The digestive system includes the digestive tract and its 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, also called the alimentary canal or gastrointestinal (GI) tract, 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 major accessory organs that have a role in digestion. These organs secrete fluids into the digestive tract.
Food undergoes three types of processes in the body:
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 the body cells in metabolism.
The digestive system prepares nutrients for utilization by body cells through six activities, or functions.
The first activity of the digestive system is to take in food through the mouth. This process, called ingestion, has to take place before anything else can happen.
The large pieces of food that are ingested have to be broken into smaller particles that can be acted upon by various enzymes. This is mechanical digestion, which begins in the mouth with chewing or mastication and continues with churning and mixing actions in the stomach.
The complex molecules of carbohydrates, proteins, and fats are transformed by chemical digestion into smaller molecules that can be absorbed and utilized by the cells. Chemical digestion, through a process called hydrolysis, uses water and digestive enzymes to break down the complex molecules. Digestive enzymes speed up the hydrolysis process, which is otherwise very slow.
After ingestion and mastication, the food particles move from the mouth into the pharynx, then into the esophagus. This movement is deglutition, or swallowing. Mixing movements occur in the stomach as a result of smooth muscle contraction. These repetitive contractions usually occur in small segments of the digestive tract and mix the food particles with enzymes and other fluids. 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 mechanical and chemical digestion takes place.
The simple molecules that result from chemical digestion pass through cell membranes of the lining in the small intestine into the blood or lymph capillaries. This process is called absorption.
The food molecules that cannot be digested or absorbed need to be eliminated from the body. The removal of indigestible wastes through the anus, in the form of feces, is defecation or elimination.
At its simplest, the digestive system is a tube running from mouth to anus. Its chief goal is to break down huge macromolecules (proteins, fats and starch), which cannot be absorbed intact, into smaller molecules (amino acids, fatty acids and glucose) that can be absorbed across the wall of the tube, and into the circulatory system for dissemination throughout the body.
Regions of the digestive system can be divided into two main parts:
- The alimentary tract
- Accessory organs.
The alimentary tract of the digestive system is composed of the mouth, pharynx, esophagus, stomach, small and large intestines, rectum and anus.
Associated with the alimentary tract are the following accessory organs: salivary glands, liver, gallbladder, and pancreas.
The mouth, or oral cavity, is the first part of the digestive tract.
It is adapted to receive food by ingestion, break it into small particles by mastication, and mix it with saliva.
The lips, cheeks, and palate form the boundaries. The oral cavity contains the teeth and tongue and receives the secretions from the salivary glands.
The pharynx is a fibromuscular passageway that connects the nasal and oral cavities to the larynx and esophagus. It serves both the respiratory and digestive systems as a channel for air and food. The upper region, the nasopharynx, is posterior to the nasal cavity. It contains the pharyngeal tonsils, or adenoids, functions as a passageway for air, and has no function in the digestive system. The middle region posterior to the oral cavity is the oropharynx. This is the first region food enters when it is swallowed. The opening from the oral cavity into the oropharynx is called the fauces. Masses of lymphoid tissue, the palatine tonsils, are near the fauces. The lower region, posterior to the larynx, is the laryngopharynx, or hypopharynx. The laryngopharynx opens into both the esophagus and the larynx.
Food is forced into the pharynx by the tongue. When food reaches the opening, sensory receptors around the fauces respond and initiate an involuntary swallowing reflex. This reflex action has several parts. The uvula is elevated to prevent food from entering the nasopharynx. The epiglottis drops downward to prevent food from entering the larynx and trachea in order to direct the food into the esophagus. Peristaltic movements propel the food from the pharynx into the esophagus.
The esophagus is a collapsible muscular tube that serves as a passageway between the pharynx and stomach. As it descends, it is posterior to the trachea and anterior to the vertebral column. It passes through an opening in the diaphragm, called the esophageal hiatus, and then empties into the stomach. The mucosa has glands that secrete mucus to keep the lining moist and well lubricated to ease the passage of food. Upper and lower esophageal sphincters control the movement of food into and out of the esophagus. The lower esophageal sphincter is sometimes called the cardiac sphincter and resides at the esophagogastric junction.
The stomach, which receives food from the esophagus, is located in the upper left quadrant of the abdomen. The stomach is divided into the fundus, body, and pyloric regions. The lesser and greater curvatures are on the right and left sides, respectively, of the stomach.
