Digestion, Absorption, and Transport: Processes and Organs in Human Nutrition

Chapter 3: Digestion, Absorption, and Transport

Introduction

This chapter explores the fundamental processes of digestion, absorption, and transport of nutrients in the human body. Understanding these processes is essential for students of nutrition, as they form the basis for how the body utilizes food for energy, growth, and maintenance.

Learning Outcomes

• Describe the processes and organs involved in digestion.

• Explain how food is propelled through the gastrointestinal tract.

• Discuss the role of enzymes and other secretions in chemical digestion.

• Describe how digested nutrients are absorbed.

• Explain how hormones and the nervous system regulate digestion.

• Explain how absorbed nutrients are transported throughout the body.

• Describe the most common digestive disorders. (Additional info: Digestive disorders are covered in later sections.)

Processes and Organs Involved in Digestion

Key Processes

• Digestion: The process of breaking down food into individual molecules small enough to be absorbed through the intestinal wall.

• Absorption: The movement of nutrients from the gastrointestinal (GI) tract into the circulatory system.

• Transport: The movement of absorbed nutrients throughout the body via the circulatory and lymphatic systems.

• Elimination: The excretion of undigested and unabsorbed food through feces.

Organs of the Digestive System

• Mouth: Begins mechanical and chemical digestion.

• Pharynx and Esophagus: Transport food to the stomach.

• Stomach: Mixes and breaks down food with gastric juices.

• Small Intestine: Major site for digestion and absorption; consists of the duodenum, jejunum, and ileum.

• Large Intestine: Absorbs water and some nutrients; forms and eliminates feces.

• Rectum: Stores feces before elimination.

Accessory Organs

• Salivary Glands: Produce saliva containing enzymes for carbohydrate digestion.

• Liver: Produces bile, processes nutrients, and detoxifies substances.

• Gallbladder: Stores and concentrates bile, releasing it into the small intestine.

• Pancreas: Secretes digestive enzymes and bicarbonate into the small intestine; regulates blood glucose.

Summary Table: Main Organs and Functions

Organ	Main Function
Mouth	Mechanical and chemical digestion begins
Esophagus	Transports food to stomach
Stomach	Mixes food, secretes acid and enzymes
Small Intestine	Digestion and absorption of nutrients
Large Intestine	Absorbs water, forms feces
Liver	Produces bile, processes nutrients
Gallbladder	Stores and releases bile
Pancreas	Secretes digestive enzymes and hormones

Example: Path of Food Through the GI Tract

Food enters the mouth, is chewed and mixed with saliva, passes through the pharynx and esophagus, enters the stomach for further breakdown, moves into the small intestine for digestion and absorption, and finally enters the large intestine where water is absorbed and waste is eliminated.

Additional info: The GI tract is approximately 23 feet long in adults, and its structure is specialized for efficient digestion and absorption.