Anatomy of Digestive System

            THE ANATOMY OF DIGESTIVE SYSTEM

The human digestive system, (also known as the GI tract, or the alimentary canal) is a series of connected organs leading from the mouth to the anus. The digestive system allows us to break down the food we eat to obtain energy and nourishment.

The alimentary canal begins at the mouth, passes through the thorax, abdomen, and pelvis and ends at the anus. It is thus a long tube through which food passes.

The digestive system -- which can be up to 30 feet in length in adults -- is usually divided into eight parts: The parts include:

The Oral cavity or mouth, the esophagus, the stomach, the small intestine and the large intestine with the salivary gland, liver, pancreas, and gallbladder adding secretions to help digestion. These organs combine to perform six tasks: ingestion, secretion, propulsion, digestion, absorption, and defecation.

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Now, let’s see the structure in detail.

Oral Cavity

The mouth is the first part of the upper GI tract and is equipped with several structures that begin the first process of digestion. These include salivary glands, teeth, tongue hard palate and the soft palate.

The Teeth: The teeth are embedded in the sockets of the alveolar ridges of the mandible and the maxilla. Teeth are used for mastication. Each individual has two sets, the temporary and the permanent teeth. There are 20 temporary teeth, 10 in each jaw. They begin to erupt when the child is about 6 months old, and the permanent teeth begin to replace the Temporary teeth in the 6th year of age and this dentition, consisting of 32 teeth, is usually complete by the 24th year.

On each side of each dental arch there are 2 incisors, 1 canine, 2 premolar and 3 molar, for a total of 32. The teeth are named after their particular roles in the process of mastication—incisors are used for cutting or biting off pieces of food; canines, are used for tearing, premolars and molars are used for chewing and grinding.

Teeth are made of a bone-like material called dentin, which is covered by the hardest tissue in the body—enamel.

The Tongue: The tongue is a fleshy and muscular sensory organ, The tongue is attached to the floor of the mouth by a ligamentous band called the frenulum and this gives it great mobility for the manipulation of food (and speech);   The tongue plays an important part in:

• mastication (chewing)
• deglutition (swallowing)
• speech
• taste

The roof of the mouth is termed the palate and it separates the oral cavity from the nasal cavity. The palate is hard at the front of the mouth since the overlying mucosa is covering a plate of bone; it is softer and more pliable at the back being made of muscle and connective tissue, and it can move to swallow food and liquids. The soft palate ends at the uvula.

The Pharynx

The pharynx is a part of the conducting zone of the respiratory system and also a part of the digestive system. It is the part of the throat immediately behind the nasal cavity at the back of the mouth and above the esophagus and larynx. The pharynx is divided for descriptive purposes into three parts, the nasopharynx, oropharynx, and laryngopharynx. The nasopharynx is important in respiration. The oropharynx and laryngopharynx are passages common to both the respiratory and the digestive systems. For the digestive system, the muscular walls of pharynx function in the process of swallowing, and it serves as a pathway for the movement of food from the mouth to the esophagus.

The Esophagus

The esophagus, commonly known as the food pipe, consists of a muscular tube through which food passes from the pharynx to the stomach. The esophagus is about 25 cm long and about 2 cm in diameter.   The upper and lower ends of the esophagus are closed by sphincter muscles. The upper cricopharyngeal sphincter prevents air from passing into the esophagus during inspiration and the aspiration of esophageal contents.  The cardiac or lower esophageal sphincter prevents the reflux of acid gastric contents into the esophagus.

The Stomach

The stomach is a J-shaped dilated portion of the alimentary tract. The stomach acts as a temporary “storage tank” for food as well as a site for food breakdown. The stomach is divided into three regions: the fundus, the body, and the antrum. Stomach size varies with the volume of food it contains, which may be 1.5 liters or more in an adult.

 

The Small Intestine

The small intestine is a muscular tube extending from the pyloric sphincter to the large intestine. It is the longest section of the alimentary tube, with an average length of 2.5 to 7 m (8 to 20 feet) in a living person. In the small intestine, the chemical digestion of food is completed and most of the absorption of nutrients takes place.

