CHAPTER 6

The Anatomy of Digestion

The human body is amazing in its structural and functional design. Like any complicated structure, it is made up of many different systems, each of which is responsible for specific activities and functions. These specialized areas are referred to as organ systems. There are ten major organ systems in the body:

respiratory (lungs, trachea)
cardiovascular (heart, arteries, veins, lymphatics)
musculoskeletal (skeletal muscles, bone)
nervous (brain, spinal cord, nerves)
integumentary (skin, hair)
immune (spleen, tonsils, bone marrow, appendix, lymph nodes)
urologic (kidneys, bladder)
endocrine (thyroid gland, pancreas, adrenal gland)
hematological (red blood cells, bone marrow)
gastrointestinal (GI)

As a gastroenterologist, I have no doubt that the GI system is the most fascinating organ system in the body. The GI system encompasses not only the GI tract (described in this chapter), but also the salivary glands, the liver, the pancreas, and the gallbladder. In the following section, I provide a brief overview of the normal anatomy and physiology of the GI tract. An understanding of the normal physiology of the GI tract creates the context for understanding problems within this complicated system that lead to symptoms of irritable bowel syndrome. In addition, having a working knowledge of the anatomy and physiology of the GI tract will help you better understand many of the medical terms used by health care providers, improving your ability to communicate with your doctor, and he or she with you. In this chapter, I also provide a brief description of normal digestion and a discussion of what causes intestinal gas.

Anatomy of the GI Tract

You may be surprised to learn that your lips and mouth are technically the beginning of your GI tract. Without these two key structures, you could not chew solid food or swallow liquids. After solid food is ingested, it must be chewed, mixed with saliva, pushed to the back of the throat (oropharynx) by the tongue, and then swallowed. Although you probably don’t think about the act of swallowing, it is a complicated process. Swallowing begins when the muscle in your upper esophagus (upper esophageal sphincter) relaxes so that chewed food can enter your esophagus when pushed backwards by your tongue. At the same time, your vocal cords snap shut, your soft palate elevates (to prevent food from going up into your nose), and your voice box (larynx) changes position. All of these actions ensure that food passes into your esophagus and not into your trachea and lungs.

The esophagus is a muscular tube approximately 10 inches in length that connects the mouth to the stomach. It is located in the chest (thoracic) cavity with the heart and lungs. Strong muscular contractions, called peristalsis, push food and liquids from your upper to lower esophagus and then into your stomach. After you begin to swallow and food enters the upper part of your esophagus, the rest of the process is performed under the direction of different divisions of the nervous system (the autonomic nervous system and the enteric nervous system) that supply the GI tract, without any conscious thought or effort on your part. Solid foods pass through your esophagus and enter your stomach in approximately 3 to 8 seconds. At the end of the esophagus is a muscular ring called the lower esophageal sphincter (LES). This muscular ring is normally contracted to prevent acid, bile, food, and other chemicals from entering the esophagus from the stomach. However, when you start swallowing, the LES relaxes to allow the rhythmic muscle contractions of the esophagus to push food into your stomach.

The stomach is a J-shaped organ located in your abdominal cavity (below the diaphragm, or breathing muscle). The stomach has many different functions: it produces acid to help break down food, stores food temporarily, mixes and grinds food up into smaller pieces, and moves food and liquids into the small intestine.

The small intestine is separated from the stomach by a thick muscular ring called the pylorus. The pylorus opens and closes in coordination with the stomach to allow food to enter the small intestine at the appropriate time. The small intestine is approximately 20 to 25 feet long and is divided into three areas: the duodenum, the jejunum, and the ileum. The last part of the small intestine, the ileum, is directly connected to the colon via the ileocecal valve. This narrow, angled connection between the small intestine and the colon prevents the backward flow of contents from one part of the GI tract to another.

