Introduction
Which 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: digestion and absorption.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 by the teeth to the creation of bile (an emulsifier1) 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 mouth 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 also know as 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 GI tract located within the mucosa. On the mucosa layer there are Villi and Micro Villi.

Submucosa: The submucosa is relatively thick, highly vascular, and serves the mucosa. The absorbed elements that pass through the mucosa are picked up from the blood vessels of the submucosa. The submucosa also has glands and nerve plexuses.
Muscularis: The muscularis is responsible for segmental contractions and peristaltic movement in the GI tract. The muscularis is composed of two layers of muscle: an inner circular and outer longitudinal layer of smooth muscle. These muscles cause food to move and churn with digestive enzymes down the GI tract.

Serosa: The last layer is a protective layer. It is composed of avascular connective tissue and simple squamous epithelium. It secretes lubricating serous fluid. This is the visible layer on the outside of the organs.

Accessory Organs

Teeth, Tongue, and Salivary Glands
1.Salivary glands
• Parotid gland, submandibular gland, sublingual gland
• Exocrine gland that produces saliva which begins the process of digestion with amylase
2. Tongue
• Manipulates food for chewing/swallowing
• Main taste organ, covered in taste buds
3. Teeth
• For chewing food up
4. Liver
• Produces and excretes bile required for emulsifying fats. Some of the bile drains directly into the duodenum and some is stored in the gall bladder.

• Helps metabolize proteins, lipids, and carbohydrates.
• Urea, chief end product of mammalian metabolism, is formed in liver from amino acids
and compounds of ammonia.
• Breaks down insulin and other hormones.
• Produces coagulation factors.
5. Gallbladder
• Bile storage.
6. Pancreas
• Exocrine functions: Digestive enzyme secretion.
• Stores zymogens (inactive enzymes) that will be activated by the brush boarder
membrane in the small intestine when a person eats protein (amino acids).
• Trypsinogen – Trypsin: digests protein.
• Chymotypsinogen – Chymotrypsin: digests proteins.
• Carboxypeptidases: digests proteins.
• Lipase-lipid: digests fats.
• Amylase: digests carbohydrates.
• Endocrine functions: Hormone secretion.
• Somatostatin: inhibits the function of insulin. Produced if the body is getting too
much glucose.
• Glucagon: stimulates the stored glycogen in the liver to convert to glucose. Produced
if the body does not have enough glucose.
• Insulin: made in the beta cells of the Islets of Langerhans of the pancreas. Insulin is a
hormone that regulates blood glucose.
7. Vermiform appendix
• There are a few theories on what the appendix does.
• Vestigal organ
• Immune function
• Helps maintain gut flora

The Digestive System

The first step in the digestive system can actually begin before the food is even in your mouth. When you smell or see something that you just have to eat, you start to salivate in anticipation of eating, thus beginning the digestive process. Food is the body’s source of fuel. Nutrients in food give the body’s cells the energy they need to operate. Before food can be used it has to be broken down into tiny little pieces so it can be absorbed and used by the body. In humans, proteins need to be broken down into amino acids, starches into sugars, and fats into fatty acids and glycerol. 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 in to the stomach.
• Chemical Digestion: several different enzymes break down macromolecules into smaller molecules that can be more efficiently absorbed. Chemical digestion starts with saliva and continues into the intestines. The digestive system is made up by the alimentary canal, or the digestive tract, and otherabdominal organs that play a part in digestion such as the liver and the pancreas. The alimentary canal is the long tube of organs that runs from the mouth (where the food enters) to the anus (where indigestible waste leaves). The organs in the alimentary canal include the mouth( for mastication),esophagus, stomach and the intestines. The average adult digestive tract is about thirty feet (30′) long. While in the digestive tract the food is really passing through the body rather than being in the body. The smooth muscles of the tubularV
digestive organs move the food efficiently along as it is broken down into absorb-able atoms
and molecules. During absorption, the nutrients that come from food (such as proteins, fats,
carbohydrates, vitamins, and minerals) pass through the wall of the small intestine and into
the bloodstream and lymph. In this way nutrients can be distributed throughout the rest
of the body. In the large intestine there is re absorption of water and absorption of some
minerals as feces are formed. The parts of the food that the body passes out through the
anus is known as feces.
Mastication
Digestion begins in the mouth. A brain reflex triggers the flow of saliva when we see or even
think about food. Saliva moistens the food while the teeth chew it up and make it easier to
swallow. Amylase, which is the digestive enzyme found in saliva, starts to break down starch
into simpler sugars before the food even leaves the mouth. The nervous pathway involved in
salivary excretion requires stimulation of receptors in the mouth, sensory impulses to the
brain stem, and parasympathetic impulses to salivary glands.
Swallowing your food happens when the muscles in your tongue and mouth move the food
into your pharynx. The pharynx, which is the passageway for food and air, is about five
inches (5″) long. A small flap of skin called the epiglottis closes over the pharynx to prevent
food from entering the trachea and thus choking. For swallowing to happen correctly a
combination of 25 muscles must all work together at the same time. Salivary glands also
produce an estimated three liters of saliva per day.

Source: Cray MI, Ch. 10 The Gastrointestinal System, Textbook of Human Physiology and Biophysics, V#1 . Atlanta Ga: IVMS 2014:259-64