The digestive system, which extends from the mouth to the anus, is responsible for receiving food, breaking it down into nutrients (a process called digestion), absorbing the nutrients into the bloodstream, and eliminating the indigestible parts of food from the body. The digestive tract consists of the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus. The digestive system also includes organs that lie outside the digestive tract: the salivary glands,the liver, pancreas, and the gallbladder.
The mouth ( oral cavity) is the first structure through which food passes in the process of digestion. Saliva, excreated into the mouth from adjacent salivary glands, is slightly acidic and is used to soften and begin chemically breaking down the food. The 32 teeth in the average adult's mouth work to mechanically break down the food.
Teeth are classified mainly by their shape and location. There are three categories of teeth:
The incisors- These are the teeth in the front of the mouth. They are shaped like chisels and are useful in biting off large pieces of food. Each person has eight of these - four on the top, four on the bottom.
The cuspids- These are the pointy teeth immediatly behind the incisors. Also called the canines, these teeth are used for grasping or tearing food. Each person has four of these - two on the top and two on the bottom.
The molars- These are flattened teeth used for grinding food. They are the furthest back in the mouth and their number can vary between people.
The pharynx (FAR - inks) is a tube that connects the mouth to both the larynx and the esophagus. It does not actively engage in the digestion process, but does aid in swallowing.
The esophagus is the tube that connects the pharynx to the stomach. It is entirely made of muscle and is between 23-25 cm (10 in). The esophagus is located directly behind the trachea and pierces the diaphragm on its way to the stomach.
The stomach (STUH- mak) is a C-shaped enlargement of the gastrointestinal (GI) tract, located directly under the diaphragm. The upper part of the stomach is a continuation of the esophagus and the lower part empties digested food into the duodenum (doo- oh- DEE- num), the first part of the small intestine. The stomach in a normal adult, when empty, is about the size of a large sausage, but varies in size when you breathe. The stomach ends with the pyloric sphincter (SFINK- ter), a valve which regulates the release of food from the stomach, to the small intestine.
The small intestine is where most of the digestion and absorption occur. The small intestine is a long tube about 6.35 m (21 ft) long and 2.5 cm (1 in) in diameter. It is divided into three sections; the duodenum (doo- oh- DE- num) which is about 25cm (10 in) long, the jejunum (jee- JOO- num) in the middle which is about 1 meter (3 ft) long, and finally the ileum (IL- ee- um) which measures 2 m (6 ft) in length. The small intestine begins at the pyloric sphincter and coils through the central and lower part of the abdomen where it eventually opens into the large intestine by way of the ileocecal (il- ee-oo- SEE- kal) sphincter.
The large intestine - begins with the ileocecal sphincter and coils up the right side of the abdomen, across the back and down the left side of the abdomen where it connects to the rectum and ends with the anus. It serves to manufacture certain vitamins, complete absorption and form and expel feces from the body. The large intestine is about 1.5 m (5 ft) long and 6.5 cm (2.5 in) in diameter and is divided in to four primary regions; the cecum (SEE- kum), colon, rectum and anal canal. The cecum is a hidden, dilated pouch, about 6 cm (2.5 in) long, located slightly below the ileocecal valve.
The colon makes up most of the large intestine and is itself divided into four regions - the ascending, transverse, descending and sigmoid portions. The ascending colon is the part moving up the right side of the body. It is connected to the transverse colon, the part of the large intestine that travels from the right to the left side of you body. The curve formed between the ascending and transverse regions of the colon is called the right colic (hepatic) flexure. The transverse colon connects on the left side of the body to the descending colon, where it forms another curve called the left colic (splenic) flexure. The sigmoid colon is located low in the abdomen. It connects to the descending colon at the left side of the body and streches to the middle of the body where it meets the rectum.
The rectum composes the last 20 cm (8 in) of the large intestine. It is located just in front of the sacrum and coccyx bones. The last 2 to 3 cm (~1 in) of the rectum is called the anal canal. The opening of the anal canal to the exterior of the body is known as the anus. The internal and external anal sphincters guard this opening.
The liver is the largest gland of the body and weighs about 1.3 kg (3.5 - 4.0 lbs) in an adult. It is located directly below the diaphragm. The liver is one of three accessory digestive organs that aid in the chemical breakdown of food. It produces and secretes bile into the gallbladder and small intestine and also receives blood from the intestine. This gland is reddish-brown in color because of its great vascularity.
The pancreas (PAN- cree - es) is a gland which aids in digestion. For more detail of its functions, please consult the endocrine system. It secretes pancreatic juice through the pancreatic duct into the duodenum. The pancreatic juice is secreted by specialized cells called acinar cells. These cells are located in clusters in the lobes of the pancreas. Each cluster is called an acinus or acini (plural). The pancreas is positioned horizontaly along the posterior curvature of the stomach. It is about 12.5 cm (6 in.) long and 2.5 cm (1 in.) thick. It has a an expanded head, centrally located body and a tapering tail.
The gall bladder is a sac-like about 7 to 10 cm (3 to 4 in) long. It is located in a depression on the underside of the liver. This organ stores and concentrates bile, which drains to it via the bile ducts , hepatic duct and cystic duct . The gall bladder receives blood from the cystic artery which branches from the right hepatic artery. The wall of the gall bladder consists of an inner layer of epithelium (skin) called the mucosal layer. The mucosal layer secretes mucous to protect the gall bladder from the basic (pH 7.6 -8.6) bile it stores. The mucosal layer is arranged into rugae (folds or ridges) similar to the stomach. Beneath the mucosal layer, there is a layer of smooth muscle.
A sphincter valve and the neck of the gall bladder allows a storage capacity of about 35 to 50 mL. When the gall bladder fills with bile, it expands to the size and shape of a small pear. Bile is continuously produced by the liver and drains to the duodenum (duo- DE- num) by way of the hepatic and common bile ducts. When the duodenum is empty, the bile is forced back up the cystic duct to the gall bladder for storage. After a meal, various stimuli cause contraction of the gall bladder and bile is released back into the common bile duct. Bile is partially an excretory product and partially a digestive secretion. It is a yellowish-green fliud composed of bile salts, bilirubin, cholesterol, and other compounds. Bile salts are used to assist in the breakdown of fat globules. Gallstones result when there are not enough salts in the bile. Gallstones can sometimes be treated with a bile salt, but often surgery is required to remove them.
Figure 21-2a: ANATOMY SUMMARY: The Digestive System