Digestive System Overview

The Digestive System Notes

Page 1

• Reference: Tortora, G. J. & Derrickson, B. H. (2008). Principles of Anatomy and Physiology, 13th Ed. John Wiley and Sons, Inc. Singapore.

Page 2: Outcomes

• Objectives: After studying this material, you should be able to:

  • Define digestion and absorption, understanding the distinction between the two processes that are crucial for nutrient availability. Digestion refers to the breakdown of food into smaller components, while absorption involves the uptake of these components into the bloodstream or lymphatic system.

  • List the functions of the Gastrointestinal (GI) tract, understanding how each function contributes to overall health and nutrition.

  • Identify the layers of the GI tract: mucosa, submucosa, muscularis, and serosa, detailing the role of each layer in digestion and nutrient absorption.

  • Identify components of the peritoneum, including the greater omentum and mesentery, and explain their significance in supporting abdominal organs and facilitating digestive processes.

  • Explain the composition of saliva and identify its origin, including the roles of different salivary glands.

  • Identify and explain the specific functions from the mouth to the stomach, detailing the processes of mechanical and chemical digestion that occur in these areas.

Page 3: Lecture Outline

• Topics included in the lecture:

  • Functions of the Digestive System, emphasizing the importance of each function in nutrient processing and bodily maintenance.

  • Layers of the GI tract, providing a deeper understanding of how structural differences affect function.

  • Peritoneum, discussing anatomical relationships and their clinical implications, such as peritonitis.

  • Digestive process from mouth to anus, outlining the journey of food and the transformation it undergoes.

  • Phases of digestion, including the cephalic phase, gastric phase, and intestinal phase, outlining hormonal and neural control mechanisms.

Page 4: Introduction

• Essential Functions of Food: Food provides necessary substances such as carbohydrates, proteins, fats, vitamins, and minerals, as well as energy for building cellular components and maintaining body functions, such as growth, repair, and metabolic activities.

• The digestive system is responsible for breaking down this food into small, absorbable nutrients for incorporation into cells, which supports life and health.

Page 5: Definitions

• Digestion: The mechanical and chemical breakdown of food into simpler molecules, enabling absorption into body cells. Mechanical digestion includes physical actions, while chemical digestion involves enzymatic reactions.

• Absorption: The process of transporting digested nutrients from the GI tract into the bloodstream or lymphatic system, occurring primarily in the small intestine.

Page 6: Functions of the GI Tract

• The GI tract and accessory organs perform six basic functions:

  1. Ingestion of food, which introduces nutrients into the GI system.

  2. Secretion (including water, acids, buffers, enzymes), which aids in breaking down food into absorbable molecules.

  3. Mixing and propulsion (motility), involving smooth muscle contractions that facilitate movement through the digestive tract.

  4. Digestion (mechanical & chemical), where food is broken down through various digestive processes.

  5. Absorption, where nutrients are taken up by the body from the GI tract.

  6. Defecation, the elimination of indigestible substances from the body.

Page 7: Mechanical Digestion

• Involves physical processes such as:

  • Chewing, which breaks larger pieces into smaller ones for easier digestion.

  • Mixing, which combines food with digestive juices to maximize efficiency.

  • Churning, particularly in the stomach, enhancing nutrient breakdown.

• These processes increase the surface area of food, facilitating chemical digestion by allowing enzymes to work more effectively, leading to better absorption of nutrients.

Page 8: Chemical Digestion

• Enzymes play a critical role in breaking down food into molecules small enough for absorption through the gut wall into the bloodstream or lymph. Important enzymes include:

  • Salivary amylase (in saliva) for starch digestion.

  • Pepsin (in gastric juice) for protein breakdown.

  • Lipases (from pancreas) for fat digestion.

Page 9: Overview of GI Tract Functions

• Mouth: Involves biting, chewing, and swallowing, beginning the digestive process.

• Pharynx & Esophagus: Functions primarily in transportation, facilitating movement of food towards the stomach through peristaltic movements.

• Stomach: Engages in mechanical and chemical digestion through muscular contractions and enzyme action, as well as absorption of water and alcohol.

Page 10: Continued GI Tract Functions

• Small Intestine: Engages in both chemical and mechanical digestion as well as absorption of nutrients into the bloodstream, featuring specialized structures like villi and microvilli that increase surface area.

• Large Intestine: Absorbs electrolytes and vitamins (B and K), and plays a key role in water reabsorption and forming feces.

