Digestive System: Anatomy and Functions

Digestive System

Main Function

The main function of the digestive system is the digestion and absorption of nutrients.

Layers of the Digestive Tract

• Mucosa: Formed by different types of epithelium depending on the zone.
• Muscularis Mucosa: Smooth muscle tissue, predominantly circular.
• Submucosa: Formed by connective tissue, blood vessels, and nerves.
• Muscularis: Formed by 2 or 3 layers of muscle tissue.
• Adventitia/Serosa: Consists of loose connective tissue or peritoneum depending on the zone.

Parts of a Tooth

• Root: Within the alveolar bone cavity.
• Crown: External, visible part of the tooth.
• Neck: Separates the root from the crown.
• Pulp Chamber: Formed by pulp, connective tissue, blood vessels, and nerves.
• Dentin: Hard tissue surrounding the pulp, covered by enamel in the crown and cementum in the root.

Parts of the Pharynx

• Nasopharynx: Communicates with the nasal cavity.
• Oropharynx: Communicates with the mouth.
• Laryngopharynx: Communicates with the larynx and esophagus.

Functions of the Pharynx

The pharynx facilitates swallowing through esophageal glands that secrete mucus. This mucus keeps the esophagus moist, allowing food to slide easily into the stomach.

Regions of the Stomach

• Cardia: Physiological sphincter that regulates the entry of food and prevents reflux into the esophagus.
• Fundus: Located to the left of the cardia.
• Body: Extends from the cardia to the lesser curvature. Area of major activity.
• Antrum: Located on the lesser curvature near the pylorus.
• Pylorus: Muscular sphincter that separates the stomach from the duodenum.

Small Intestine

Approximately 6 meters long, located in the central portion of the abdomen.

• Duodenum: Tube from the pylorus to the angle of Treitz. Receives bile from the liver and gallbladder, and pancreatic juice from the pancreas. The common bile duct and pancreatic duct join at the ampulla of Vater, regulated by the sphincter of Oddi.
• Jejunum and Ileum: Second and third portions of the small intestine. The ileum is more vascularized and ends at the ileocecal valve, which regulates the passage of chyme from the ileum to the cecum of the large intestine.

Role of Intestinal Villi

Intestinal villi increase the surface area for contact between the small intestine and nutrients, enhancing absorption.

Large Intestine

The last portion of the digestive tract.

• Cecum: Bag-shaped structure with the appendix. The appendix has no known digestive function but is involved in immunological processes.
• Colon: Divided into ascending, transverse, descending, and sigmoid portions. It ends at the rectum.
• Rectum: Last portion of the large intestine, ending at the anus. It contains two sphincters: the internal anal sphincter (smooth muscle, involuntary) and the external anal sphincter (striated muscle, voluntary).

Functions of the Large Intestine

• Absorption of water
• Elimination of waste products
• Supports the activity of bacteria that digest proteins and other cells, producing vitamin K, B12, thiamine, riboflavin, and gases.

Salivary Glands

Function

To produce and secrete saliva into the oral cavity.

Classification

• Major Salivary Glands: Parotid, submandibular, sublingual.
• Minor Salivary Glands: Smaller and more numerous, located on the tongue, lips, and inside of the cheeks.

Liver

Functions

• Produces and secretes bile to emulsify fats for easier digestion.
• Stores glycogen, vitamins (A, D, E, K, B12), minerals (iron, copper), and folic acid.
• Synthesizes proteins, fats, and carbohydrates.
• Metabolizes various substances, including medications and bilirubin.

Gallbladder

Function

Stores and concentrates bile.

Pancreas

Functions

• Endocrine Gland: Produces hormones like insulin and glucagon.
• Exocrine Gland: Secretes digestive enzymes and bicarbonate.

Function of Insulin

Reduces blood glucose levels (hypoglycemic effect). Promotes the utilization of carbohydrates by the body.

Function of Glucagon

Increases blood glucose levels (hyperglycemic effect). Mobilizes glycogen into glucose.

Gastrointestinal Processes in the Mouth

• Chewing: Performed by the teeth, tongue, and cheeks to mechanically break down food.
• Chemical Digestion: Initiated by saliva secreted by the salivary glands.
• Swallowing: The passage of chewed and moistened food (bolus) from the mouth to the pharynx.

Enzymes in Saliva

• Serous (Watery) Secretion: Contains ptyalin (salivary amylase) that acts on carbohydrates.
• Mucous Secretion: Lubricates food and the oral mucosa, protecting it and facilitating swallowing.

Functions of Saliva

• Protects the oral mucosa
• Cleans teeth
• Begins digestion of carbohydrates through amylase
• Helps form the food bolus
• Lubricates the oral cavity and pharynx
• Dissolves chemical substances in food, facilitating taste perception
• Has an immune function

Process of Swallowing

The process of moving the bolus from the mouth to the stomach consists of three stages:

1. Stage 1: Food moves from the mouth to the pharynx. Food moistened with saliva forms the bolus and is pushed into the oropharynx.
2. Stage 2: The nasopharynx is blocked, preventing the bolus from entering the respiratory tract. The epiglottis covers the larynx as the bolus enters the laryngopharynx.
3. Stage 3: The bolus moves through the esophagus into the stomach. Peristaltic waves (involuntary contractions) propel the bolus through the esophagus. Mucous glands in the esophagus lubricate and protect the esophageal mucosa, facilitating the movement of the bolus to the stomach.

Functions of the Stomach

• Stores food from the swallowing process.
• Mixes food with gastric secretions to form chyme.
• Secretes hydrochloric acid, water, intrinsic factor, and enzymes (pepsinogen and gastrin).
• Regulates emptying of chyme into the duodenum through the pylorus.

Components of Gastric Juice

• Mucus: Protects the stomach lining.
• Hydrochloric Acid: Softens proteins, eliminates microorganisms.
• Intrinsic Factor: Aids in the absorption of vitamin B12.
• Enzymes:
  • Pepsinogen (converted to pepsin): Digests proteins into peptides.
  • Gastric Lipase: Digests fats or lipids.