The colon, also called the large intestine, is a muscular tube approximately four feet long. The colon begins in the right, lower part of the abdomen, where it is connected to the small intestine through an area called the cecum (the cecum also includes the opening of the appendix; see Figure 6.1). As the colon moves into the upper abdomen (here it is called the ascending colon), it takes a sharp turn (the hepatic flexure) near the liver and then travels across the abdomen from the right side to the left side (this part of the colon is called the transverse colon). Near the spleen, the colon makes another sharp turn (the splenic flexure) and heads down into the lower abdomen (here it is first called the descending colon and then the sigmoid colon). The sigmoid colon eventually becomes the rectum, which merges into the anal canal.

Normal Digestion

A common misconception about the digestive process is that it only involves the stomach. In reality, many different parts of the GI tract help change the food that you eat into a form that your body can use.

The digestive process begins as soon as you start to chew your food. Chewing breaks food down into smaller pieces and lubricates it with saliva, making it easier to swallow. Saliva contains amylase, a digestive enzyme that breaks down starch and glycogen (the storage form of sugar). After you chew and swallow your food, peristalsis pushes the food through the esophagus and into the stomach, where it may remain for some time. During this period, strong muscular contractions of the stomach help grind up the food and mix it with stomach acid—a process that can take several hours, depending on the size of the meal, the proportion of liquids to solids, and the nutritional content of the meal. Larger meals leave the stomach more slowly than smaller meals, and liquid meals (such as soups) leave the stomach faster than solid meals (such as sandwiches). In addition, meals higher in fat content are pushed from the stomach more slowly than meals containing little fat.

The stomach only sends food into the small intestine when the food particles are small enough to be further broken down by digestive enzymes and absorbed. Food particles are typically 2 to 3 mm in size before they leave the stomach. When particles of food are small enough, they enter the small intestine in small amounts—approximately 1/2 to 1 teaspoon (2.5 to 5 ml) at a time. This liquefied food then mixes with fluid and secretions from the small intestine and pancreas.

Figure 6.1. Anatomy of the Gastrointestinal Tract
The gastrointestinal tract begins at the mouth and ends at the anal canal. The esophagus is a 10-inch-long muscular tube that runs from the mouth to the stomach. The esophagus lies in the thoracic (chest) cavity. Shortly before reaching the stomach, the esophagus passes through an opening in the diaphragm. The diaphragm is the muscle that is vital for breathing and separates the thoracic cavity from the abdominal cavity. The remainder of the GI tract resides within the abdominal and pelvic cavities. The stomach empties into the small intestine. The small intestine is approximately 20 to 25 feet long and is responsible for absorbing nutrients (vitamins, minerals, proteins, carbohydrates, and fats). The small intestine ends at the ileocecal valve, which connects the end of the small intestine (the ileum) to the beginning portion of the colon (the cecum). The appendix is attached to the cecum. The colon is approximately 4 to 5 feet long. The ascending colon extends from the lower right, in the pelvic cavity, up into the abdominal cavity, where it turns near the liver (the hepatic flexure). As the transverse colon, it then crosses from right to left in the abdominal cavity, turns at the spleen (the splenic flexure), and continues, as the descending colon, down into the pelvic cavity, where it turns again and becomes the sigmoid colon. The GI tract terminates in the rectum and anal canal.

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The pancreas is an organ that is especially important to the digestive process because it produces many different enzymes (amylase, lipase, trypsin, chymotrypsin, and elastase) that further break down small particles of food. The liver secretes a substance called bile that also helps digest food, specifically helping in the digestion and absorption of lipids (fats).

The liquid material comprised of secretions from the digestive organs is called chyme (pronounced “kyme”; it rhymes with “lime”). Chyme is propelled through the small intestine by peristalsis, which is crucial to digestion in the small intestine. As the muscles of the small intestine contract, liquid chyme is exposed to the very large surface area of the small intestine, allowing fluids and nutrients to be slowly absorbed. The small intestine functions, to some degree, like a sponge, and it has many tiny villi that greatly increase its surface area. If the 20 to 25 feet of small intestine were removed and completely spread out, they would just about cover the surface of a tennis court. The primary function of the small intestine is to absorb nutrients, including vitamins, minerals, fats, carbohydrates, and proteins. At the end of the small intestine, the remaining liquid chyme passes into the large intestine, where the material will be further concentrated and eventually eliminated as stool. The time it takes for the first bite of food to travel from the stomach to the colon varies dramatically because of volume, fat content, and proportion of liquids to solids. On average, this process occurs within 3 hours of eating a meal.