• Rectum and Anus: Responsible for defecation, the final elimination of indigestible substances from the body.

Page 11: Layers of the GI Tract

1. Mucosa: The innermost layer that secretes enzymes and absorbs nutrients. It has specialized cells for selective uptake of substances.

2. Submucosa: Contains blood and lymphatic vessels as well as neurons that provide input for GI tract operations, part of the enteric nervous system.

3. Muscularis: Composed of smooth muscle, facilitating mixing and movement through segmentation (local contractions) and peristalsis (wave-like movements).

4. Serosa: The outer layer that secretes serous fluid to reduce friction, providing protection to underlying structures.

Page 12: Mucosa

• Functions:

  • Secretes digestive enzymes and hormones that regulate organ functions and digestion.

  • Contains goblet cells that produce mucus to protect intestinal walls and aid in the passage of food.

  • Plays a role in immune response through associated lymphoid tissues.

Page 13: Submucosa

• Contains:

  • Blood and lymphatic vessels that transport absorbed nutrients to the body.

  • Neurons that form the enteric nervous system, controlling GI tract movements and secretions efficiently.

Page 14: Muscularis

• Composed of:

  • Circular and longitudinal smooth muscle fibers that are responsible for contractions.

  • Controls mixing and propelling food through segmentation and peristalsis, essential for optimal digestion and absorption.

Page 15: Serosa

• A serous membrane that secretes fluid to reduce organ friction, ensuring smooth movement of organs against each other during digestive processes.

Page 16: Peritoneum

• Visceral Layer: Covers the organs and helps to support them.

• Parietal Layer: Lines the walls of the body cavity, contributing to the positioning of the organs.

• Peritoneal Cavity: Contains potential space with serous fluid that allows for movement and reduces friction between organs.

Page 17: Peritoneum Folds

• Lesser Omentum and Mesocolon are two key folds that support and stabilize abdominal organs.

• Mesentery: A double layer of peritoneum that binds the small intestine to the posterior abdominal wall and supports blood vessels, lymphatics, and nerves that supply the intestines.

Page 18: Greater Omentum & Mesentery

• Greater Omentum: Drapes over the intestines like a "fatty apron," containing adipose tissue that may play roles in energy storage, immune response, and inflammation regulation.

Page 20: Salivary Glands

• Salivary glands include:

  • Parotid: Largest, secreting a strong enzyme-rich saliva.

  • Submandibular: Produces both mucus and serous fluid, aiding digestion.

  • Sublingual: Mostly produces mucus-rich saliva for lubrication.

• Function: Produce saliva that aids in chemical digestion in the mouth, moistening food for easier swallowing and beginning the enzymatic breakdown of carbohydrates.

Page 21: Composition of Saliva

• Saliva is 99.5% water, containing:

  • Mucus for lubrication that facilitates swallowing.

  • Bicarbonate ions to buffer acidic foods, maintaining a neutral environment for enzymes.

  • Enzymes like salivary amylase and lysozyme that digest starch and destroy bacteria respectively, contributing to oral health.

Page 22: Salivation

• Increased Salivation: Triggered by:

  • The sight, smell, sound, and memory of food or stimulation of the tongue, primarily through the parasympathetic nervous system.

• Reduced Salivation: May occur during fear or anxiety, mediated by sympathetic nervous system activation, leading to a dry mouth condition, which can affect digestion.

Page 23: Pharynx

• A funnel-shaped tube extending from the nasal cavity to the esophagus and larynx, crucial for both digestive and respiratory pathways.

• Consists of skeletal muscle and is lined by a mucous membrane, aiding in swallowing and preventing food from entering the airway.

• Divisions:

  • Nasopharynx (respiration only)

  • Oropharynx & Laryngopharynx (both digestive and respiratory functions) that facilitate the movement of food into the esophagus.

Page 24: Esophagus

• A muscular tube that connects the pharynx to the stomach, facilitating the passage of food through coordinated muscular contractions called peristalsis.

• Location: Anterior to vertebrae, and posterior to the trachea and heart, containing mucosal sphincters at the upper and lower ends to prevent reflux.

Page 25: Stomach

• Components of the stomach include:

  • Regions: Cardia, Body, Pylorus, and the Pyloric Canal, each serving distinct digestive functions.

  • Function: Acts in both mechanical and chemical digestion, mixing food with gastric juices, while also absorbing certain substances like water and alcohol, setting the stage for further digestion in the small intestine.

• More details to follow in the next section.