by Andrew Lameiras
The digestive system is "the system that makes food possible to absorb into the body". Critical to good health, the digestive system allows humans to take in essential nutrients and materials through the digestion of the food they eat. Essentially, digestion can be divided into the taking in of the food (ingestion), the break down of that food (digestion), the intake of these broken down elements (absorption), and the elimination of the unused remains (defecation). How the food is broken down, where the food is used, and how it is used or stored are all aspects of metabolism. Metabolism is
"the chemical processes that occur within a living organism in order to maintain life". With both the digestive system and metabolism, humans can enjoy and utilize the food we eat everyday.




PART I: Anatomy and Physiology of the Digestive System

external image digestive-system-1_82.gif

Anatomy of the Digestive System

The digestive system is made up of two sets of organs: the alimentary canal and the accessory digestive organs. The alimentary canal is the continuous pathway food goes through, where it is taken in, broken down, absorbed, and eliminated. Accessory digestive organs aid in the processes of the alimentary canal.

Organs of the Alimentary Canal

The organs which make up the alimentary canal are: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. These organs make up a continuous tube of muscle which pushes food along as it is acted upon by the digestive forces of the organs. The entire alimentary canal is open at both ends, and spans a total of thirty feet if relaxed. The system is almost always tense, however, because of the powerful contractions of the organs involved. The alimentary canal is also known as the gastrointestinal tract.

Mouth

The mouth is the portal for food to enter the digestive system. This oral cavity is essentially made up of the lips, cheeks, tongue, palates, teeth, and uvula. The lips act as protection for the cavity, while the cheeks, palates, and tongue form the walls and roof of it. The uvula hangs from the roof of the mouth in the back of the oral cavity. The oral cavity proper is the area of the mouth containing the teeth and gums. The tongue is a single, powerful muscle attached to the floor of the mouth by the lingual frenulum.
The mouth works to masticate, or chew, food when it enters the digestive system. The teeth work to crush food as the walls and lips of the mouth keep food in place. The tongue aids in manipulating food in the mouth, while mixing it with saliva. The tongue also holds the taste buds which allow for our sense of taste. Thanks to these receptors, organisms have a sensual drive to want to eat certain foods.

Pharynx

After being masticated in the mouth, the wet ball of food, known as a bolus, is pushed into the pharynx during swallowing. The pharynx is made up of multiple parts, essential to both the intake of food and air. These parts include the oropharynx and the laryngopharynx.
The muscular walls of the pharynx work to propel food further through the digestive system. The use of alternating contractions of muscles to accomplish this is known as peristalsis.

Esophagus

From the pharynx, peristalsis forces food to the stomach through the esophagus. This multi-layered, muscular tube is approximately 10 inches long, and leads to the stomach.

Stomach

The esophagus leads to the stomach, the C-shaped digestive organ often associated with eating.The bolus of food from the esophagus passes through the cardioesophageal sphincter to enter the bulk of the stomach. The stomach is about ten inches long, and its diameter is determined by its capacity. Completely full, the stomach can hold a gallon of food. It is insulated by a tough layer of fat and mucus, protecting the organ from the corrosive acids inside of it. Once food has been thoroughly broken down, it passes through the pyloric sphincter in the form of chyme.
The stomach contains acids and enzymes, making up the gastric juice responsible for the digestion of many materials. For example, pepsin and hydrochloric acids work to break up proteins. These substances are often secreted by the glands of the body, such as the pancreas. Other substances are created by gastric glands, located int he gastric pits which line the stomach wall.

Small Intestine

From the pyloric sphincter, chyme is passed through the ileocecal valve into the small intestine. This organ is responsible for the bulk of digestion, and is a long muscular tube with an average length of anywhere from eight to eighteen feet while inside a human. To fit within the human body, the small intestine is tightly coiled, resting within the middle of the abdomen, surrounded by the large intestine. The small intestine is made up of three sections: the duodenum, the jejunum, and the ileum.
Almost all food absorption takes place in the small intestine. Thanks to its immense surface area, the small intestine is well suited to digest the food that passes through it. The villi of this organ take in the digested nutrients, sending them throughout the body via portal veins.

Large Intestine

The large intestine is the larger and thicker section of the lower digestive system which frames the small intestine. It is comprised of multiple subdivisions: the cecum, the appendix, the ascending, transverse, and descending colon, the sigmoid colon, the rectum, and the anal canal. The large intestine absorbs most of the water from the digested and extracted foodstuffs. With the water taken from it, the waste is defecated from the body as fecal matter, useless to the body.

Accessory Digestive Organs

Salivary Glands

The glands responsible for secreting saliva in the oral cavity are the parotid, submandibular, and sublingual salivary gland. Salivary amylase is found in saliva; it begins the digestion of carbohydrates ingested. Saliva also contains mucus and other serous fluids. These aid the creation of a bolus in the mouth, and prevent damage to the esophagus by dry and rough foodstuffs eaten.

Teeth

Teeth are sets of bone-like protrusions in the oral cavity responsible for the physical breakdown of foodstuffs. A tooth consists of a crown covered in tough enamel. The root f the tooth, which connects nerves from the body into these structures, is covered in cementum. Dentin comprises the bulk of the tooth, and gives it its durable and bone-like structure. The pulp cavity of the tooth contains the bulk of the nerves and blood vessels.

Pancreas

The pancreas is an important accessory organ which aids in the digestion of multiple types of foodstuffs. It lies in the mesentary between the stomach and small intestine. It excretes pancreatic juice into the duodenum. This juice contains enzymes in an alkaline fluid, capable of breaking down all types of food.

Liver