The Gastrointestinal System

Lesson 4 Overview

In this lesson, you’ll learn the basic principles of the gastrointestinal, urinary, and reproductive systems. The gastrointestinal system is mainly responsible for the absorption and digestion of the food that you eat. It also helps with distribution of the nutrients to various organs as well as elimination of byproducts in the form of stool. The urinary system maintains an appropriate water and electrolyte balance and eliminates waste in the form of urine. The reproductive system is responsible for maintaining normal levels of various reproductive hormones to keep all the reproduction-related processes functioning.

You’ll also gain knowledge of various diseases of the gastrointestinal, urinary, and reproductive systems and the available treatment options. While discussing treatment of these disorders, you’ll learn about the medications used to treat each disorder. Medications named throughout this lesson are listed as the generic name with the brand name provided in parentheses.

 

Lesson Objectives

• Describe the gastrointestinal system and its disorders and treatment options
• Describe the urinary system and its disorders and treatment options
• Describe the reproductive system and its disorders and treatment options

 

Introduction

Every time you enjoy your favorite meal or snack, your gastrointestinal (GI) system is hard at work. Your GI system processes the nutrients and minerals that are important for your survival. It also disposes of unnecessary waste. You'll learn about the GI system's anatomy, physiology, common diseases, and their treatments.

 

The Digestion Process

The GI system is also sometimes referred to as the digestive system and comprises two related sets of organs: the GI tract and the accessory organs. 

The GI tract is a long, continuous tube that stretches from the mouth to the anus. In between are the pharynx, esophagus, stomach, small intestine, large intestine, and rectum. Food passes straight through this tract in a line.

The accessory organs are the salivary glands, liver, and pancreas, which all help the mechanical and chemical digestion of food as it passes through the gastrointestinal tract.

 

The process of digestion involves four major functions:

1. Ingestion is the process of putting food in the mouth, chewing, and swallowing.
2. Digestion is the process of physically and chemically breaking chunks and large molecules of food into small molecules that will pass through cell membranes. Digestive enzymes are substances that break down complex nutrients:                                                                                                  - Proteins into simpler amino acids                                                                                                                                                                                                  - Sugars into simple sugars, such as glucose                                                                                                                                                                                  - Fats into fatty acids and triglycerides 
3. Absorption is the process of transferring digested food from intestinal membranes into the blood circulation, which takes most nutrients straight to the liver for further processing and/or storage.
4. Elimination is the process of passing undigested and unabsorbed food out of the body in the form of stool.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

Major Organs of the GI System

Mouth.

Your mouth's job is to ingest food and prepare it for digestion. The mouth is also referred to as the oral cavity or the buccal cavity, formed from the term "bucca," meaning "cheeks." Once you take food in through your mouth, your teeth, tongue, and saliva work together to chew, process, and swallow the food.

 

Pharynx

The back of the mouth is called the throat or pharynx. The pharynx is the common channel through which air, food, and liquids must pass through to either go to larynx (voice box), the trachea (windpipe), or the esophagus (food pipe). Swallowing is a complex action that requires coordination of the tongue, soft palate (the roof of your mouth), and the muscles of the pharynx. The epiglottis is a flap of cartilage that fold down to cover the larynx during swallowing to prevent food from entering the larynx, and therefore the lungs. 

 

Esophagus

The esophagus is 10- to 12-inch long tube that extends from the pharynx to the stomach. Once food passes from the pharynx to enter the esophagus, muscle movements called peristalsis move the food down into the stomach. Sometimes, food can irritate your esophagus and cause the esophagus muscles to contract and expel food. The lower esophageal sphincter maintains enough pressure to prevent stomach content from refluxing back into the esophagus.  

 

Stomach

The esophagus empties food straight into the stomach. The stomach has three parts:

1. The fundus is the broad portion near the esophagus.
2. The body is the central portion.
3. The pylorus (or antrum) is the lower, narrow section that joins with the small intestine.

