Digestive System Lesson: Anatomy & Function

Lesson Overview

• Digestive Processes
• Layers of the GI Tract
• Peritoneum and Mesenteries
• Oral Cavity and Salivary Glands
• Teeth and Support Structures
• Swallowing and the Esophagus
• Stomach Structure and Secretions
• Small Intestine and Villi
• Accessory Organs
• Large Intestine Functions
• Neural and Hormonal Regulation

The digestive system is responsible for breaking down food into nutrients that the body can absorb and use. It consists of a long alimentary canal (gastrointestinal tract) through which food passes, and various accessory organs that assist in digestion. The anatomy (structure) of each part of this system is closely related to its physiology (function).

In essence, food is taken in, broken down both mechanically and chemically, nutrients are absorbed, and waste is eliminated. The following sections provide a detailed look at the components of the digestive system and how they work together to digest food and maintain bodily function.

Digestive Processes

Food undergoes several key processes as it travels through the digestive system. Each process is vital to converting a bite of food into absorbable molecules:

• Ingestion: Intake of food into the mouth.
• Secretion: Digestive fluids and enzymes are released by glands.
• Mixing and propulsion: Muscular movements mix and push contents through the tract.
• Digestion: Physical and chemical breakdown of food.
• Absorption: Nutrients move into blood or lymph vessels.
• Defecation: Waste is expelled from the body.

Each section of the tract has a specific role in these processes.

Layers of the GI Tract

From the esophagus through the stomach, intestines, and rectum, the walls of the gastrointestinal tract share a common structural plan of four layers. Each layer has a distinct role in supporting digestion and absorption. The layers, listed from innermost (closest to the food in the lumen) to outermost, are:

The lamina propria provides support and contains MALT (mucosa-associated lymphoid tissue), important for immune defense. The muscularis propels food and helps with mixing. The serosa produces fluid to reduce friction.

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Digestive System Quiz: How Much Do You Know About The Human Digestive System?

Peritoneum and Mesenteries

The abdominal organs are covered and supported by folds of the peritoneum:

• Falciform ligament: Attaches liver to the front abdominal wall.
• Greater omentum: Hangs from stomach, containing fat and lymph nodes.
• Lesser omentum: Connects stomach and liver.
• Mesentery: Anchors the small intestine, carries vessels.
• Mesocolon: Anchors parts of the large intestine, contains vessels.

These structures provide physical stability and house nerves and vessels.

Oral Cavity and Salivary Glands

Digestion begins in the mouth:

• Teeth: Mechanically break food into smaller pieces.
• Salivary glands: Secrete saliva with enzymes like amylase.
• Tongue: Positions food and helps form the bolus.
• Palatopharyngeal arch: Forms boundary to the pharynx.
• Soft palate and uvula: Close off nasal passages during swallowing.

Teeth and Support Structures

• Deciduous molars are replaced by bicuspids during development.
• Periodontal ligament secures teeth in sockets.
• Gingivae protect the underlying tissues.

This arrangement enables effective mastication.

Swallowing and the Esophagus

Swallowing (deglutition) occurs in three phases:

• Oral phase: Voluntary movement by the tongue.
• Pharyngeal phase: Reflex phase, food passes through the pharynx.
• Esophageal phase: Involuntary peristalsis moves food to stomach.

The upper esophageal sphincter has skeletal muscle, allowing voluntary control.

Stomach Structure and Secretions

The stomach has distinct regions and secretory cells:

• Rugae: Allow stomach expansion.
• Parietal cells: Produce hydrochloric acid.
• Chief cells: Secrete pepsinogen for protein digestion.
• G cells: Secrete gastrin to stimulate gastric activity.

Food becomes chyme through churning and mixing with gastric juice.

Small Intestine and Villi

The small intestine is the major site of digestion and absorption:

• Villi: Projections increasing surface area for absorption.
• Microvilli: Form brush border, contain digestive enzymes.
• Lacteals and blood vessels: Absorb fats and other nutrients.

Peyer's patches, part of MALT, are found especially in the ileum.

Accessory Organs

• Liver: Produces bile to emulsify fats.
• Gallbladder: Stores and releases bile when needed.
• Pancreas: Secretes enzymes (amylase, lipase, proteases) and bicarbonate.

These organs deliver their secretions to the duodenum.

Large Intestine Functions

Key roles of the large intestine:

• Water and electrolyte absorption
• Vitamin synthesis by gut flora (e.g., vitamin K)
• Formation and storage of feces
• Defecation: Involves the external anal sphincter (skeletal muscle)

Peristalsis and bacterial fermentation occur here.

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Digestive system anatomy and physiology quiz questions with answers

Neural and Hormonal Regulation

Digestion is controlled by:

• Parasympathetic nerves: Stimulate digestion and secretions.
• Sympathetic nerves: Inhibit digestion during stress.
• Gastrin: Stimulates acid production.
• Secretin: Stimulates bicarbonate release.
• CCK (cholecystokinin): Stimulates bile and enzyme release.

These systems coordinate digestive phases across different organs.