Evacuation

Evacuation of stool from the rectum (a process called defecation) is normally an easy process that functions smoothly. Many people have a complete, spontaneous bowel movement each day without straining, pain, or feelings of incomplete evacuation. However, defecation is a complicated, learned process that requires an intact nervous system (central, autonomic, enteric, and peripheral) and normal muscle function (within the GI tract and the pelvic floor). The process of defecation is also greatly influenced by societal norms, familial customs, and personal behavior.

Defecation involves certain steps that must occur in a precisely coordinated sequence. One, stool must be propelled from the sigmoid colon into the rectum (requiring normal motility in the colon). Two, distention (stretching) of the rectum by stool must be detected by the body and the brain (requiring an intact nervous system). Third, the person’s body and brain must recognize that it is a socially appropriate time to defecate. (People can normally block the urge to defecate if it occurs at an inappropriate time, such as during a meeting or a car ride.) Four, the person will usually assume a squatting or sitting position, thereby straightening the anorectal angle and making evacuation easier. Five, the internal anal sphincter normally automatically relaxes when the rectum is stretched, and then the person must consciously relax the external anal sphincter. Finally, the person performs a valsalva maneuver, increasing intra-abdominal and intra-rectal pressure and permitting the evacuation of stool. A valsalva maneuver is when you take a deep breath and contract your abdominal muscles without letting your breath out.

Although having a bowel movement should be an easy process, it now seems quite complicated, doesn’t it? It’s little wonder that toilet training is so difficult for some children. In addition, because all of these complicated steps must occur in a precise sequence, injury to any one part of the system can affect the entire process, leading to constipation, diarrhea, or incontinence.

Normal Colon Function

Many people think the colon is simply a pipeline for material to pass through on the way from the small intestine to the rectum. But the colon has a variety of functions. One, it helps concentrate stool by absorbing large amounts of water. The colon can easily absorb 4 to 5 liters of fluid per day, if necessary. It also has the ability to absorb electrolytes (for example, sodium, potassium, and chloride) and some nutrients. Two, the colon is responsible for the continued breakdown, fermentation, and absorption of certain carbohydrates. Three, the colon acts as a reservoir to store stool (one of the major functions of the sigmoid colon).

The colon is normally quite active; it contracts and relaxes in a coordinated pattern throughout the day. Specific patterns of neuromuscular activity within the colon depend on the time of day, whether you are awake or sleeping, the volume and timing of your most recent meal, your level of physical activity, the presence of coexisting medical problems, medication use, and the time of your most recent bowel movement. Normal colonic motility, or movement, involves mixing the liquid material from the ileum within the ascending colon and then propelling the material from the right side of the colon to the sigmoid colon and rectum, where it can be concentrated further, stored, and then evacuated. The normal transit of material through the colon takes approximately 36 hours (although it can differ dramatically from one person to the next), which is equally divided between the right colon (ascending colon), transverse colon, and left colon (descending colon, sigmoid colon, and rectum).

Normal Pelvic Floor Function

Aside from the GI tract, there are other areas of the body that must function normally to avoid problems such as diarrhea, constipation, pain, and bloating. The pelvic floor is a group of muscles that support the internal organs of the lower abdomen and pelvis (the urethra, bladder, vagina, cervix, uterus, prostate gland, anal canal, and rectum; see Figures 6.2 and 6.3). The muscles of the pelvic floor include the pubococcygeus, iliococcygeus, and puborectalis. When viewed in cross-section, these muscles form a gently sloped funnel that stretches from the coccyx (tail bone) to the pubic bone. When viewed from the front (anterior) to the back (posterior), the pelvic floor muscles run from both sides of the inner hip bones and blend together in the midline.

