54 Digestive System Introduction

The digestive system is essentially a tube within our body, from mouth to anus. It includes the stomach and intestines, as well as accessory organs such as the liver, gall bladder and pancreas. The space within any tubular body structure, such as blood vessels or this digestive tract, is known as a lumen. During embryological development, embryonic cells (endoderm) forming the primitive yolk sac, turn outside-in (invaginates) forming the anal opening to the outside. This endoderm layer becomes the epithelial tissue lining of the future gastrointestinal (GI) tract and many associated organs. Later in the development process, the mouth opening breaks through the outer layer of embryonic cells (ectoderm) from the opposite side, meeting the endoderm. This ectoderm lines the mouth and forms the salivary glands. As a result, anything inside the space or lumen of the GI tract can technically be described as still being “external” to the body tissues. So, the caustic process of digestion is occurring in an “external” tube, without destroying the “internal” body tissues themselves. Then the products of digestion are absorbed from this “external environment” into the body cells and tissue fluids.

As early as the 2nd century, early anatomists understood the basic structure and important function of the digestive system. Many of the terms we still use for anatomy were developed then and during the medieval period, when they described the importance of the stomach and intestines for proper nutrition and digestion to maintain health. They also recognized the association of the liver and gall bladder to produce and store bile, although they didn’t correctly understand its physiology. It was identified as one of the four body fluids (blood, phlegm, yellow bile and black bile) that Hippocrates proposed were the physiological foundation for different human emotions and behaviors; thus they were called “humors”. The imbalance of these four humors was thought to be the cause of physical and mental illnesses. This and many of the early physiological theories have since been discarded. However, their idea that the stomach was an animate organ with ability to think or feel doesn’t seem totally ridiculous as you learn about how modern science is discovering the physiological importance of a specific part of the nervous system that resides entirely in our gut. By the renaissance period, physiologists were focusing their research on the chemical basis of digestion occurring inside the GI tract.

Overview of the Digestive System

Here is a preview of each of the modules to come:

• Structures and Functions, will explore how the major organs of the digestive system are able to accomplish system level functions:
  • movement through the length of the digestive system
  • complementary mechanisms for digestion in different segments
  • structural components enhancing efficient absorption
  • accessory organs that contribute secretions to the GI tract

Pay attention to not only what structures are similar and what are unique in the different sections, but how this contributes to the overall function of the system as a whole.

• Levels of Organization, will examine this structure and function in more detail, progressing through the major levels of organization in the human organism:
  • nutritive and digestive molecules
  • cells of the stomach, small and large intestine, liver, and pancreas
  • tissue layers that make up the GI tract, from mouth to anus

Pay attention to the function of cells for both secretion and absorption of specific substances in the different parts of the digestive tract and accessory organs. What these specific cells produce and how they move materials across cell membranes is critical to understanding the function of the system as a whole.

• Homeostasis, will delve into how this system contributes to the body’s natural tendency to maintain a stable internal environment:
  • how hormones and nerves control and coordinate the digestive system
  • what happens when digestive system malfunctions

Pay attention to how not only is the rate of movement from one section to another controlled, but what is released into the lumen is controlled based on monitoring of content of food eaten. Don’t get the endocrine secretion of hormones into the blood for control and regulation confused with the exocrine secretion of substances into the lumen for the digestion and absorption processes – both are happening.

• Integration of Systems, will investigate which systems are subsets of larger systems, and how they function together in harmony and conflict:
  • how digestive system interacts with the other body systems
  • how other systems affect the digestive system functions

Pay attention to the relationship between proper movement and secretion for digestion and the subsequent absorption and transport of materials for metabolism. This links the digestive tract and accessory digestive organs to the other systems.