The gastrointestinal system is one of the largest endocrine organs in the body. The main function of the gastrointestinal system is to supply the cells of the body with nutrients. This is an article with 11 Facts About the Gastrointestinal System Every Nursing Student Should Know.
The gastrointestinal tract is a pathway that begins at the mouth and ends at the anus. The upper portion of the gastrointestinal tract includes the oral cavity, teeth, tongue, salivary glands, pharynx and esophagus. The lower portion of the gastrointestinal tract includes the stomach, small intestine, and large intestine. The accessory organs include the liver, gallbladder, and pancreas. The gastrointestinal tract is also known as the alimentary tract, the alimentary canal, and the digestive tract. The functions of the gastrointestinal tract are:
The journey through the gastrointestinal tracts begins with ingestion. Ingestion is when food enters the mouth. The process of digestion begins with mastication. Mastication is the mechanical aspect of digestion and involves the chewing of the food. Digestion is the breakdown of large food particles into smaller molecules. Secretion is the process in which acids, buffers, and enzymes are used to further break down food particles. The next step is absorption. Absorption is the movement of the nutrient from the digested food into the bloodstream. Finally, the journey ends with excretion. Excretion is the elimination of any material unused by the body.
Fact #1: Digestion Begins in the Mouth.
Digestion happens as food is moved through the gastrointestinal tract. It prepares food for absorption into the cells of the body. Digestion begins in the mouth where food is broken down into smaller pieces and mixed with saliva. Saliva is a mixture of mucus, water and Ptyalin, a salivary amylase. Ptyalin begins the digestion of starches. Thereby, the mucus and water help lubricate the food to assist in the swallowing. The act of swallowing is voluntary and controlled by the cerebral cortex and the medulla oblongata. When a person swallows the epiglottis covers the tracheal opening to keep the person from aspirating. After the food is swallowed it travels down the esophagus.
The esophagus transports food from the mouth to the pharynx. It is a hollow tube that is normally collapsed until a food bolus is swallowed. When food moves from the mouth, it moves through the pharyngoesophageal sphincter which relaxes so food can enter. The lining of the esophagus is covered by stratified squamous epithelium cells. These epithelium cells make the esophagus resistant to abrasion, temperature extremes and irritation by chemicals.
The smooth muscle of the esophageal lining contracts and propels the food toward the stomach. Esophageal peristalsis is the process of propelling the food downward through the esophagus. The food moves from the esophagus through the lower esophageal sphincter into the stomach. Mucus is excreted by the walls of the esophagus so the food can move down easier.
The lower esophageal sphincter is an opening between the esophagus and the stomach. And, as a bolus of food moves down to the lower end of the esophagus, it relaxes and allows the food to enter the stomach. When the food enters the stomach the lower esophageal sphincter closes to prevent the reflux of the stomach contents back into the esophagus.
Fact #2: The Stomach is a Major Site of Digestion.
The stomach is a muscular bag that expands, lined with mucous membranes. It is located on the left side of the abdominal cavity. The stomach can expand because of the folds in the stomach lining called rugae. The functions of the stomach are:
- To temporarily hold the food received.
- To secrete gastric acid and enzymes which digests food. The mixture of food and gastric juices is called chyme.
- To regulate how fast the digested food enters into the small intestine.
- To absorb a very small amount of water and substances.
The Parts of the Stomach
The stomach begins at the bottom of the esophagus and ends at the opening of the small intestine. It has four parts and two curvatures.
- The first part is the cardia. The cardia is called that because it is located near the heart. The cardia region surrounds the lower esophageal sphincter. Also, the lower esophageal sphincter is sometimes called the cardiac sphincter.
- The fundus is to the side and slightly higher than the cardia area. This is the dome-shaped top of the stomach. This portion temporally holds the food as it enters the stomach.
- The body is the middle section of the stomach. Also, it is the largest section of the stomach.
- The pyloric region is the terminal end of the stomach. The pylorus performs most of the digestion. The pyloric sphincter is where the digested food enters into the small intestine.
- The lesser curvature is the concave curves or sunken side of the stomach.
- The greater curvature is the convex curve or round side of the stomach.
So, the stomach receives swallowed food from the esophagus. Then it secretes a highly acidic fluid from its glands. This acid is called hydrochloric acid (HCl). One function of hydrochloric acid is to assist in the breakdown of food into components that can be easily absorbed. The second function of hydrochloric acid is to assist in the destruction of bacteria that is swallowed. Pepsin is an important enzyme secreted by the gastric mucosa for protein digestion. Another enzyme secreted by the gastric mucosa is the intrinsic factor. The intrinsic factor combines with vitamin B12 so that the vitamin can be absorbed in the ileum.
