Anatomy of the Digestive System
The organs of the digestive tract include the oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and large intestine. The accessory digestive organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
The mouth is formed by the cheeks, lips, hard and soft palates, and tongue. The functions of the mouth are to ingest food, initiate digestion (mechanically through chewing; chemically through enzymes in the saliva), and pass the food into the pharynx. The teeth are used for crushing and grinding food and the tongue moves food around when chewing and also assists in swallowing food.
Most of the saliva is produced by three pairs of salivary glands (parotid, submandibular, and sublingual) outside of the oral cavity and is transported to the mouth via salivary ducts. Numerous minor salivary glands (called buccal glands) are located in the palatal region of the oral cavity. Besides digesting certain foods, saliva also functions as a solvent in cleansing the teeth.
The pharynx serves as a common passageway for both the respiratory and digestive system. When the food leaves the pharynx, it enters the esophagus.
The esophagus connects the pharynx to the stomach. It is a collapsible muscular tube about 10 inches long. At the end of the esophagus (before entering the stomach) is the gastroesophageal sphincter which acts like a valve to help prevent the stomach contents from going back into the esophagus. This is not considered a true sphincter because it does at times allow stomach contents to enter the esophagus. Some mammals, like rodents, have a true gastroesophageal sphincter and can not regurgitate (this is why poisoning is an effective measure in killing mice and rats).
The stomach is a J-shaped pouch that empties its content into the small intestine. It can be divided into three regions: the fundus, body, and pylorus. A pyloric sphincter prohibits backflow from the small intestine into the stomach. The functions of the stomach are to store food as it is mechanically churned with gastric secretions, initiate digestion of proteins, carry on limited absorption, and move food (a pasty material called chyme) into the small intestine.
The small intestine is about 1 inch wide and about 12 feet long (in a cadaver, it will measure about twice this length because the muscle wall is relaxed). It is divided into three regions: the duodenum, jejunum, and ileum. Contents pass from the ileum into the large intestine through the ileocecal valve. The small intestine serves as the major site of digestion and absorption in the digestive tract. Most chemical digestion takes place in the duodenum.
The large intestine is about 2.5 inches wide (thus giving it the name "large") and about 5 feet long. It is divided into four regions: the cecum, colon (ascending, transverse, and descending), rectum, and anal canal. The large intestine absorbs water and electrolytes from the chyme and passes waste products out of the body through the anal canal.
Liver, Gallbladder, and Pancreas
These three accessory digestive organs aid in the chemical breakdown of carbohydrate, protein, and fat. The liver produces and secretes bile into the gallbladder and small intestine. The pancreas (as an exocrine gland) secretes pancreatic juice into the duodenum.