INTRODUCTION: Digestive system
The digestive system, also called the alimentary canal or gastrointestinal tract, begins with the mouth, where food enters the body, and ends with the anus, where solid waste material leaves the body. The primary functions of the organs of the digestive systems are threefold.
First, complex food material taken into the mouth must be digested, or broken down, mechanically and chemically, as it travels through the gastrointestinal tract (passageway). Digestive enzymes speed up chemical reactions and help in the breakdown (digestion) of complex nutrients. Complex proteins are digested to simpler amino acids; complicated sugars are reduced to simple sugars, such as glucose; and large fat molecules (triglycerides) are broken down to fatty acids and glycerol.
Second, the digested food must be absorbed into the blood stream by passing through the walls of the small intestine. In this way, valuable nutrients, such as sugar and amino acids, can travel to all the cells of the body. Cells then catabolize (burn) nutrients in the presence of oxygen to release energy stored with the food. Cells also use amino acids to anabolite (build) large protein molecules needed for growth and development. Although the walls of the small intestine also absorb fatty acids and glycerol, these nutrients enter lymphatic vessels rather than blood vessels. Digested fats eventually enter the blood stream as lymph vessels join with blood vessels in the upper chest region.
The third function of the digestive system is the elimination of solid waste materials that cannot be absorbed into the blood stream. The large intestine concentrates these solid wastes, called feces, and the wastes finally pass out of the body through the anus.
Anatomy & Physiology
Oral Cavity
The alimentary canal begins with the oral cavity, or mouth.
The cheeks form the walls of the oral-shaped cavity and the lips surround the opening to the cavity.
The hard palate forms the anterior portion of the roof of the mouth and the muscular soft palate lies posterior to it. Rugae are the irregular ridges in the mucous membrane covering the anterior portion of the hard palate. Hanging from the soft palate is a small soft tissue called the uvula. The word uvula means little grape. The structure functions to aid in producing sound and speech.
The tongue extends across the floor of the oral cavity, and muscles attach it to the lower jaw bone. It moves the food around during mastication (chewing) and deglutination (swallowing). Papillae, small raised areas on the tongue, contain taste buds that are sensitive to chemical nature of foods and allow discrimination of different tastes as food moves across the tongue.
The tonsils are masses of lymphatic tissue located in depressions of the mucous membranes on both sides if the oropharynx (part of the throat near the mouth). They act as filters to protect the body from invasion of microorganisms and produce lymphocytes, which are white blood cells able to fight disease.
The gums are made of fleshy tissue and surround the sockets of the teeth. Each dental arch has 16 permanent teeth. There are 32 permanent teeth in the entire oral cavity.
Dentists use special terms to describe the surfaces of teeth. The labial surface (labio/o means lip), for incisor and canine teeth, is nearest the lips. The buccal surface (bucc/o means cheek), for premolar and molar teeth, is adjacent to the cheek. Some dentists refer to both the labial and buccal surfaces as the facial surface (faci/o means face). Opposite to the facial surface, all teeth have a lingual surface (lingu/o means tongue). Mesial surface of a tooth lies nearest the median line and the distal surface, farthest from the medial line. Premolars and molars have an additional occlusal surface (occlusion means to close) that comes in contact with a corresponding tooth in the opposing arch. The incisors and cuspids have a sharp incisal edge.
A tooth consists of a crown, which shows above the gumline and a root, which lies within the bony tooth socket. The outermost protective layer of the crown, the enamel, protects the tooth. Enamel is a dense, hard, white substance – the hardest substance in the body. Dentin, the main substance of the tooth, lies beneath the enamel and extends throughout the crown. Yellow in color, dentin is composed of bony tissue that is softer than enamel. The cementum covers, protects and holds the tooth in place in the tooth socket.
The pulp lies underneath the dentin. It is soft and delicate tissue and fills the center of the tooth. Blood vessels, nerve endings, connective tissue, and lymphatic vessels are within the pulp canal (also called the root canal). Root canal therapy is often necessary when disease or abscess (pus collection) occurs in the pulp canal. A dentist opens the tooth from above and cleans the canal of infection, nerves, and blood vessels. The canal is then disinfected and filled with material to prevent the entrance of microorganisms and decay.
Three pairs of salivary glands surround the oral cavity. These exocrine glands produce a fluid called saliva that contains important digestive enzymes. Saliva is released from the parotid gland, submandibular gland, and sublingual gland on each side of the mouth. Narrow ducts carry the saliva into the oral cavity.
Pharynx The pharynx or throat is a muscular tube, about five inches long, lined with a mucous membrane. It serves as a common passage for air traveling from the nose (nasal cavity) to the windpipe (trachea) and food traveling from the oral cavity to the esophagus. When swallowing (deglutination) occurs, a flap of tissue, the epiglottis, covers the trachea so that food cannot enter and become lodge there.
Esophagus The esophagus, meaning swallowing (phag/o) inward (eso-), is a 9- to 10-inch muscular tube extending from the pharynx to the stomach. Rhythmic contractions of muscles in the wall of the esophagus propel food toward the stomach. Peristalsis, meaning constriction (-stalsis) surrounding (peri-), is this involuntary, progressive, rhythm-like contraction of the esophagus and the other gastrointestinal tubes. The process is like squeezing a marble (the bolus, or semi-solid mass of food) through a rubber tube.
Stomach Food passes from the esophagus into the stomach. The stomach is composed of an upper portion called the fundus, a middle section known as the body, and a lower portion, the antrum. Rings of muscles called sphincters control the openings into and leading out of the stomach. The lower esophageal sphincter (cardiac sphincter) relaxes and contracts to move food from the esophagus into the stomach, whereas the pyloric sphincter allows food to leave the stomach when it is ready. Folds in the mucous membrane (mucosa) lining the stomach are called rugae. The rugae contain digestive glands that produce the enzyme pepsin (to begin digestion of proteins and hydrochloric acid.
