Module 1: Introduction to Gastrointestinal (GI) Tract

LEARNING OBJECTIVES

• Describe the gastrointestinal (GI) tract
• Identify the parts of the GI tract and accessory organs
• Explain the functions of the digestive system
• Describe the basic structure of the GI tract wall
• Understand the regulation of GI functions
• Explain why barium contrast is used in GI imaging

MODULE

This module introduces the gastrointestinal (GI) tract, its structure, functions, and importance in barium contrast studies. Understanding the GI tract is essential before learning any barium contrast examination, because these procedures focus on visualizing and evaluating the GI organs using X-ray imaging.

OVERVIEW

The gastrointestinal (GI) tract is a vital body system responsible for digestion, absorption of nutrients, and elimination of waste. A clear understanding of the GI tract is essential before learning barium contrast examinations, since these procedures are designed to visualize and evaluate the GI organs using X-ray imaging.

The digestive system performs both mechanical and chemical processes to break down food into basic units that can be absorbed and used by the body.

LESSON 1

The digestive system:

• Take in food (ingestion)
• Break down food (digestion)
• Absorb nutrients and water (absorption)
• Remove waste from the body (defecation)

Food and body tissues are made of the same basic chemicals, but they are arranged differently. Digestion is needed to break food into small pieces so the body can rebuild them into tissues.

Medical Specialties

• Gastroenterology – study and treatment of diseases of the stomach and intestines
• Proctology – diagnosis and treatment of disorders of the rectum and anu

ORGANIZATION OF THE DIGESTIVE SYSTEM

The digestive system consists of two major groups of organs:

1. GASTROINTESTINAL (GI) TRACT / ALIMENTARY CANAL

• A continuous hollow tube from the mouth to the anus
• Passes through the thoracic and abdominopelvic cavities
• About 5–7 meters long in living adults (7–9 meters in cadavers)

PARTS OF THE GI TRACT

The GI tract is divided into two main parts:

• Upper gastrointestinal tract
• Lower gastrointestinal tract

The upper GI tract includes the:

• Mouth – chewing; saliva begins digestion
• Pharynx – passageway for food
• Esophagus – moves food to stomach by peristalsis
• Stomach – mixes food; begins protein digestion
• Duodenum – first part of small intestine

The lower GI tract includes the:

• Small intestine – main site of digestion and absorption
• Large intestine (colon) – absorbs water; forms stool
• Rectum – stores feces
• Anus – releases waste

2. ACCESSORY DIGESTIVE ORGANS

These organs help digestion but are not part of the GI tube:

• Teeth – physical breakdown of food into smaller pieces
• Tongue – chewing and swallowing
• Salivary glands – saliva
• Liver – produces bile
• Gallbladder – stores bile
• Pancreas – digestive enzymes

FUNCTIONS OF THE GI TRACT

The digestive system performs six basic processes:

• Ingestion – taking food into the mouth
• Secretion – release of enzymes, acid, mucus, and bile
• Mixing and propulsion – movement of food through the tract
• Digestion – mechanical and chemical breakdown of food
• Absorption – nutrients enter the blood and lymph
• Defecation – elimination of waste

BASIC STRUCTURE OF THE GI TRACT WALL

Most parts of the GI tract have four layers:

1. Mucosa (Inner Layer)

This is the innermost layer. It helps in digestion, absorption, and secretion.

Epithelium

• Mouth, pharynx, esophagus, and anal canal
  → covered by non-keratinized stratified squamous epithelium
  → protects against friction from food
• Stomach and intestines
  → covered by simple columnar epithelium
  → helps in secretion and absorption
• Contains enteroendocrine cells
  → these cells release hormones that control digestion

Lamina propria

• A soft connective tissue layer
• Contains blood vessels and lymphatic vessels
• Contains MALT (mucosa-associated lymphatic tissue)
  → helps protect the GI tract from infection

Muscularis mucosae

• A thin muscle layer
• Forms small folds
• These folds increase surface area for better digestion and absorption

2. Submucosa

A connective tissue layer that supports and binds the mucosa to the muscle layer

Contains:

• Blood vessels
• Lymphatic vessels
• Glands
• Lymphatic tissue
• Contains the submucosal plexus
  → a nerve network that controls secretion and blood flow

3. Muscularis (Muscle Layer)

This layer is responsible for movement of food.

• Mouth, pharynx, and upper esophagus
  → made of skeletal muscle
  → allows voluntary control (swallowing)
• Rest of the GI tract
  → made of smooth muscle
  → works automatically (involuntary)

Muscle layers

• Inner circular layer – squeezes food
• Outer longitudinal layer – pushes food forward
• Contains the myenteric plexus
  → nerve network that controls peristalsis and motility

4. Serosa / Adventitia (Outer Layer)

Serosa

• Outer protective covering
• Made of connective tissue and simple squamous epithelium

Adventitia

• Made of connective tissue only
• Found in the esophagus, which does not have a serosa

For short tip:

• Mucosa – absorption and secretion
• Submucosa – blood vessels, nerves, glands
• Muscular layer – smooth muscle for peristalsis
• Serosa / Adventitia – outer protective covering

These layers affect how the GI tract appears in contrast studies.

IMPORTANCE OF IMAGING IG TRACT

Diseases of the GI tract may cause symptoms such as:

• Difficulty swallowing
• Abdominal pain
• Vomiting
• Diarrhea or constipation
• Blood in stool

WHY IS BARIUM CONTRAST NEEDED?

The GI tract is made of soft tissue

Soft tissues are not clearly visible on a plain X-ray

Barium contrast studies allow healthcare professionals to:

• See the shape of the GI tract
• Observe movement (motility)
• Detect narrowing, blockage, or abnormal structures
• Evaluate swallowing and bowel function

FIG 2. Plain X-ray vs Barium contrast X-ray comparison

Plain X-ray	GI organs are poorly visible
X-ray with Barium	GI organs are clearly outlined and detailed

REGULATION OF GI FUNCTIONS

1. Neural (Nerve) Control

• Enteric nervous system – the “local brain” in the GI tract; controls digestion directly inside the gut.
• Autonomic nervous system – connects the GI tract to the brain and spinal cord; controls digestion automatically.

GI Reflexes (automatic responses)

• Control secretion (enzymes, acid, mucus) and the movement of food
• Help move the stool and control defecation
• Slow digestion when there is pain

2. Hormonal Control

• Local hormones – released by cells in the GI tract to help digestion
• External hormones – from other glands (like thyroxine or cortisol) also influence digestion

Tip

• Nerves → control movement and secretion
• Hormones → control speed and activity of digestion

DEFINITION OF TERMS

• Barium sulfate – Contrast agent used in GI studies
• Contrast media – A substance used to make organs visible on X-ray
• Digestion – The process of breaking down food
• Gastrointestinal tract – The tube where food is digested and absorbed
• Motility – Movement of food through the GI tract
• Peristalsis – Wave-like muscle movement