The intrinsic factor must be present for the absorption of vitamin B12. The absence of vitamin B12 leads to pernicious anemia.
Peristaltic contractions continue in the stomach to propel the stomach contents toward the pylorus. To enter the small intestine the digested food must pass through the pyloric sphincter. Large food particles that cannot pass through the pyloric sphincter are turned back to continue to be broken down. At this point, the partially digested food mixed with the gastric secretions is called chyme. The chyme continues the process by moving into the small intestine.
Fact #3: The Major Function of the Small Intestine is Absorption.
The small intestine extends from the stomach to the large intestine. It secretes several digestive enzymes and some hormones. These enzymes assist in the final digestion of chyme. The small intestine is divided into three parts.
- The duodenum
- The jejunum
- The ileum
The duodenum is the first branch of the small intestine and the shortest. The digestive process continues in the duodenum. Secretions in the duodenum come from the accessory digestive organs. The accessory digestive organs are the pancreas, liver, and gallbladder. These organs are covered later in this article. Also, there are glands in the walls of the small intestine that secrete digestive enzymes.
Secretions from the pancreas have a high alkaline property that assists in neutralizing the acid that enters the duodenum. The enzymes secreted by the pancreas are trypsin, amylase, and lipase. Trypsin assists in the digestion of proteins. Amylase assists in the digestion of starches. And, lipase assists in the digestion of fats.
Bile is secreted by the liver and stored in the gallbladder. Villi are the small finger-like projections that secrete digestive enzymes and absorb nutrients in the small intestine. Amino acids, fatty acids, ions, simple sugars, vitamins, and water are absorbed into the bloodstream from the small intestine. Peristalsis and segmentation continue in the small intestine propelling the contents of the stomach into the colon.
Segmentation is the back and forth mixing of chyme and digestive juices. Peristalsis is the muscular contractions that propel chyme.
Jejunum and Ileum
The jejunum and ileum make up the largest portion of the small intestine. The jejunum is the middle section of the small intestine. There are specific locations throughout the small intestine in which nutrients are absorbed. Absorption is accomplished by diffusion and active transport. The ileum is the terminal end of the small intestine. It is connected to the large intestine via the ileocecal valve.
Diffusion is a method in which particles in the fluid move from an area of higher concentration to an area of lower concentration. This results in the equal distribution of particles.
Active transport is the movement of particles across a cell membrane against a concentration gradient. This is accomplished by chemical activity.
Fact #4: The Large Intestine is Responsible for Water and Electrolyte Reabsorption.
Waste products left after absorption are moved to the terminal ileum and slowly move into the colon via the ileocecal valve. The ileocecal valve opens and closes due to the peristaltic waves from the small intestine. The large intestine begins at the ileocecal valve and ends at the anus. The large intestine borders the small intestine. The functions of the large intestine are:
- The reabsorption of water.
- The reabsorption of electrolytes.
- The compacting and preparing waste material for elimination.
Very little absorption takes place in the large intestine. There are three sections of the large intestine. They are the cecum, colon and the rectum. The colon is divided into the ascending, transverse, descending and sigmoid colon. The ascending colon travels up the right side of the abdomen. And, the transverse colon travels across the abdomen below the liver and the stomach. Then, the descending colon travels down the right side of the abdomen. The descending colon turns into the sigmoid colon. Finally, the sigmoid colon extends to the rectum.
The cecum receives food that has not been digested by the ileum. The appendix is attached to the cecum. The appendix is also called the vermiform appendix. Bacteria in the large intestine further breakdown waste material, undigested contents, and unabsorbed materials.
The colon has two types of secretions. The first secretion is an electrolyte secretion. This is a bicarbonate solution that neutralizes the bacteria action in the colon. Secondly, there is a mucus secretion. The mucus secretion protects the mucosa of the colon from the contents of digestion.
Passage through the colon is very slow. This allows for the reabsorption of water and electrolytes. This is the major function of the large intestine. The waste material eventually reaches the rectum.
Fact # 5: Defecation is Under Voluntary and Involuntary Control.
When the waste material reaches the rectum it is semi-solid. Water has been removed. The waste material is called fecal matter or feces. When the fecal matter fills the rectum it becomes distended.
The brown color of the feces is a result of the breakdown of bile by the bacteria in the intestine. Also, the bacteria are responsible for much of the odor of the feces. The gases formed in the intestine are a mixture of methane, hydrogen sulfide and ammonia. These gases are either expelled or detoxified by the liver.