When empty, the stomach resembles the size and shape of a large eggplant. It's about 12 inches long with the widest part measuring about six inches around.

On the outside of the stomach is a mucosa, a membrane that lines various cavities in the body and surrounds internal organs, composed of layers of epithelial cells surrounding a layer of loose connective tissue. The stomach's mucosa folds into structures called rugae, or a series of ridges. As the stomach fills up, the rugae unfold so the stomach can stretch and smooth out.

You've probably seen (or felt) just how much a full stomach can expand. It can hold more than one quart at one time. Overeating can make the stomach push against the diaphragm, causing discomfort that takes a while to wear off. Food stays in the stomach for about three hours.

The three layers of muscle (longitudinal, oblique, and circular) in the stomach allow it to contort and twist to mechanically break food apart. As the food is churned about, two sphincters prevent the food from spilling out from either end of the stomach.

As previously stated, the lower esophageal sphincter prevents food from refluxing into the esophagus. The pyloric sphincter prevents food from proceeding into the small intestine before the gastric process of digestion is complete.

While the stomach churns the food, thousands of tiny glands in the mucous membranes of the stomach's lining secrete gastric juices, including pepsin and hydrochloric acid (HCI). These enzymes mix in with the churned food to form a soupy mixture called chyme.  The pepsin breaks down proteins, and hydrochloric acid maintains an acidic environment in the stomach that kills bacteria. Peristalsis of the stomach muscles forces the chyme to continue down the digestive tract, through the pyloric sphincter, and into the small intestine.

 

Small Intestine

The small intestine (or small bowel) has a deceiving name; it's actually about 20 feet long! But its diameter, at approximately 1 inch, is much smaller than that of the large intestine (which averages 2 1/2 inches in width). Food passes through three parts of the small intestine in the following order.

The duodenum is the top part of the small intestine that extends from the stomach. It's about 12 inches long and curves around the head of the pancreas. Besides receiving food from the stomach, the pancreatic duct and the common bile duct from the liver both open into the duodenum to empty digestive juices.

The jejunum is the middle part of the small intestine. It's about 8 feet long and is covered in villi—projections that help to absorb nutrients.

The ileum is the final section of the small intestine. It's 10 to 12 feet long, and it absorbs the nutrients that aren't absorbed in the jejunum.

 

Pancreas

The pancreas is a gland located behind the stomach, nestled in the curve of the duodenum. As you've already learned in the previous lesson, the beta cells of the pancreas secrete insulin directly into the bloodstream. As an accessory organ to the digestive system, it secretes the most important digestive juice, called pancreatic juice. Pancreatic juice is rich in enzymes that digest all three major kinds of food: carbohydrates, proteins, and fats. The main enzymes are trypsin, lipase, and amylase. Trypsin breaks down protein, whereas lipase breaks down fat. Amylase, also secreted by the salivary glands, breaks down carbohydrates. The pancreas also secretes sodium bicarbonate (a base) to neutralize the gastric acid.

 

Liver

The liver, the largest gland in the body, is also one of the most complex organs. It weighs about 3.5 pounds and is somewhat triangular. It performs over 500 jobs and produces more than 1,000 essential enzymes.

The liver's important jobs are to

• Store blood and filter out toxic elements
• Convert sugars into glycogen (starch), which it stores
• Maintain a normal blood sugar level
• Maintain a normal sex hormone level in the blood
• Break down fats and temporarily store fatty acids
• Store vitamins B12, A, D, E, and K
• Destroy old RBCs, storing useful components and sending others to the kidneys for disposal in urine
• Produce fibrinogen and prothrombin, proteins that facilitate blood clotting

As the main "supply center" of glucose in the body, the liver has a lot of freedom in how it delegates nutritional duties. It can convert excess fats and proteins into sugar for immediate use or sugars and starches into fat and ship them off to storage in other parts of the body. The blood vessels that take blood to the liver from the intestine comprise what's called the hepatic portal system.

