Digestive System

Digestive System

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Digestive System...

The Digestive System:

The digestive system includes the digestive tract and it's accessory organs, which process food into molecules that can be absorbed and utilized by the cells of the body.

Food is broken down bit by bit until the molecules are small enough to be absorbed and the waste products are eliminated.

The digestive tract is also called the alimentary canal and the gastrointestinal tract. It consists of a long, continuous tube that extends from the mouth to the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The tongue and teeth are accessory structures located in the mouth. The salivary glands, liver, gallbladder, and pancreas are not part of the digestive tract, but are major accessory organs of the digestive system that play a role in the digestive process. These secrete fluids into the digestive tract.

FUNCTIONS OF THE DIGESTIVE TRACT

Food undergoes three types of processes in the body:

digestion

absorption

metabolism

Digestion and absorption occur in the digestive tract. After the nutrients are absorbed, they are available to all cells in the body and are utilized by these cells in metabolism. Metabolism is the sum of all the chemical reactions that take place in the body including the utilization of nutrients.

The digestive system prepares nutrients for utilization by body cells through six activities:

ingestion- the first activity of the digestive system is to take in food. This needs to take place before anything else can happen.

mechanical digestion- large pieces of food need to be broken down into smaller ones so that various enzymes can begin another activity in the chain of events. Mechanical digestion begins in the mouth with chewing or mastication of the food and continues with the churning and mixing actions within the stomach.

chemical digestion- complex molecules are transformed by chemical digestion into smaller molecules that can be easily absorbed and utilized by the cells.

movements- after ingestion and mastication, the food particles move from the mouth into the pharynx, then into the esophagus. This movement is deglutition or swallowing. The movements that propel the food particles through the digestive tract are called peristalsis. These are rhythmic waves of contractions that move the food particles through the various regions in which the mechanical and chemical digestion takes place.

absorption- The simple molecules that result from chemical digestion pass through cell membrane of the lining in the small intestine into the blood or lymph capillaries. This process is called absorption.

elimination- the food molecules that cannot be digested need to be eliminated from the body. The removal of indigestible wastes through the anus, in the form of feces, is called defecation.

GENERAL STRUCTURE OF THE DIGESTIVE TRACT

The long, continuous tube that is the digestive tract is about 9 meters in length. It opens to the outside at both ends, through the mouth at one end and through the anus at the other. Although tgere are variations in each region, the basic structure of the wall is the same throughout the entire length of the tube.

The wall of the digestive tract has four layers:

mucosa- or the mucous membrane layer, is the most innermost of the wall. It lines the lumen of the tract. In certain regions the mucosa secrete mucus, digestive enzymes, and hormones. Ducts from other glands pass through the mucosa to the lumen. In the mouth and the anus the thickness of the mucosa is needed to protect against potential for abrasion, while the stomach and the intestines have a thin layer to allow for secretion and absorption.

submucosa- is a thick layer of loose connective tissue that surrounds the mucosa. This layer also contains blood and lymphatic vessels, nerves and some glands. Blood vessels supply nourishment to the surrounding tissues. Blood and lymph carry away absorped nutrients that are the end products of digestion.

muscularis- which consists of two layers of smooth muscle. When the two layers of muscle tissue contract, the diameter of the tube is decreased. This muscular layer contraction is called peristalsis.

serous layer or serosa- is the outermost covering of the digestive tube. The serosa of the digestive tract is also called the visceral peritioneum. It secretes a serous fluid for lubrication so that the abdominal organs move smoothly against each other to prevent friction. The loops of the digestive tract are anchored to the posterior wall of the abdominal cavity by a large double fold of peritoneal tissue called the mesentary.

REGIONS OF THE DIGESTIVE TRACT

Mouth- or the oral cavity is the first part of the digestive tract. It is adapted to receive food by ingestion. The food will be broken into small particles by mastication and mix with saliva. The lips, cheeks, and palate form the boundaries. The oral cavity contains the teeth and the tongue, and receives the secretions from the salivary glands.

Lips and cheeks- help to hold the food in the mouth and keep it in place for chewing or mastication. They are also used in the formation of words for speech. The reddish color of lips is due to the transparent tissue lying just above the many blood vessels in the lips. This is why we can use the lips as a tester for the symptom of cyanosis or bluing of a tissue due to lack of oxygen. Lips also have nerve endings which help us identify the temperature and texture of foods.

Palate- is the roof of the oral cavity. It separates the oral cavity from the nasal cavity. The anterior portion is called the hard palate and it is supported by bone. The posterior portion or the soft palate is made up of connective tissue and skeletal muscle tissue. Posteriroly, the hard palate ends in a projection called the uvula. During swallowing, the soft palate and the ubula move uward to direct food away from the nasal cavity and into the oropharynx. Cleft palate is a condition in which the bones in the hard palate do not fuse completely together during prenatal development. This leaves an opening between the nasal and oral cavities. An infant with this problem has difficulty creating enough suction for proper feeding. Cleft palate can usually be corrected with a series of surgical procedures and with dental work as the child develops.

