FunctionsThe digestive system begins with the mouth or the oral (buccal) cavity, where food enters the body, and terminates at the anus where solid waste materials leave the body. The functions of the digestive system are threefold.1. It prepares food for absorption2. It absorbs food into the bloodstream3. It eliminates solid waste materials
Preparation of Food for AbsorptionThe ingested food is in a form that cannot reach the cells because of its inability to pass through the intestinal mucosa into the bloodstream. Therefore, the consumed food must be prepared for absorption.Digestive enzymes are are substances that speed up chemical reactions and help in the breakdown (digestion) of complex nutrients:a) Complex proteins are digested to simpler amino acids;b) Complex sugars are reduced to simpler sugars, such as glucose;c) Large fat molecules are broked down to fatty acids and triglycerides.Thus, we can define digestion as a complete process of changing the chemical and physical composition of food in order to facilitate assimilation of the nourishing ingredients of food by the cells of the body.
Absorption of Food into the BloodstreamThe digested food must be absorbed into the bloodstream by passing through the walls of the small intestine. Within the cells, nutrients are catabolized (burned) in the presence of oxygen to release energy stored in the food. Amino acid nutrients are used to anabolize (build) large protein molecules which are needed for growth and development of cells. Fatty acids and triglycerides are also absorbed through the walls of the small intestine but enter lymphatic vessels rather than blood vessels. These digested fats eventtually enter the bloodstream as lymph vessels join blood vessels in the upper chest region.
Elimination of Solid Waste MaterialsThe third function of the digestive system is to eliminate solid waste materials that cannot be absorbed into the bloodstream. These waste materials are called feces (stools). They are concentrated in the large intestine and finally pass out of the body through the anus. This process is called defecation.
The Oral Cavity (Mouth)The digestive system is composed of the gastrointestinaltract and accessory digestive organs. The gastrointestinaltract begins at the oral cavity or mouth. Digestion begins inthe mouth, where chemical and mechanical digestionoccurs. Saliva or spit, produced by the salivary glands(located under the tongue and near the lower jaw), isreleased into the mouth. Saliva begins to break down thefood, moistening it and making it easier to swallow. Adigestive enzyme (called amylase) in the saliva begins tobreak down the carbohydrates (starches and sugars). Oneof the most important functions of the mouth is chewing.Chewing allows food to be mashed into a soft mass that iseasier to swallow and digest later.
TongueThe structures within the oral cavity are cheeks orbucca and the tongue and its muscles, whichextend across the floor of the mouth. The mainfunctions of the tongue are manipulation of foodduring chewing process, deglutition orswallowing, speech production, anddetermination of taste. The surface of the tonguehas rough elevations; these elevations are tastebuds. The sense organs are called papillae andare capable of perceiving a variety of flavorsfound in food, such asbitterness, sweetness, saltiness and sourness.
Teeth Types of teeth• The functions of teeth vary, depending on their individual shape and size and their location in the jaws. The three basic food processing functions of the teeth are cutting, holding or grasping, and grinding.• Incisors are single-rooted teeth with a relatively sharp thin edge referred to as the incisal edge. Located in the front of the mouth, they are designed to cut food without the application of heavy forces. Central (front teeth) and lateral (distal to the centrals) teeth are incisors
-• Canines, also known as cuspids, are located at the corner of the arch. They are designed for cutting and tearing foods, which require the application of force.• Premolars are a cross between canines and molars. An older term for premolar is bicuspid. The pointed cusps hold and grind the food. They have a broader surface for chewing food. There are two sets of premolars in the permanent dentition and NO premolars in the primary dentition.
-• Molars are much larger than premolars. The molars have more cusps than other teeth that are used to chew or grind up food. There are two sets of molars in the primary dentition and three sets of molars in the permanent dentition.
Tooth StructureTeeth are covered by hard enamel, which gives them a white and smooth appearance. The enamel is the hardest substance in the body. Beneath the enamel is the main structure of the tooth, the dentin. It is yellowish and is composed of bony tissue which is softer than enamel. Dentin is surrounded by a thin layer of modified bone called cementum. In the innermost part of the tooth is the pulp, a soft, delicate layer, which stores the nerves and blood vessels of the tooth. The teeth are embedded in pink fleshy tissue known as gums, or gingiva.
Other Structures• Some of the other structures located within the mouth are the hard and soft palates. The hard palate lies in the anterrior portion of the roof of the mouth, while the soft palate lies in its posterior portion. The soft palate forms a partition between the mouth and nasopharynx and is continuous with the hard palate. Rugae are the irregular ridges in the mucous membrane covering the anterior portion of the hard palate. The entire buccal cavity, like the rest of the digesttive tract, is lined with mucous membrane.
-• After the food is chewed, it is formed into a round, sticky mass called a bolus. The bolus is pushed by the tongue from the mouth into the pharynx. Its downward movement is guided into the pharynx or throat by the soft, fleshy V-shaped tissue hanging from the soft palate, which is called uvula. Uvula also functions to aid in speech production.