The liver is the only organ in the body that can actually regenerate if it's damaged. The liver only needs one-quarter of its cells to completely regenerate.

 

Large Intestine

The large intestine (or large bowel), at five feet long, is larger in diameter than the small intestine. The chief function of the large intestine is to receive waste material from the small intestine, absorb water and sodium from the waste material before it passes out of the body, and incubate bacteria that produce essential vitamins.

The large intestine forms a three-sided frame around the small intestine. At the joint of the small and large intestines, there's a pouch called the cecum. The ileocecal valve (a sphincter-like structure) regulates the passage of material from the ileum of the small intestine to the cecum of the large intestine.

When material enters the large intestine, most of the digestible nutrients have already been absorbed by the small intestine. The chyme has become fecal matter, which may take one to three days to move from the ileocecal valve to the rectum, the terminal portion of the large intestine.

The portion of the large intestine between the cecum and the rectum is called the colon. The ascending colon, transverse colon, descending colon, and sigmoid colon are named for the shapes of various sections of the colon. The ascending colon "climbs" straight up from the cecum to the liver; the transverse colon goes "across" the abdomen; the descending colon goes "down" along the flank; and the sigmoid colon, which makes an S shape, empties into the rectum. At either end of the rectum are anal sphincters. The internal sphincter will relax automatically when the rectum fills with feces, stimulating the desire to defecate. The external sphincter, or anus, is controlled voluntarily.

 

GI Disorders

Gastrointestinal inflammation is a common disease that can occur from ingesting food toxins, sustaining injury, or developing an infection from bacteria such as H. pylori. An increased amount of acid from certain drugs (steroids, aspirin, Motrin) can also irritate the stomach. H. pylori. can induce gastric or stomach ulcers as well.

Treatment is targeted to remove the offending agent with pain relievers, anti-inflammatory medications, and antibiotics.

 

Flatulence is commonly called gas. Gas causes abdominal pain and discomfort. A decrease in GI motility and certain foods and drugs can cause accumulation of gas bubbles in the GI tract.

Treatment includes using simethicone (Mylicon, Gas-X, Phazyme).

 

Inflammatory bowel disease (IBD) is the chronic inflammation of the GI tract. Two common types of diseases associated with IBD are called ulcerative colitis and Crohn's disease, which can affect any part of the GI tract.

Common symptoms of IBD include

• Diarrhea
• Constipation
• Abdominal pain
• Weight loss

Commonly used medications to treat IBD are

• Sulfasalazine (Azulfidine) 
• Methylprednisolone (Solu-Medrol)
• Cyclosporine (Sandimmune) 
• Infliximab (Remicade)
• Mesalamine (Rowasa)
• Metronidazole (Flagyl)

 

Pancreatitis is an inflammation of the pancreas. Most cases are caused by alcohol ingestion or gallstones. 

Symptoms of this condition include

• Severe upper abdominal pain
• Nausea
• Loss of appetite

Treatment for acute pancreatitis requires hospital admission for intravenous (IV) fluids, antibiotics, and medication to relieve pain and inflammation. In the case of gallstone pancreatitis, the gallbladder may be surgically removed.

 

Cirrhosis is chronic liver failure often caused by alcoholism combined with malnutrition. However, infections and poisons can also cause deterioration of the liver. Cirrhosis takes a long time to develop, but its symptoms appear abruptly. Liver damage can also happen due to untreated viral infections of the liver, such as hepatitis.

Common symptoms of liver disease include

• Abdominal pain
• Vomiting of blood
• Jaundice (the yellowing of skin due to buildup of a protein called bilirubin in the blood)
• Swelling
• Ascites (fluid accumulation in the peritoneal cavity)

Acetaminophen (Tylenol) is a common OTC drug used widely to treat pain and inflammation. An overdose of Tylenol can cause severe liver damage requiring hospitalization and an antidote called acetylcysteine. 

