Human Digestive System Overview

Chapter 3: Digestion, Absorption, and Metabolism

• This chapter covers the processes of digestion, absorption, and metabolism.

Digestion and Absorption

• Digestion is the process of breaking down food into smaller components that can be absorbed into the body.
• Absorption is the process of taking these substances into the interior of the body.
• The gastrointestinal (GI) tract is a hollow tube that aids in digestion and absorption.

Key Terms

• Digestion: the process of breaking down food into small enough components to be absorbed by the body
• Absorption: process of taking substances into the interior of the body
• GI Tract: A continuous hollow tube, consisting of the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus
• Transit Time: the amount of time it takes for food to pass through the GI tract, which ranges from 24-72 hours, depending on the type of food. Proteins take longer than carbohydrates.

Structure of the GI Tract

• Lumen: The inside portion of the GI tract, where food is still undigested
• Mucosa: The innermost layer of the GI tract lining, consisting of mucosal. Cells that provide protection and aid in absorption. They have a short lifespan and need nutrients to stay alive and produce mucus.
• Connective Tissue: A layer of tissue that provides structural support and helps with secretion and muscle contractions
• Smooth muscle: Layers of smooth muscle aid in peristalsis to move food through the tract

Overview of the Digestive System

• The mouth mixes food with saliva and breaks down food.
• The salivary glands create saliva.
• The pharynx swallows food mixed with saliva.
• The esophagus moves food to the stomach.
• The stomach churns and mixes food with acid and protein-digesting enzymes.
• The liver produces bile for fat digestion and absorption.
• The pancreas releases bicarbonate to neutralize substances and produces enzymes to digest carbohydrates, proteins, and fats.
• The gallbladder stores bile and releases it into the small intestine when needed.
• The small intestine completes digestion and absorbs nutrients.
• The large intestine absorbs water and some vitamins and minerals.
• The anus allows waste to leave the body.

Digestive Secretions

• Digestive secretions are the substances that assist with digestion in the GI tract.
• Mucus: A viscous material produced by cells in the mucosal lining of the gut helps moisten, lubricate, and protect the GI tract
• Enzymes: Protein molecules that speed up chemical reactions involved in digestion and do not change during the reactions.

Digestive Enzymes

• Enzymes are essential for carbohydrate, protein, lipid and other compound digestion
• Specific enzymes and their functions are detailed in the table.

Digestive Hormones

• Hormones coordinate and control digestive processes.
• Gastrin, somatostatin, secretin, CCK, and gastric inhibitory peptide are hormones that play different roles in digestion.

The GI Tract and Barrier Function

• Barrier function: protecting the GI tract from harmful substances absorption.
• The GI tract limits the absorption of toxins and disease-causing organisms due to the protective role of gastrointestinal and immune cells.

Absorption Mechanisms

• Several mechanisms enable nutrient absorption in the small intestine.
• Simple Diffusion: Movement of compounds from an area of high concentration to low concentration, without needing energy
• Osmosis: water moving to equalize the concentration of dissolved substances across a membrane, without energy use
• Facilitated Diffusion: Movement of molecules that cannot pass freely across cell membranes with the help of carrier. Molecules are moved from high to low concentration but need energy
• Active Transport: Movement of substances against their concentration gradient, requiring energy.

Small Intestine

• Contains villi and microvilli, increasing surface area for nutrient absorption
• The lumen is covered with villi and has microvilli.

Segmentation

• The alternating contraction and relaxation of muscles in the small intestine mix and break down food, maximizing exposure to enzymes for absorption.

Digestion of the Large Intestine

• Processes waste material and absorbs water and some vitamins
• Contains a section called the colon and the rectum
• Nutrients and fecal material may remain for longer periods of time in the large intestine.

Summary of Digestion

Detailed summary of carbohydrate, protein and lipid digestion.