Digestive System: Structure and Function (Chapter 23, Marieb Human Anatomy & Physiology)

The Digestive System

Overview of the Digestive System

The digestive system is responsible for breaking down food into nutrients, which are then absorbed into the blood and transported to cells throughout the body. It consists of the alimentary canal (gastrointestinal or GI tract) and accessory digestive organs.

• Alimentary canal (GI tract): A continuous muscular tube that digests food and absorbs nutrients. Includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

• Accessory digestive organs: Aid in the digestive process but are not part of the GI tract. Includes teeth, tongue, salivary glands, liver, pancreas, and gallbladder.

Key Terms:

• Digestion: The process of breaking down food into smaller molecules for absorption.

• Absorption: The movement of nutrients from the GI tract into the blood or lymph.

• Defecation: The elimination of indigestible substances as feces.

Main Organs of the Digestive System

Organ	Accessory digestive organ	Alimentary canal
Esophagus		✓
Mouth		✓
Small intestine		✓
Gallbladder	✓
Pancreas	✓
Stomach		✓
Large intestine		✓
Pharynx		✓
Tongue	✓
Liver	✓
Salivary glands	✓

Table: Classification of digestive system organs as accessory or part of the alimentary canal.

Major Processes of the Digestive System

Digestive Processes

The digestive system carries out six essential activities to process food:

1. Ingestion: Taking food into the mouth.

2. Propulsion: Moving food through the GI tract, including swallowing (voluntary) and peristalsis (involuntary, alternating waves of contraction and relaxation).

3. Mechanical breakdown: Physically preparing food for digestion by chewing, mixing, and segmentation (local constrictions of the intestine).

4. Digestion: Enzymatic breakdown of complex food molecules into their building blocks.

5. Absorption: Passage of digested end products from the lumen of the GI tract through mucosal cells into the blood or lymph.

6. Defecation: Elimination of indigestible substances from the body as feces.

Example: After eating a meal, food is chewed (mechanical breakdown), mixed with saliva (chemical digestion), swallowed (propulsion), digested in the stomach and intestines, nutrients are absorbed, and waste is eliminated.

Summary of Digestive Tract Activities

• Mechanical breakdown: Chewing (mouth), churning (stomach), segmentation (small intestine).

• Propulsion: Swallowing (oropharynx), peristalsis (esophagus, stomach, small and large intestines).

• Digestion: Enzymatic breakdown of food in the stomach and small intestine.

• Absorption: Nutrients and water absorbed in the small and large intestines.

• Defecation: Elimination of feces from the large intestine.

Key Concepts

• Alimentary canal: Also called the GI tract, it is the main pathway for food and waste.

• Accessory organs: Aid in digestion but are not part of the main tube.

• Peristalsis: Involuntary waves of muscle contraction that move food along the GI tract.

• Segmentation: Rhythmic local constrictions of the intestine that mix food with digestive juices.

Additional info: The digestive system is essential for providing the body with nutrients and energy, and its coordinated processes ensure efficient breakdown, absorption, and elimination of food substances.