The role of the stomach is to prepare the food chemically and mechanically so that it can be received in the small intestine for digestion and absorption into the bloodstream. Food does not enter the bloodstream through the walls of the stomach. The stomach controls the passing of foods into the first part of the small intestine so that it proceeds only when it is chemically ready and in small amounts. Food leaves the stomach in 1 to 4 hours or longer depending upon the amount and type of food eaten.
Small intestine (small bowel) The small intestine (small bowel) extends for 20 feet from the pyloric sphincter to the first part of the large intestine. It has three parts. The first section is the duodenum, only 1 foot in length, which receives food from the stomach as well as bile from the liver and gallbladder and pancreatic juices from the pancreas. Enzymes and bile help to digest food before it passes into the second part of the small intestine. The jejunum, about 8 feet long. The jejunum connects with the third section, the ileum, about 11 feet long. The ileum attaches to the first part of the large intestine.
Millions of tiny, microscopic projections called villi line the walls of the small intestine. The tiny capillaries (microscopic blood vessels) in the villi absorb the digested nutrients into the bloodstream and lymph vessels.
Large Intestine (Large Bowel) The large intestine extends from the end of the ileum to the anus. It is divided into 6 parts: ascending colon, transverse colon, descending colon, sigmoid colon and rectum. The cecum is a pouch on the right side that connects to the ileum at the ileocecal valve (sphincter). The appendix hangs from the cecum. The appendix has no clear function and can become inflamed and infected when it is clogged or blocked. The colon (large intestine), about 5 feet long, has three divisions. The ascending colon extends from the cecum to the undersurface of the liver, where it turns to the left (hepatic flexure or bend) to become the transverse colon. The transverse colon passes horizontally to the left toward the spleen, and turns downward, splenic flexure into the descending colon. The sigmoid colon, shaped like an S (resembling the Greek letter sigma, which curves like an S), lies at the distal end of the descending colon and leads into the rectum. The rectum terminates in the lower opening of the gastrointestinal tract, the anus.
The large intestine receives the fluid waste products of digestion (the material unable to pass into the bloodstream) and stores these wastes until they can be released from the body. Because the large intestine absorbs most of the water within the waste material, the body can expel solid feces (stools). Defecation is the expulsion or passage of feces from the body through the anus. Diarrhea, watery stools, can result from lack of absorption of the water through the walls of the large intestine.
Liver, Gallbladder, and Pancreas
Three important additional organs of the digestive system – the liver, gallbladder, and pancreas – play crucial roles in the proper digestion and absorption of nutrients.
The liver, located in the right upper quadrant (RUQ) of the abdomen, manufactures a thick, yellowish-brown, sometimes greenish, fluid called bile. Bile contains cholesterol (a fatty substance), bile acids, and several bile pigments. One of these pigments called, bilirubin, is produced from the breakdown of hemoglobin during normal red blood cell destruction. Bilirubin then travels via the blood stream to the livers where it is conjugated (combined) with another substance so that it can be added to bile. Thus, conjugated bilirubin enters the intestine with bile. In the colon, bilirubin is degraded by bacteria into a variety of pigments that give feces a brownish color. Bilirubin and bile leave the body in feces.
If bilirubin cannot leave the body, it remains in the bloodstream, causing jaundice (hyperbilirubinemia), yellow discoloration of the skin, whites of the eyes, and mucous membranes.
The liver continuously release bile, which then travels through the hepatic duct to the cystic duct. The cystic duct leads to the gallbladder, a pear-shaped sac under the liver, which stores and concentrates the bile for later use. After meals, in response to the presence of food in the stomach and duodenum, the gallbladder contracts, forcing the bile out the cystic duct into the common bile duct. Meanwhile, the pancreas secretes pancreatic juices (enzymes) that travel vial the pancreatic duct to join with the common bile duct just as it enters the duodenum. The duodenum thus receives a mixture of bile and pancreatic juices.
Bile has detergent-like effect on fats in the duodenum. It breaks apart large fat globules, creating more surface area so that enzymes from the pancreas can digest the fats. This is called emulsification. Without bile, most of the fat taken into the body would remain undigested.
The liver, besides producing bile, has several other vital and important functions. Some of these are:
1. Keeping the amount of glucose (sugar) in the blood at normal level. The liver removes excess glucose from the bloodstream and stores it as glycogen (starch) in liver cells. When the blood sugar levels become dangerously low, the liver can convert stored glycogen back into glucose via a process called glycogenolysis. In addition, the liver can also convert proteins and fats into glucose, when the body needs sugar, by a process called gluconeogenesis.
2. Manufacture of some blood proteins, particularly those necessary for blood clotting.
3. Release of bilirubin, a pigment in bile.
4. Removal of poisons (detoxification) from the blood.
The portal vein brings blood to the liver from the intestines. Digested foods pass into the portal vein directly after being absorbed into the bloodstream from the small intestine, thus giving the liver the first chance to use the nutrients.
The pancreas is both an exocrine and an endocrine organ as an exocrine gland, it produces enzymes to digest starch, such as amylase (amyl/o = starch, -ase = enzyme), to digest fat, such as lipase (lip/o = fat), and to digest proteins, such as protease (prote/o = protein). These pass into the duodenum through the pancreatic duct.
As an endocrine gland (secreting into the bloodstream) the pancreas secretes insulin. This hormone, needed to help release sugar from the blood, acts as a carrier to bring glucose into cells of the body to be used for energy.