Healthy pelvic floor muscles support the internal organs, keeping them in proper position and assisting them in normal functions. These muscles play an especially important role in the health and normal function of the bladder and rectum: they assist in the complete emptying of both of these organs. In addition, pelvic floor muscles prevent leakage from the bladder (urinary incontinence) and rectum (fecal incontinence). If a person’s pelvic floor muscles are injured, or if the muscles do not work in a coordinated manner, then she or he may experience constipation, fecal incontinence, or urinary incontinence.

Figure 6.2. Anatomy of the Pelvic Floor
This cross-sectional view shows the muscles of the pelvic floor and the organs supported by it in a woman. The pelvic floor consists of a group of muscles that form a funnel-shaped structure stretching from the pubic bone at the front to the coccyx (the tail bone) at the back and to the pelvic bones on the sides. The muscles of the pelvic floor are the pubococcygeus, iliococcygeus, and puborectalis. The organs supported by the pelvic floor in women are (from front to back) the bladder, vagina, uterus, anal canal, and rectum; in men, the bladder, prostate gland, and anorectal area are supported by the pelvic floor.

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Abnormal Colon Function

Constipation and diarrhea are not diseases; they are symptoms of abnormal colon function. Although several different conditions may cause these symptoms (see Tables 6.1 and 6.2), IBS is one of the most common causes of either constipation or diarrhea. How does IBS cause these two contradictory processes?

People who have IBS can develop constipation for many different reasons. One, the movement of material through the colon may be very slow (this can occur when either the muscles or the nerves that supply the colon have been injured). Two, there may be poor coordination between the muscles and nerves in the colon. (The muscle and nerve systems may be normal, but if their actions are not coordinated, they won’t function normally as a group.) Three, people who have IBS may become constipated because their pelvic floor muscles do not function or coordinate normally. In some people who have IBS, especially younger women, the complex signals required to have a normal bowel movement become mixed up or confused (see pelvic floor dyssynergia, described later in the chapter). Finally, some people who have IBS and chronic constipation have GI tracts that function normally, even when evaluated by many different tests (see Chapter 8). These people are said to have normal transit constipation, which is constipation (infrequent stools, bloating, fullness, abdominal pressure) without evidence of a mechanical obstruction or any abnormalities in colonic motility or pelvic floor function. Understandably so, normal transit constipation is a difficult concept for patients and physicians to comprehend. Normal transit constipation may represent an undefined neurochemical or hormonal problem; alternatively, it may represent a sensory disorder. This type of constipation is an active area of research within the field of gastroenterology, and physicians may have a better understanding of why this condition develops sometime in the next decade.

Figure 6.3. Pelvic Floor Changes during Evacuation
Many steps need to occur in an intricate and properly timed sequence in order to evacuate stool easily and effectively. In A, the puborectalis muscle is contracted, producing a tight “turn” at the junction of the anal canal and rectum. This turn, called the anorectal angle, is usually approximately 90 degrees when evacuation is not occurring. The relative tightness of the angle helps maintain continence. At the time of defecation, the puborectalis muscle relaxes, opening up the anorectal angle to approximately 135 degrees (as in B). This wider angle, in combination with relaxation of the external anal sphincter and the internal anal sphincter, makes evacuation of stool much easier.