Excretion, elimination or defecation begins with a distended rectum. Defecation is a reflex that involves voluntary and involuntary control. When waste material is in the rectum, sensory nerve endings cause the desire to defecate. Defecation is considered to be a spinal reflex which involves parasympathetic nerve fibers. The nerve endings cause a reflex contraction of the rectum and relaxation of the internal anal sphincter.
The external anal sphincter is under conscious control of the cerebral cortex. Normally, the external anal sphincter is always contracted. During defecation, the external anal sphincter voluntarily opens and the contents are expelled.
The Valsalva maneuver can also promote defecation. The Valsalva maneuver involves the contraction of the chest muscles and closing of the epiglottis. This stimulates the contraction of the abdominal muscles. In turn, this causes an increase in the intraabdominal pressure. Thereby, the contraction of the abdominal muscles causes the emptying of the colon.
The Valsalva maneuver is usually contraindicated in patients with a head injury, eye surgery, cardiac problems, hemorrhoids, abdominal surgery, and liver cirrhosis with portal hypertension to name a few. Always check for contraindications for your patient before teaching the Valsalva maneuver.
Fact #6: The Abdomen is the Largest Cavity in the Body.
The abdomen contains the majority of the digestive tract and the abdominal organs. The majority of the absorption and digestion of food occurs in the abdomen. The top of the abdomen lies under the diaphragm and is protected by the lower ribs. And, the bottom of the abdomen ends just above the pelvis. Besides the few lower ribs, the abdomen is unprotected by bone. This allows for the distention of the abdomen during pregnancy and after a big meal.
The abdominal wall protects and supports the abdomen. The abdominal wall is a collection of different muscle layers. Also, the abdominal wall supports the abdominal contents. The abdomen contains the digestive organs. These organs are the stomach, liver, gallbladder, pancreas, and intestine. Additional organs that are located in the abdominal cavity include the spleen and kidneys.
The peritoneum covers the contents of the abdominal cavity. The peritoneum is a thin lubricating sheet of tissue. A part of the peritoneum is the greater omentum. It covers the contents from the stomach to the small bowels. The greater omentum prevents the spread of infection to the other organs. It also protects the abdominal contents from heat loss and injury.
Fact #7: The abdomen is divided into nine regions and four quadrants.
The Nine Regions
To map the nine regions of the abdomen, imaginary lines like those used to map the thorax can be used. The nine regions method is helpful to define specific structures within the abdomen during a physical assessment. You can narrow the cause of a problem by knowing the organs present in a specific region. The regions of the abdomen are divided by two horizontal planes and two vertical planes. The horizontal planes are the subcostal and transtubercular. And, the vertical planes are the right midclavicular and the left midclavicular. The nine regions are:
- Right hypochondrium
- Left hypochondrium
- Right lumbar
- Left lumbar
- Right inguinal
- Left inguinal
The Four Quadrants
Mapping the four quadrants is a good technique used for assessing bowel sound. This method of mapping uses imaginary lines just like the nine regions method. The quadrants are divided by one horizontal plane and one vertical plane. The horizontal plane is the transumbilical. This plane crosses through the umbilicus. And, the vertical plane is the medial plane. It extends from the xiphoid process to the symphysis pubis. The four quadrants are:
- Right upper quadrant
- Left upper quadrant
- Right lower quadrant
- Left lower quadrant
Fact #8: The Gastrointestinal Tract Secretes Several Digestive Enzymes and Digestive Hormones.
The hormones active in digestion include gastrin, secretin, and cholecystokinin.
Gastrin is produced in the stomach when the stomach becomes distended. It is secreted by the gastric glands. Gastrin stimulates the increased secretion of gastric juices. This hormone causes increased motility of gastric content, thereby increasing the emptying of the stomach.
Secretin is produced in the duodenum. This hormone stimulates the release of bicarbonate and water from the pancreas. Also, secretin stimulates the release of bile from the liver. Secretin inhibits stomach contractions.
Cholecystokinin is produced in the duodenum. It stimulates the release of digestive enzymes from the pancreas. It also stimulates the release of bile from the gallbladder. This hormone slows gastric emptying.
Digestive enzymes break down starch, protein, and fat as part of the digestive process.
Enzymes that break down starch.
Ptyalin is salivary amylase. The salivary glands produce Ptyalin.
The pancreas produces amylase.
The intestine produces maltose, sucrase, and lactase.
Enzymes that break down proteins
The gastric mucosa produces Pepsin and hydrochloric acid.
