Page 2 - 86 human physiology part-2

The Gastrointestinal System

12 THE GASTROINTESTINAL SYSTEM

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Introduction

W hich organ is the most important organ in the body? Most people would say the heart or the

brain, completely overlooking the gastrointestinal tract (GI tract). Though definitely not the
most attractive organs in the body, they are certainly among the most important. The 30+ foot long tube
that goes from the mouth to the anus is responsible for the many different body functions which will be
reviewed in this chapter. The GI tract is imperative for our well being and our life-long health. A non-
functioning or poorly functioning GI tract can be the source of many chronic health problems that can
interfere with your quality of life. In many instances the death of a person begins in the intestines.

The old saying "you are what you eat" perhaps would be more accurate if worded "you are what
you absorb and digest". Here we will be looking at the importance of these two functions of the
digestive system: absorption and digestion.

The Gastrointestinal System is responsible for the breakdown and absorption of various foods
and liquids needed to sustain life. Many different organs have essential roles in the digestion of food,
from the mechanical disrupting of the teeth to the creation of bile (an emulsifier) by the liver. Bile
production of the liver plays a important role in digestion: from being stored and concentrated in the
gallbladder during fasting stages to being discharged to the small intestine.

In order to understand the interactions of the different components we shall follow the food on its
journey through the human body. During digestion two main processes occur at the same time.

• Mechanical digestion: larger pieces of food get broken down into smaller pieces while
being prepared for chemical digestion. Mechanical digestion starts in the mouth and continues
into the stomach.
• Chemical digestion: starts in the stomach and continues into the intestines. Several different
enzymes break down macromolecules into smaller molecules that can be absorbed.

The GI tract starts with the mouth and proceeds to the esophagus, stomach, small intestine
(duodenum, jejunum, ileum), and then to the large intestine (colon), rectum, and terminates at the anus.
You could probably say the human body is just like a big donut. The GI tract is the donut hole. We will
also be discussing the pancreas and liver, and accessory organs of the gastrointestinal system that
contribute materials to the small intestine.

Layers of the GI Tract

The GI tract is composed of four layers, or tunics. Each layer has different tissues and functions.
From the inside out they are called: mucosa, submucosa, muscularis, and serosa.

Mucosa: The mucosa is the absorptive and secretory layer. It is composed of simple epithelium
cells and a thin connective tissue. There are specialized goblet cells that secrete mucus throughout the

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