The small intestine extends from the pyloric sphincter to the ileocecal valve, where it empties into the large intestine. The small intestine finishes the process of digestion, absorbs the nutrients, and passes the residue on to the large intestine. The liver, gallbladder, and pancreas are accessory organs of the digestive system that are closely associated with the small intestine.
The small intestine is divided into the duodenum, jejunum, and ileum. The small intestine follows the general structure of the digestive tract in that the wall has a mucosa with simple columnar epithelium, submucosa, smooth muscle with inner circular and outer longitudinal layers, and serosa. The absorptive surface area of the small intestine is increased by plicae circulares, villi, and microvilli.
Exocrine cells in the mucosa of the small intestine secrete mucus, peptidase, sucrase, maltase, lactase, lipase, and enterokinase. Endocrine cells secrete cholecystokinin and secretin.
The most important factor for regulating secretions in the small intestine is the presence of chyme. This is largely a local reflex action in response to chemical and mechanical irritation from the chyme and in response to distention of the intestinal wall. This is a direct reflex action, thus the greater the amount of chyme, the greater the secretion.
The salivary glands, liver, gallbladder, and pancreas are not part of the digestive tract, but they have a role in digestive activities and are considered accessory organs.
Three pairs of major salivary glands (parotid, submandibular, and sublingual glands) and numerous smaller ones secrete saliva into the oral cavity, where it is mixed with food during mastication. Saliva contains water, mucus, and enzyme amylase. Functions of saliva include the following:
- It has a cleansing action on the teeth.
- It moistens and lubricates food during mastication and swallowing.
- It dissolves certain molecules so that food can be tasted.
- It begins the chemical digestion of starches through the action of amylase, which breaks down polysaccharides into disaccharides.
The liver is located primarily in the right hypochondriac and epigastric regions of the abdomen, just beneath the diaphragm. It is the largest gland in the body. On the surface, the liver is divided into two major lobes and two smaller lobes. The functional units of the liver are lobules with sinusoids that carry blood from the periphery to the central vein of the lobule.
The liver receives blood from two sources. Freshly oxygenated blood is brought to the liver by the common hepatic artery, a branch of the celiac trunk from the abdominal aorta. Blood that is rich in nutrients from the digestive tract is carried to the liver by the hepatic portal vein.
The liver has a wide variety of functions and many of these are vital to life. Hepatocytes perform most of the functions attributed to the liver, but the phagocytic Kupffer cells that line the sinusoids are responsible for cleansing the blood.
Liver functions include the following:
- synthesis of bile salts
- synthesis of plasma protein
- carbohyrate metabolism
- lipid metabolism
- protein metabolism
The gallbladder is a pear-shaped sac that is attached to the visceral surface of the liver by the cystic duct. The principal function of the gallbladder is to serve as a storage reservoir for bile. Bile is a yellowish-green fluid produced by liver cells. The main components of bile are water, bile salts, bile pigments, and cholesterol.
Bile salts act as emulsifying agents in the digestion and absorption of fats. Cholesterol and bile pigments from the breakdown of hemoglobin are excreted from the body in the bile.
The pancreas has both endocrine and exocrine functions. The endocrine portion consists of the scattered islets of Langerhans, which secrete the hormones insulin and glucagon into the blood. The exocrine portion is the major part of the gland. It consists of pancreatic acinar cells that secrete digestive enzymes into tiny ducts interwoven between the cells. Pancreatic enzymes include anylase, trypsin, peptidase, and lipase. Pancreatic secretions are controlled by the hormones secretin and cholecystokinin.
- Adenoiditis: Inflammation of the adenoids. The symptoms include snoring and mouth breathing.
- Pharyngitis: Inflammation of the pharynx(throat). The symptoms include pain in the throat, dysphagia(difficulty in swallowing), and fever.
- Laryngitis: Inflammation of the larynx. The symptoms include pain in the throat, dysphasia(difficulty in speaking), and fever.
- Aphagia: Inability to swallow.
- Aphasia: Inability to speak.
- Stomatitis: Inflammation of the mouth.
- Canker sore: Also called aphthous ulcers. A canker sore is an open sore in the mouth, which appears as a painful white or yellow ulcer surrounded by a bright red area. A canker sore is benign (not harmful).