The pancreas produces Trypsin.
The intestine produces aminopeptidase and dipeptidase.
Enzymes that break down fat.
The mucosa of the pharynx produces pharyngeal lipase.
The gastric mucosa produces steapsin.
The pancreas produces pancreatic lipase.
The liver produces bile. The gallbladder stores bile.
Fact # 9: The Liver is responsible for fats, carbohydrate and protein metabolism.
The liver is located below the diaphragm in the upper right quadrant of the abdomen. It is one of the largest organs in the abdominal cavity. The liver is divided into two parts, the right and left lobe. And, it also has two smaller lobes underneath called the caudate and quadrate.
The functional unit of the liver is the lobule. The liver receives a large blood supply every minute. The hepatic portal vein collects blood from the digestive tract and carries it to the liver for processing. The hepatic artery supplies the liver with oxygen-rich blood. The liver is not just a digestive accessory organ. The liver has many functions in the body. It controls numerous chemical reactions. Also, the liver produces substances that are crucial for sustaining life. The main function that the liver performs in the digestive system is the metabolism of fats, carbohydrates, and proteins.
Metabolism is the chemical process used to eliminate waste. There are two phases to metabolism. The first phase is anabolism. This is the conversion of smaller molecules into larger molecules. Secondly, the liver performs catabolism. This is the destructive phase. Catabolism is when larger molecules are converted into smaller molecules. For example, glycogen which is a larger molecule is converted into glucose which is a smaller molecule.
Through oxidation (decomposition) of fatty acids, the liver supplies the body with energy. The duodenum secrets secretin which stimulates the liver. The liver manufactures bile which is crucial for the digestion of fats. The detergent action of bile salts breaks the fat into tiny pieces so that it is easier for the digestive enzymes to work. Emulsification is the process of breaking up fats into tiny pieces.
The liver controls carbohydrate metabolism by maintaining normal blood glucose levels. When there is excess glucose in the blood the liver takes up the glucose and converts it into glycogen via glucogenesis. The liver stores glucose as glycogen. When blood glucose levels begin to fall glycogen is converted back into glucose by a process called glycogenolysis. The glucose is transported to the tissues via the blood.
The liver degrades amino acids by the process of deamination. Deamination is the removal of an amine group via hydrolysis. Amino acids make up the protein. The by-product of deamination is ammonia. The liver converts ammonia to urea. The urea leaves the liver, enters the circulation and travels to the kidneys for excretion.
Fact #10: The Gallbladder is an Accessory Organ of the Gastrointestinal System
When bile is produced by the liver it travels to the gallbladder for storage until needed. It travels via the hepatic duct. The gallbladder is a sac-shaped organ. It is located right under the right lobe of the liver. The gallbladder is responsible for storing the bile produced by the liver. It concentrates the bile by reabsorbing some of the water.
Fatty acids activate the gallbladder. The hormone cholecystokinin is released when fatty acids enter the duodenum. This causes the walls of the gallbladder to contract and squeeze bile into the duodenum. The bile travels by way of the cystic duct through the common bile duct to the duodenum. Bile is a yellowish-brown or a yellowish-green fluid that contains acids, mucus, fluid, and pigment. Bile emulsifies fat so that it can be dispersed in water.
Fact #11: The Pancreas is an Endocrine and Exocrine Gland
The pancreas is located posterior (behind) the stomach. It extends from the duodenum to the spleen. The pancreas is an exocrine and an endocrine gland. The endocrine function of the pancreas is to secrete the hormones insulin and glucagon. These hormones maintain glucose homeostasis. The exocrine function of the pancreas is for gastrointestinal function.
The functional unit of the pancreas is the acinus. The acinus synthesizes, store, and secrete digestive enzymes. The exocrine portion of the gland secrets buffers and digestive enzymes into the duodenum via the pancreatic ducts. The main pancreatic duct is the duct of Wirsung. This duct joins the common bile duct and terminates in the duodenum at the ampulla of Vater.
The sphincter of Oddi surrounds the ampulla of Vater. The sphincter of Oddi controls the rate at which the pancreatic enzymes and bile flow into the duodenum. The buffers secreted by the pancreas neutralizes the acidity of the chyme. This keeps the intestinal wall from damage. The duodenum releases cholecystokinin which activates the release of digestive enzymes from the pancreas.
The gastrointestinal system takes in, digests, absorbs and eliminates food for the purpose of supplying nutrients to the cells of the body. The digestive organs and the accessory organs accomplish these functions. These 11 Facts About the Gastrointestinal System will help with your understanding of this system.
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