Tongue- is the largest and most movable organ in the oral cavity. Most of the tongue's substance is made of skeletal muscle. The major attachment for the tongue is the root or the posterior region which is anchored to the hyoid bone. Remember that from the skeletal system? The anterior portion portion is relatively free but is connected to the floor of the mouth by the frenulum. The surface of the tongue is covered with projections called papillae. In addition to providing friction for manipulating food the papillae contain the taste buds. Masses of lymphoid tissue provide defense against bacteria as it enters the mouth, these are called lingual tonsils. The muscles in the tongue allow for manipulation of the food to form a ball or bolus to make it easier for swallowing. A person with a short lingual frenulum is called tongue-tied. The movement of the tongue is abnormally limited, which causes difficulty in speech. Surgically cutting the frenulum corrects this problem.

Teeth- Two different sets of teeth develop in the mouth. The first set usually begins to appear at the age of 6 months and continues to develop until about the age of 3. This set is called the primary or deciduous teeth (baby teeth). There are about 20 in all. The secondary or permanent teeth usually begin to appear around the age of 6-7 years of age. This set contains a total of 32 teeth. The third molars are the last teeth to erupt. They are sometimes called “wisdom teeth” and they begin to erupt between the ages of 17 - 25. These teeth may remain embedded in the jaw bone. In some cases, wisdom teeth are absent all together. Different teeth are shaped to handle food in different ways. The incisors are chiseled and have sharp edges for biting food; cuspids or acnines are cone shaped and have points for grasping and tearing food; bicuspids (premolars) and molars have flat surfaces with rounded projections for crushing and grinding. Although different types of teeth have different shapes and functions, each tooth has three distinct parts: the crown, the neck, and the root. The crown is the visible portion of the tooth which is covered by enamel. The root is the portion that is embedded in the sockets of the mandible and the maxilla. The neck is the small region in which the crown and the root meet next to the gums or gingiva. The central core of the tooth is the pulp cavity which consists of connective tissue, blood vessels, and nerves. The pulp cavity is surrounded by dentin which forms the bulk of the tooth. Dentin is surrounded by a thin layer of calcified connective tissue called cementum which attaches the root to the ligaments. Tooth enamel, the hardest substance in the body, surrounds the dentin in the crown of the tooth. Caries is the medical term for a cavity.

Salivary glands- There are three pairs of major salivary glands and numerous small ones that secrete saliva into the oral cavity, where it is mixed with food during mastication. The parotid glands are the largest of the salivary glands. The submandibular glands are located on the floor of the mouth. The sublingual glands are in the floor of the mouth. Mumps is a viral infection of the parotid glands. The infection causes inflammation in the gland, which makes opening the mouth and chewing difficult. If the disease occurs in post adolescent males, the infection may spread to the testes, which in severe cases, may lead to sterility.

Saliva contains water, mucus, and the enzyme amylase. Functions of saliva include the following:

cleansing action on the teeth

moisten and lubricates food

dissolves certain food molecules so that the food can actually be tasted

begins the chemical digestion of starches through the action of the amylase

Pharynx- is the passageway that connects the nasal and oral cavities. It leads to the larynx and the esophagus and serves as a channel for both the respiratory system and the digestive system. (air and food) It contains the pharyngeal tonsils or the adenoids as they are often called. The pharynx is subdivided into three sections: nasopharynx, oropharynx and the laryngopharnyx. Food is forced into the larynx by the tongue. When the food reaches the opening, an involuntary swallowing reflex is initiated. The uvula is the elevated to prevent food from entering the nasopharynx. The epiglottis drops downward to prevent food from entering the the larynx and to direct food into the esophagus.

Esophagus- is a collapsible muscular tube, about 25 cm long that serves as a passageway between the pharynx and the stomach. As it travels downward it is anterior to the vertebral column and posterior to the trachea. It passes through an opening in the diaphragm, called the esophageal hiatus, and it empties into the stomach. There is an upper and lower esophageal sphincter, the lower sphincter is called the cardiac sphincter due to it's location.

Stomach- receives food from the esophagus and is located in the left upper quadrant of the abdomen. It's capacity varies but in an average adult it holds 1.5 liters although some individuals may hold up to 4 liters. The stomach is divided into four regions:

cardiac region- small region around the opening of esophagus.

fundus region- due to the shape of the stomach it is the most superior region, it balloons above the cardiac region to form a temporary storage area

body- is the main portion of the stomach

pyloric region- which acts as a valve between the stomach and the small intestine

When the stomach is empty there are folds in the lining of the stomach called rugae.

These rugae allow for expansion and elasticity of the muscle folds for food intake.

A hiatal hernia occurs when the stomach protrudes into the thoracic cavity through a weakened area of the diaphragm. Symptoms of this condition include pain in the upper abdomen and “heart burn” due to the reflux of stomach acid into the espophagus, especially when the person is lying down.