PharynxThe pharynx is a muscular tube which serves as a common passageway for air from the nasal cavity to the larynx (voicebox), as well as for food going from the mouth to the esophagus. The pharynx is divided into three major sections:1. The nasopharynx (the throat behind the nose)2. The oropharynx (the throat behind the moouth)3. The laryngopharynx (the throat above the larynx)
-• The laryngopharynx is further divided into two tubes; one which leads to the lungs, called the trachea (wind pipe), and one which leads to the stomach, called the esophagus. A small flap of tissue, the epiglottis, covers the trachea. The main function of the epiglottis is to prevent food from entering the trachea, thus allowing all food to be channeled to the stomach through the esophagus
Esophagus• Anatomically and functionally, the esophagus is the least complex section of the digestive tract. Its role in digestion is simple: to convey boluses of food from the pharynx to the stomach. The esophagus begins as an extension of the pharynx in the back of the oral cavity. It then courses down the neck next to the trachea, through the thoracic cavity, and penetrates the diaphragm to connect with the stomach in the abdominal cavity. It uses rhythmic, wave-like muscle movements (called peristalsis) to force food from the throat into the stomach.
StomachThe stomach, which is attached to the end of the esophagus by way of the cardiac sphincter, is a stretchy sack shaped like the letter J. It is located in the abdominal cavity directly below the diaphragm. It has three important jobs:1. To store the food we have eaten2. To break down the food into a creamy mixture3. To slowly empty that creamy mixture into the small intestine
-The stomach is like a mixer, churning andmashing together all the small balls of foodthat came down the esophagus into smallerand smaller pieces. It does this with help fromthe strong muscles in the walls of the stomachand gastric juices that also come from thestomachs walls. These juices contain enzymesand hydrochloric acid.
-Pepsin breaks down protein, rennin curdlesmilk, while hydrochloric acid kills bacteria thatmight be in the eaten food. The creamymixture (digested food) which is emptied intothe small intestine through the pyloricsphincter is called chyme. The sphincter isdesigned to open partially so that largeparticles are kept in the stomach for furthermixing and breaking down.
Small Intestine (small bowel)Most digestion and absorption of food occursin the small intestine. The small intestine is anarrow, twisting tube that occupies most ofthe lower abdomen between the stomach andthe beginning of the large intestine. It extendsabout 20 feet in length. The small intestineconsists of 3 parts: the duodenum (the C-shaped part), the jejunum (the coiledmidsection), and the ileum (the last section).
-• The small intestine has 2 important functions. First, the digestive process is completed here by enzymes and other substances made by intestinal cells, the pancreas, and the liver. Glands in the intestine walls secrete enzymes that breakdown starches and sugars. The pancreas secretes enzymes into the small intestine that help breakdown carbohydrates, fats, and proteins.
-The liver produces bile, which is stored in thegallbladder. Bile helps to make fat molecules(which otherwise are not soluble in water)soluble, so they can be absorbed by the body.Second, the small intestine absorbs thenutrients from the digestive process. Theinner wall of the small intestine is covered bymillions of tiny fingerlike projections calledvilli.
-• The villi are covered with even tinier projections called microvilli. The combination of villi and microvilli increase the surface area of the small intestine greatly, allowing absorption of nutrients to occur. Undigested material travels next to the large intestine.
The Large Intestine• The large intestine forms an upside down U over the coiled small intestine. It begins at the lower right-hand side of the body and ends on the lower left-hand side. The large intestine is about 5-6 feet long. It has 3 parts: the cecum, the colon, and the rectum. The cecum is a pouch at the beginning of the large intestine.
-This area allows food to pass from the smallintestine to the large intestine. The colon iswhere fluids and salts are absorbed and extendsfrom the cecum to the rectum. The large intestineconsists of the following sections:cecum, ascending colon, transversecolon, descending colon, sigmoidcolon, rectum, anal cana, and anus.The last part of the large intestine is therectum, which is where feces (waste material) arestored before leaving the body through the anus.
-• The main job of the large intestine is to remove water and salts (electrolytes) from the undigested material and to form solid wastes that can be excreted. Bacteria in the large intestine help to break down the undigested materials. The remaining contents of the large intestine are moved toward the rectum, where feces are stored until they leave the body through the anus as a bowel movement.
Liver• The liver is the largest gland, and the largest solid organ in the body, weighing some 1.8 kg in men and 1.3 kg in women. It holds approximately 13% (about one pint or 0.57 liter) of our total blood supply at any given moment and is estimated to have over 500 functions. The liver is dark reddish brown in color and is divided into two main lobes (the much larger right and the smaller left) which are further subdivided into approximately 100,000 small lobes, or lobules. It is located beneath the diaphragm in the right upper quadrant (RUQ) of the abdominal cavity.
Some of the Liver`s Functions• processing digested food from the intestine• controlling levels of fats, amino acids and glucose in the blood• combating infections in the body• clearing the blood of particles and infections including bacteria• neutralizing and destroying drugs and toxins• manufacturing bile• storing iron, vitamins and other essential chemicals• breaking down food and turning it into energy• manufacturing, breaking down and regulating numerous hormones including sex hormones• making enzymes and proteins which are responsible for most chemical reactions in the body, for example those involved in blood clotting and repair of damaged tissues (e.g. prothrombin, fibrinogen).
Pancreas• The pancreas is both an endocrine gland (producing several important hormones, including insulin, glucagon, and somatostatin), as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to the small intestine. These enzymes help in the further breakdown of the carbohydrates, protein, and fat in the chyme.
Gallbladder• The gallbladder is a pear-shaped sac, usually stained dark green by the bile it contains. It is located in the hollow underside of the liver. Its duct, the cystic duct, joins the hepatic duct from the liver to form the common bile duct, which enters the duodenum. The gallbladder receives bile from the liver and then concentrates and stores it. It secretes bile when the small intestine is stimulated by the entrance of fats.