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Table 6.1. Common Causes of Constipation

Anal fissure Anatomical obstruction Colorectal cancer Compression of the colon caused by an abdominal or pelvic mass Irritable bowel syndrome Medications Metabolic disorders Muscular disorders

Neurologic disorders Pelvic floor dysfunction Psychiatric disorders Rectal prolapse Rectocele Slow intestinal motility Stricture caused by Crohn’s disease Stricture caused by diverticulitis Stricture caused by ischemic colitis

Table 6.2. Common Causes of Diarrhea

Abnormal GI tract motility

Dietary changes (for example, excess fiber; lactose, fructose, or gluten intolerance)

Functional bowel disorders (for example, IBS)

Increased secretion of fluid in the small intestine

Infections (viral, bacterial, parasite)

Inflammatory bowel disease (Crohn’s disease or ulcerative colitis)

Injury to the lining of the GI tract that prevents normal absorption

Medications

Metabolic disorders (for example, hyperthyroidism, long-standing diabetes)

Osmotic agents (sugars that can’t be absorbed, such as sorbitol)

Prior gallbladder surgery

Short gut syndrome

People who have IBS and diarrhea may also have malfunctioning colons. The colon may contract too vigorously, especially in the sigmoid colon, which can lead to cramps and spasms in the rectosigmoid area and a sense of urgency to defecate. In some people who have IBS and diarrhea, materials move too quickly through the colon, which minimizes how much time the colon has to absorb water and concentrate stool. When materials move too quickly through the colon, people will have softer, more liquid stools.

Abnormal Pelvic Floor Function

People who have pelvic floor dysfunction often complain of excessive straining, prolonged or excessive time spent in the bathroom attempting to have a bowel movement, and feelings of incomplete evacuation (“I went a little but still feel like I need to go more”). Some people need to assist evacuation with digital stimulation (inserting a finger into the rectum), pushing on the perineal body (the small muscular area just in front of the rectum), or vaginal splinting (placing a finger or fingers in the vagina to assist with defecation).

Pelvic floor dyssynergia is one of the most common pelvic floor disorders to cause constipation. In people who have pelvic floor dyssynergia (primarily women), the internal anal sphincter does not relax properly and/or the external anal sphincter contracts inappropriately during attempted defecation. People may develop the sensation that they need to have a bowel movement, but when they push or strain, they tighten the external anal sphincter muscle and block the normal evacuation of stool. This condition does not respond well to medications; it is treated most effectively with physical therapy and a bowel retraining program.

There are many other problems that can develop in the pelvic floor or anorectal area and lead to constipation: rectal prolapse (a portion of the lining of the rectum is pushed out, usually with severe straining), intussusception (the lining of the rectum folds on itself and impedes or prevents normal defecation), or the formation of a rectocele (a bulging of the rectal wall; this usually occurs in the anterior direction, toward the vagina). Less commonly, people who are constipated may have descending perineum syndrome (abnormal descent of the pelvic floor) or weakened muscle contractions in the rectum.

Intestinal Gas

What’s Normal?

Gas in the intestinal tract is normal, although it can be uncomfortable or embarrassing for some people. Intestinal gas is usually one of five types of gas: nitrogen, oxygen, carbon dioxide, hydrogen, or methane. Most intestinal gas is nitrogen, with oxygen being the second most common.

Whereas nitrogen and oxygen are present in the intestinal tract because they have been swallowed, carbon dioxide, hydrogen, and methane are present because they are formed within the GI tract. Gas may develop after a person ingests different foods or liquids. People who drink large amounts of carbonated beverages may find that they have more problems with upper intestinal gas than people who drink only water or noncarbonated beverages. Most intestinal gas develops during the normal digestive process, when sugars and simple carbohydrates are broken down and completely absorbed while other sugars and carbohydrates are only partially broken down. The remaining sugars and carbohydrates are then fermented by bacteria in the intestinal tract, which produces hydrogen and, to a lesser degree, methane. As proteins and fats are broken down during the digestive process, they may produce carbon dioxide and small amounts of methane. Less commonly, gas can develop within the GI tract as a result of a chemical reaction. For example, gastric acid can react with bicarbonate, creating an end product of carbon dioxide. The carbon dioxide may be expelled during a burp, or it may diffuse through the stomach wall and into the bloodstream, from which it is breathed out during normal respiration.