The small intestine has three subdivisions: the duodenum, the jejunum, and the ileum,

The duodenum: It is about 25 cm long and curves around the head of the pancreas. Secretions from the gall bladder and pancreas are released into the duodenum through a common structure, the hepatopancreatic ampulla.

The jejunum: It is the middle section of the small intestine and is about 2 meters long.

 The ileum, or terminal section, is about 3 meters long and ends at the ileocaecal valve, which controls the flow of material from the ileum to the caecum.

The Large Intestine

The large intestine is much larger in diameter than the small intestine but shorter in length. It is About 1.5 m (5 feet) long, it extends from the ileocecal valve to the anus. Its major functions are to dry out indigestible food residue by absorbing water and to eliminate these residues from the body as feces. It frames the small intestines on three sides and has the following subdivisions: cecum, appendix, colon, rectum, and anal canal.

The caecum: The saclike cecum is the first part of the large intestine. It is usually about 13 cm long and has the same structure as the walls of the colon but contains more lymphoid tissue.

The ascending colon : The ascending colon travels up the right side of the abdominal cavity and makes a turn, the right colic (or hepatic) flexure, to travel across the abdominal cavity.

The transverse colon: The ascending colon makes a turn and continuous to be the transverse colon as it travels across the abdominal cavity.

Descending colon. It then turns again at the left colic (or splenic) flexure, and continues down the left side as the descending colon.

The sigmoid colon: This part describes an S-shaped curve in the pelvis then continues downwards to become the rectum

The Rectum and the Anal Canal

 

It is a slightly dilated section of the colon about 13 cm long. It leads from the sigmoid colon and terminates in the anal canal.

The anal canal is a short passage about 3.8 cm long in the adult and leads from the rectum to the exterior.

Two sphincter muscles control the anus; the internal sphincter, consisting of smooth muscle fibers, is under the control of the autonomic nervous system and the external sphincter, formed by skeletal muscle, is under voluntary control.

Accessory Organs of the Human Digestive System

Various secretions are poured into the alimentary tract, some by glands in the lining membrane of the organs and some by glands situated outside the tract. The latter are the accessory organs of digestion and their secretions pass through ducts to enter the tract. They consist of:

1.     3 pairs of salivary glands

2.     Pancreas

3.     Liver and the biliary tract.

Salivary Gland: Salivary glands are accessory exocrine gland. The salivary gland includes parotid gland, Submandibular gland, and sublingual gland. These glands secrets saliva into the oral cavity through salivary ducts.

The parotid gland is largest among all salivary gland and situated on the side of the face immediately below and in front of external ear.

The sublingual gland is the smallest one and this almond shaped gland situated beneath the mucous membrane of the floor of the mouth.

The walnut size submandibular or submaxillary are located in the floor of the mouth.

The product of the salivary glands, is saliva and Saliva is the combined secretions from the salivary glands and the small mucus-secreting glands of the lining of the oral cavity. About 1.5 liters of saliva is produced daily

Pancreas: The pancreas is a pale grey gland and about 12 to 15 cm long and is situated in the epigastric and left hypochondriac regions of the abdominal cavity. The pancreas is both an exocrine and an endocrine gland. The function of the exocrine pancreas is to produce pancreatic juice containing enzymes that digest carbohydrates, proteins, and fats and the function of the endocrine pancreas is to secrete the hormones insulin and glucagon, which are principally concerned with control of blood glucose levels.

Liver: The liver is the largest gland in the body, weighing between 1 and 2.3 kg. It is Located under the diaphragm, more to the right side of the body.

The liver has four lobes. The two most obvious are the large right lobe and the smaller, wedge-shaped, left lobe. The other two, the caudate and quadrate lobes, are areas on the posterior surface. The liver’s digestive function is to produce bile. Bile is a yellow-to-green, watery solution containing bile salts, bile pigments, cholesterol, phospholipids, and a variety of electrolytes.

Gallbladder: The gallbladder is a small, thin-walled green sac that snuggles in a shallow fossa in the inferior surface of the liver. When food digestion is not occurring, bile backs up the cystic duct and enters the gallbladder to be stored.