GI Tract

The nostrils and lips transition between the outside of the head and the inside of the body, specifically - the upper aero-digestive tract.
The nose opening connects at the very back of the mouth. The front of the mouth is called the oral cavity, the back -is called the pharynx. Smell receptors at the top of the nose are important in stimulating appetite and providing taste sensation.
The salivary glands make saliva, the fluid that moistens the mouth and aids in swallowing food. Shown in yellow, there are 3 pairs of major salivary glands: one in each cheek, one on each side below the chin, under the jaw - and a pair of small ones on the floor of the mouth, towards the front (under the tongue).
Taste buds cover much of the upper surface of the tongue, and are also in the pharyx. The tonge and muscles of the cheek and pharynx moved the food down in swallowing.
The esophogus travels from the neck, through the chest, and so to the stomach in the abdomen, and has various layers of muscles that help in moving food from pharynx to stomach.
The stomach's thick muscular walls knead the food once it arrives, mashing it in acidic fluid (gastric acid) and dissolving it with the enzymes that are the first major step in digestion. Hormones made by the full stomach give a feeling of saity (feeling full & satisfoed), and the empty stomach makes grehlin - a hormone that stimulates hunger and appetite.
The Pancreas gland makes digestive juice that allows the small intestine to absorb food particles. Pancreatic juice is alkaline enough that it not only neutralizes the acid from the stomach, but raises the pH to alkaline levels.
The liver makes bile salts that are sent into the intestine through the bile ducts, and also receives the nutrient rich blood that drains from the intestine in the portal circulation. The small intestine includes the duodenum, ileum, and jejunum.
The gall bladder stores the bile and sends it in bursts into the intestine.
The colon (large intestine) receives the contents of the last part of the small intestine. By that time, most useable nutrients have already been absorbed. By the time the contents wind their way to the descending colon, water has been almost entirely absorbed as well - and the stool is formed.
The rectum holds stool for elimination.
The anus is a transition between the outside of the body and the rectum. A circular muscle allows elimination of stool to occur voluntarily.

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Gastrointestinal Tract GI Tract Gut Alimentary Tract
The gastrointestinal tract is an elaborate tube -one that starts at the mouth and ends at the anus. On the way, food and drink are taken in, and propelled downward. The entire digestive system works to deplete ingested meals of water and nutrients, converting foodstuffs into the simple organic substances that are easily absorbed through the tube's inner lining. Various wastes of the body are added to the remains of the food as it moves along. These wastes mix with the undigestible remnants of our meals, and both mix with micoroganisms shed from their berth in the lining of the lower part of the intestines, then exit through the anus as stool (feces). The digestive system is all about nutrition - but has other roles, as well. Our bodies not only contain living cells that require the nutrients absorbed in the gut to carry out metabolism, and keep us alive - our bodies also contain pools of gels and liquids that are composed of very exact proportions of salts (electrolytes) and water. In fact, although many of these streams and reservoirs (think bloodstream, lymph, and cerebrospinal fluid - as just a few examples) and gelatinous areas (like the lens of the eye) need to be a very particular consistency and make-up to be healthy, within each cell there are also pools of gels and liquids that must be kept just as pristine for the health of the cell. These requirements make the water and electrolyte content within the body critically important - and it is the GI tract that is the main provider of both to the body. Each of the way stations along the alimentary tract has functions and forms specific to that portion of the gut, but all work in a co-ordinated fashion - together. Physical relationships and chemical signal systems are intimately shared among them. And so, it is a rare ailment of one portion of the tract that does not strongly affect neighboring regions, or alter the workings of the digestive tract as a whole - unless that ailment is a very mild and transitory one. Further, since the central nervous system and major endocrine glands are bound up in nets of feedback loops with the gut at many levels, the activity of the gastrointestinal tract affects the body as a whole, and the activity of the person - on the whole, affects the gastrointestinal tract and digestion.
The nostrils and lips transition between the outside of the head and the inside of the body, specifically - the upper aero-digestive tract. The nose opening connects at the very back of the mouth. The front of the mouth is called the oral cavity, the back -is called the pharynx. Smell receptors at the top of the nose are important in stimulating appetite and providing taste sensation. The salivary glands make saliva, the fluid that moistens the mouth and aids in swallowing food. Shown in yellow, there are 3 pairs of major salivary glands: one in each cheek, one on each side below the chin, under the jaw - and a pair of small ones on the floor of the mouth, towards the front (under the tongue). Taste buds cover much of the upper surface of the tongue, and are also in the pharyx. The tonge and muscles of the cheek and pharynx moved the food down in swallowing. The esophogus travels from the neck, through the chest, and so to the stomach in the abdomen, and has various layers of muscles that help in moving food from pharynx to stomach. The stomach's thick muscular walls knead the food once it arrives, mashing it in acidic fluid (gastric acid) and dissolving it with the enzymes that are the first major step in digestion. Hormones made by the full stomach give a feeling of saity (feeling full & satisfoed), and the empty stomach makes grehlin - a hormone that stimulates hunger and appetite. The Pancreas gland makes digestive juice that allows the small intestine to absorb food particles. Pancreatic juice is alkaline enough that it not only neutralizes the acid from the stomach, but raises the pH to alkaline levels. The liver makes bile salts that are sent into the intestine through the bile ducts, and also receives the nutrient rich blood that drains from the intestine in the portal circulation. The small intestine includes the duodenum, ileum, and jejunum. The gall bladder stores the bile and sends it in bursts into the intestine. The colon (large intestine) receives the contents of the last part of the small intestine. By that time, most useable nutrients have already been absorbed. By the time the contents wind their way to the descending colon, water has been almost entirely absorbed as well - and the stool is formed. The rectum holds stool for elimination. The anus is a transition between the outside of the body and the rectum. A circular muscle allows elimination of stool to occur voluntarily.	The GI Tract is also called the Alimentary Canal. One way to think of it is as hollow tube, the opening within the tube is called the lumen. The walls of that tube are composed differently at different points along the alimentary canal, the stomach is different from the esophagus above it or the small intestine below it. Glands, including the salivary glands and the pancreas, and many others, empty their secretions into the lumen to process food. Much of the alimentary tract, which is also called the gut, is studded with lymphoid tissue. This lymphoid tissue provides immune defense against ingested germs. The liver manufactures bile for digestion, and also acts as a detoxification center - among other things. The nutrient rich blood that drains from the wall of the small intestine, which is the the main absorption area for foodstuffs , goes directly to the liver through a special circulation called the portal circulation. That's good for the body - as the liver can detoxify materials that might hurt other tissues. It's sometimes not so good for the liver - as substances like alcohol end up in much higher levels there than anywhere else. The diagram above flattens the elaborate 3-dimensional gastrointestinal tract into a 2-dimensional schema. Created by Mariana Ruiz, it is the clearest diagram that I have ever seen, even so, it is symbolic rather than truly representational.