- Stomatitis herpetic: Herpetic stomatitis is a viral infection of the mouth characterized by ulcers and inflammation. Apthous ulcers are different, since they are not caused by the same virus.
- Esophagitis: Esophagitis is inflammation, irritation, and swelling of the esophagus, the tube that leads from the back of the mouth to the stomach
- Achalasia: There is a sphincter in between esophagus and stomach which allows the food to enter into the stomach from esophagus. Achalasia is an abnormal condition in which the cardiac sphincter does not relax much to enter food into the stomach from the esophagus.
- Gastritis: Inflammation of the gastric mucosa is called gastritis.
Gastritis can be caused by bacterial or viral infection or backflow of bile into the stomach (bile reflux).
Gastritis can also be caused by irritation from:
- Medications (such as aspirin or NSAIDs)
- Chronic vomiting
- Excess hydrochloric acid secretion (such as from stress)
- Eating or drinking caustic or corrosive substances (such as poisons)
Gastritis can occur suddenly (acute gastritis) or gradually (chronic gastritis).
- Diarrhea: Loose, watery, and frequent stools is called diarrhea. Some common causes of diarrhea include mild viral infection, food poisoning, crohn’s disease, ulcerative colitis, and some drugs (drug induced diarrhea).
- Constipation: Difficult delayed defecation (passing of stools) is called constipation. Constipation may involve pain during the passage of a bowel movement, the inability to pass a bowel movement after straining or pushing for more than 10 minutes, or no bowel movements after more than 3 days.
- GERD: (Gastroesophageal reflux disease). It is an abnormal condition in which the food or liquid travels from the stomach back up into the esophagus (the tube from the mouth to the stomach) due to mal functioning of the cardiac sphincter. This partially digested material is usually acidic and can irritate the esophagus, often causing heartburn and other symptoms.
- Gastric ulcer: Ulcer is an open sore. A gastric ulcer is a break in the normal tissue lining the stomach. The symptoms are as follows
- Abdominal pain
- May wake you at night
- May be relieved by antacids or milk
- May occur 2 to 3 hours after a meal
- May be worse if you don’t eat
- Abdominal indigestion
- Vomiting, especially vomiting blood
- Blood in stools or black, tarry stools
- Weight loss
- Duodenal ulcer: It is a break in the normal tissue lining the duodenum (the first part of the small intestine).
- Achlorhydria: A condition of no HCl in the stomach leading to diseases.
- Cholelithiasis: Cholelithiasis is the presence of gallstones in the gallbladder without any associated symptoms.
- Choledocholithiasis: Choledocholithiasis is the presence of a gallstone in the common bile duct. The stone may consist of bile pigments and/or calcium and cholesterol salts that are formed in the biliary tract.
- Cholestasis: Cholestasis is any condition in which bile excretion from the liver is blocked, which can occur either in the liver or in the bile ducts. The symptoms include:
- Jaundiced (yellow) skin or eyes
- Inability to digest certain foods
- Nausea, vomiting
- Right upper quadrant abdominal pain
- Clay-colored or white stools
- Diverticulitis: Formation of abnormal side pouch like structure is called diverticulosis. Diverticulitis is inflammation of this abnormal pouch (diverticulum) in the intestinal wall, usually found in the large intestine (colon).
- Hematochezia: Bright red blood per rectum (BRBPR), or red or maroon colored stool is called hematochezia.
- Melena: Black tarry stool that contains blood is called melena.
The main difference between hematochezia and melena is that in hematochezia the bleeding spot will be in the lower GI tract, where as in melena the bleeding spot will be in the upper GI tract.
- Steatorrhea: Discharge of fat in large amounts in stools due to indigestion of fat(due to pancreatic disease) or malabsorption.
- Crohn’s disease: Crohn’s disease is a chronic inflammatory disease that can affect any part of the gastrointestinal tract but most commonly occurs in the ileum (the area where the small and large intestine meet).
- Ileus: Intestinal obstruction involves a partial or complete blockage of the bowel that results in the failure of the intestinal contents to pass through. The mechanical causes include.
