the salivary glands, pancreas, liver, and gallbladder.
• The process of digestion involves mechanical and chemical breakdown of food.
• Digestive hormones play a crucial role in regulating the digestive process.
• Absorption of nutrients occurs primarily in the small intestine.