 

Diarrhea is defined as watery, loose, and frequent stools with or without abdominal pain. Diarrhea can result from various illnesses such as colon cancer, inflammatory bowel syndrome, AIDS, and diabetes. Food toxins can also result in acute diarrhea. Certain medications such as antibiotics, antihypertensive medications, and antacids can also cause diarrhea.

Patients are instructed to keep themselves hydrated with electrolyte-containing solutions to prevent dehydration from continuous diarrhea. Diarrhea due to food poisoning isn't treated with anti-diarrheal drugs, as diarrhea can help remove the toxin from the body.

 

Constipation is defined as the difficult or infrequent passing of stools. Patients suffering from constipation have stools that are hard or firm and thus difficult to pass. Constipation can be associated with other medical conditions such as diabetes, thyroid disorders, inflammatory bowel disease, and pregnancy.

Constipation can also be induced by certain medications such as antacids, iron, diuretics, and opiates.

You can prevent constipation by drinking plenty of fluids, eating a fiber-rich diet, and exercising regularly.

 

Nausea is defined as an uneasiness or discomfort of the stomach with the urge to vomit. Increased salivation commonly occurs with nausea, and nausea may precede vomiting. Vomiting, also called emesis, is the act of throwing up the contents of one's stomach through the mouth.

Nausea and vomiting can be caused by motion sickness, migraines, stomach inflammation or infection, and food poisoning. Nausea and vomiting are also common side effects of many drugs. Chemotherapy medications often cause a severe form of nausea and vomiting. Nausea or emesis may also be triggered by motion. All methods of travel have a tendency to cause motion sickness. Morning sickness is nausea and vomiting that occur very commonly during the first trimester of pregnancy.

Motion sickness can be prevented by taking anti-nausea or anti-emetic medications before traveling. Patients undergoing chemotherapy administration are usually given antiemetic and anti-nausea medications prophylactically.

 

Gastro-esophageal reflux disease (GERD) is a chronic but treatable disease that results from stomach acid backing up into the esophagus.

Common risk factors for GERD include pregnancy, obesity, smoking, and surgery. Common symptoms of GERD are

• Heartburn
• Coughing
• Chest pain
• Difficult breathing
• Regurgitation

GERD can be prevented with such lifestyle modifications as

• Weight loss
• Exercise
• Smoking cessation
• Consumption of smaller meals
• Staying upright for two to three hours after meals
• Avoiding chocolate, alcohol, and caffeine

 

A peptic ulcer is an open lesion or wound in the mucosal lining of the GI tract including the esophagus, stomach, and intestine. Peptic ulcers can be caused by cigarettes, alcohol, NSAIDs, aspirin, steroids, and H. pylori. The most frequent and severe complication of peptic ulcer disease (PUD) is GI bleeding. PUD may also occur due to complications of untreated, chronic GERD.

Treatment usually involves lifestyle modifications such as avoiding alcohol, cigarettes, and drugs that are known to cause peptic ulcer disease. Refer to the Medications to Treat GERD table. The medications used to treat both GERD and PUD are the same. However, they may be used in different combinations and strengths for each disease. PUD may require combining antacids with antibiotics.

 

Key Points

• Two common types of diseases associated with IBD are ulcerative colitis and Crohn's disease.
• Nausea and vomiting can be caused by motion sickness, migraines, stomach inflammation or infection, and food poisoning.
• The pancreas secretes juice that's rich in enzymes and helps to digest all three major kinds of food: carbohydrates, proteins, and fats. The main enzymes are trypsin, lipase, and amylase.
• Three main parts of the small intestine are the duodenum, the jejunum, and the ileum.
• The ascending colon, transverse colon, descending colon, and sigmoid colon describe the shapes of various sections of the colon.
• The liver is the only organ in your body that can regenerate itself from only 1/4 of its cells.
• The small intestine is about 20 feet long, which is much longer than the large intestine at 5 feet long. The diameter of the small intestine is much smaller, approximately 1 inch wide, when compared to the large intestine, which averages 2 1/2 inches wide.
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