- Postoperative adhesions or scar tissue
- Impacted feces (stool)
- Tumors blocking the intestines
- Granulomatous processes (abnormal tissue growth)
- Intussusception (telescoping of the intestine)
- Volvulus (twisted intestine)
- Foreign bodies (ingested materials that obstruct the intestines)
- Volvulus: A volvulus is a condition in which the bowel becomes twisted upon itself. This twisting causes obstruction of the intestine, and also often cuts off the blood supply to the intestine. This can cause death of some or the entire intestine, which is often fatal.
- Intussusception: Telescoping of the intestine is called intussusception. Intussusception is caused by part of the gut being pulled inward. This can obstruct the passage of digestive contents through the intestine. Strangulation, in which the blood supply is cut off, can occur in the segment of intestine that has been pulled inside. This can cause the tissue to die.
- Anal fissure: An anal fissure is a small split or tear in the anal mucosa that may cause painful bowel movements and bleeding. There may be blood on the outside of the stool or on the toilet tissue following a bowel movement.
- Anal fistula: Abnormal opening that connecting two internal organs or an internal organ to the outside Abnormal tube like passage near the anus which may communicate with the rectum.
- Ascites: Abnormal accumulation of fluid in the peritoneal cavity or abdominal cavity. (Dropsy—-old name)
- Cachexia: A general weight loss and wasting occurring in the course of a chronic disease or emotional disturbance
I. LFTs: Liver function tests:
a. SGPT: Serum glutamic pyruvic transaminase.
b. SGOT: Serum glutamic oxaloacitic transaminase.
Presence of these enzymes in the blood indicates liver disease and danger to the liver cells.
c. Alkaline Phosphotase: Presence of these enzymes in the blood indicates liver cancer.
d. Serum bilirubin: Elevated levels of bilurubin in the blood indicate jaundice.
II. Stool analysis:
a. Stool culture: Faces are placed in a growth medium to test for microscopic organisms that are in
large number/abnormally present.
b. Stool guaiac: (Hemoccult test) Test to determine the presence of blood in the faces by adding the guaiac to the stool. Positive reaction indicates presence of blood in the stools.
- Barium enema: (Lower GI series) Barium sulphate is introduced into the rectum and colon by enema, X- rays are taken of it to find out the ulcers extra growths, and constrictions.
- Barium swallow: (upper GI series) Barium sulphate is swallowed and x- rays of the upper GI are taken.
- Cholangiography: Process of recording the blood vessels of the gall bladder.
- CT Scan of abdomen: computed tomography. That is cut section. A series of X- rays are taken and processed by a computer to show a cross sectional image of internal organs.
- Abdominal Ultrasonography: Ultra sound waves are passed into the abdomen and a record is made of the echoes as they bounds of abdominal viscera.
- Liver scan: Radioactive material is injected intravenously and when the liver cells take it up, using a scanner can do scanning.
- Gastro intestinal endoscopy: Process of visualizing the internal part of the stomach and intestine.
- Liver biopsy: A tissue is taken for microscopic examination.
- Nasogastric intubation: Process of introducing a NG tube into the stomach through the nose.
- Paracentesis/ Abdominocentesis: surgical puncture and aspiration of the fluid from the abdominal cavity/peritoneal cavity.
alk – phos : Alkaline phoshotase
a.c. : Before meal.
BE : Barium enema.
BRBPR : Bright red blood per rectum.
BM : Bowel movements. (Bone marrow in oncology)
BX : Biopsy.
C T scan : Computerized tomography scan (cut section of each and every tissue)
EGD : Esophagogastroduodenoscopy.
FBS : Fasting blood sugar.
GB : Gall bladder.
GE : Gastroesophageal
GERD : Gastro esophageal reflux disease
GI : Gastrointestinal
HCl : Hydrochloric acid
H & E : Hematoxylin and eosin (stains for specimens on microscopic slides)
H/O : History of
IVC : Intravenous cholangiography
IBD : Inflammatory bowel disease
LLQ : Left lower quadrant
LRQ : Lower right quadrant
LUQ : Left upper quadrant
LFTs : Liver function tests
n.p.o : Nothing per os (nothing by mouth)
p.c. : After meals
PUD : Peptic ulcer disease
RLQ : Right lower quadrant
R/O : Rule out
RUQ : Right upper quadrant
TPN : Total parenteral nutrition
UGI : Upper gastrointestinal
ULQ : Upper left quadrant
URQ : Upper right quadrant