- Digestion

The digestive system, also known as the Gastrointestinal Tract (GI) or the Alimentary Tract, is a long tube that is located between the mouth and the rectum.The digestive system is made up of several organs. Each organ is responsible for a specific task when digesting food. The organs that make up the digestive system are: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. There are also other organs called the accessory organs. The accessory organs consist of the liver, gall bladder, and pancreas.

The digestive system mixes, mashes, and breaks down foods into subunits that the body can use. Food when first eaten cannot reach your cells (because it cannot pass through the intestinal walls to the bloodstream and, if it could, it would not be in a useful chemical state. The job of the gut is to modify our food physically and chemically and dispose of the unusable stuff as waste.

Digestion begins as you take your first bite of food and ends as your body disposes of unusable products. The primary digestive organs do most of the work of digesting solid foods and the accessory organs help process the nutrients, by producing bile, and secreting digestive enzymes. The physical and chemical modification of your food depends on both the exocrine and endocrine secretions of the accessory organs and the controlled movement of food through the primary digestive tract.

Mouth - Food is first processed in the mouth, This process involves the teeth and tongue. Swallowing involves coordinated activity of tongue, soft palate pharynx and esophagus. The partially munched food moves down the esophagus.

Esophagus -The esophagus is a muscular tube approximately 20 cm (8 inches) in length that passes from the neck, behind the major blood vessels and heart in the chest, to penetrate the diaphragm (at the hiatus), and joins the stomach at another muscular valve (lower esophageal sphincter or LES) The LES is important in preventing Heart burn, a condition that occurs when acid from the stomach enters the esophagus and literally eats it away.

The muscle bands of the esophagus contract sequentially like a snake to propel food actively from top to bottom. Food does not simply fall down through the esophagus by gravity. Indeed it is possible to swallow food and fluid successfully while standing on your head!

Stomach - The stomach is C shaped, and on the left side of the abdominal cavity (because liver is on right). The stomach is a muscular bag, which can distend with large meals, and acts as the reservoir for food. Its movements help churn up the food into a soft thick soup. The stomach lining contains cells which produce acid and other digestive enzymes, which help break down the food. The acid also serves as an antiseptic, attacking the bacteria which we all swallow with our food.

The lining of the stomach is also rich with nerve endings. Some tell us when we are full, to reduce further food intake. Others stimulate the production of digestive juices needed for further food breakdown in the small intestine.The exit from the stomach is called the pylorus. This is a muscular valve which opens and closes to allow the partially digested food into the intestine for further digestion and absorption.

Small Intestine - The duodenum is the first part of small intestine. It is about 10" long and curves around head of pancreas. The Duodenum is also the entry point of the common bile duct. Chemical degradation of small controlled amounts of food begins here, as enzymes secreted by pancreas and duodenum itself are aided by emulsifying bile as the food is broken down. This process and absorption of most of the nutrients occurs and continues in the rest of the small Intestine; The Jejunum which is about 8 feet long and the Ileum (~10 feet long) continue the digestive process. The small intestine is where most of the absorption of digested food occurs. Simple sugars and amino acids are absorbed through intestinal epithelium and enter the blood stream to be used by cells.

Large Intestine - The illium (small intestine) terminates at the caecum and the beginning of the large intestine. The caecum is a small saclike evagination, important in some animals as a repository for bacteria/other organisms able to digest cellulose. The rest of the large intestine has three longitudinal muscle bands with bulges in the wall (haustra) between them.

The large intestine resorbs water then eliminates drier residues as feces. The different sections of your large intestine are the ascending colon, transverse colon, descending colon, then sigmoid colon which leads to the anus. The anus has voluntary and involuntary sphincters and also has the ability to distinguish whether contents are gas or solid.

Accessory digestive organs:

Pancreas - Is both an endocrine and an exocrine gland. The exocrine part of the pancreas produces many enzymes which enter the duodenum via the pancreatic duct and are the primary digestive enzymes for the entire digestive tract. The endocrine part produces insulin and Glucagon, the blood sugar regulators.

Liver and gallbladder - Bile, a watery greenish fluid is produced by the liver and secreted via the hepatic duct and cystic duct to the gall bladder for storage, and thence on demand via the common bile duct to an opening near the pancreatic duct in the duodenum. It contains bile salts, bile pigments, cholesterol, and phospholipids. Bile salts and phospholipids emulsify fats, the rest are just being excreted. Gallstones are usually cholesterol based, may block the hepatic or common bile ducts causing pain and or jaundice.

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