Researchers have developed special devices to measure the amount of gas within the GI tract. Their studies have shown that the normal individual has 200 to 300 ml of gas within their intestinal tract at any one time (although not a perfect comparison, a typical 12-oz can of soda contains approximately 350 ml of liquid). Over the course of 24 hours, people release approximately 750 ml of gas from their rectum (technically referred to as flatus). Flatus typically occurs 10 to 15 times per day in a normal person.

Gas and the Upper GI Tract

Some people have problems with upper intestinal gas and recurrent or persistent belching and burping (the technical name for belching and burping is eructation). Nearly all gas present in the upper GI tract (that is, the stomach and upper small intestine) is there because people swallowed it, typically because they ate or drank too fast. Eating or drinking in the car, eating while talking on the phone, or eating while walking to a meeting are some of the most common ways of unintentionally swallowing a large amount of air. Some people easily swallow 2 to 3 liters of air during a single meal. Swallowing air (called aerophagia) can develop in some people as a nervous habit. In others, it may occur because oral stimulation (smoking, chewing gum, sucking on candies or mints) causes large amounts of saliva to be produced, which then leads to repetitive swallowing of both saliva and air.

Belching occurs when gas within the stomach produces a sensation of upper abdominal fullness, pressure, or discomfort. To release this gas, the lower esophageal sphincter must relax, temporarily forming a common cavity or connection between the stomach and the esophagus. This connection allows gas to rise up and move from the stomach into the esophagus. The upper esophageal sphincter then reflexively relaxes, and a noisy release of gas occurs. Belching or burping after a meal is considered a compliment in some countries but rude in others. Belching is not usually dangerous, but it can become part of a vicious cycle. Venting gas from your stomach may initially provide relief from the sensation of fullness or pressure, but some people end up swallowing more air at the end of the burp or belch. This air then creates more pressure or discomfort in the upper abdomen, forcing the person to belch again and subsequently swallow more air. This cycle can be broken in most people who have aerophagia by first recognizing the behavior and then modifying the maladaptive behavior that leads to air swallowing.

Gas and the Lower GI Tract

Some people seem to have a lot of intestinal gas, usually as a result of the breakdown and fermentation of dietary substances in their GI tract. Carbohydrates not completely broken down and absorbed in the small intestine will eventually pass into the colon. At this point, the bacteria that normally reside in the colon will ferment these undigested substances, leading to the production of lower intestinal gas. Typical undigested substances include the nonabsorbable carbohydrates stachyose and raffinose (found in beans and legumes), lactose (milk sugar), and poorly absorbed carbohydrates such as fructose and sorbitol (found in fruit juices, sports drinks, “energy” drinks, and fruit).

You may be surprised to learn that people who have IBS generally have the same amount of intestinal gas as people who do not have IBS. This fact has been confirmed by x-rays and by measuring the amount of gas in the intestinal tracts of both groups of people (those who have and those who do not have IBS) who complain of feeling bloated or gassy. Many people who have IBS, however, seem to be very sensitive to even small amounts of gas within their intestinal tract. This gas may make them feel bloated or distended, and it can occasionally cause crampy pain and discomfort.

That some people who have IBS are hypersensitive to gas in the intestinal tract should not be surprising, since we know that people who have IBS are generally more sensitive to pain throughout their intestinal tract, compared to people who do not have IBS. This hypersensitivity to distention caused by gas in the GI tract was confirmed by several research studies. During these studies, a small tube was inserted into the colon of both healthy volunteers and people who had IBS. Increasing amounts of gas were then infused through the tube, and the study subjects were asked to indicate when they could begin to sense the gas distending their colon and when they considered the pressure uncomfortable or painful. These studies all showed that people who had IBS sensed the distention sooner and felt discomfort at lower amounts of pressure during the procedure than did the healthy volunteers. People who had IBS reported that even small amounts of gas in the colon were uncomfortable or painful, whereas the healthy volunteers did not have any complaints of pain or discomfort. These studies support the view that people who have IBS sense things differently in their GI tract, whether it be gas, peristalsis, or pain.

Finally, data collected during the last several years have shown that many people who have IBS, especially those with chronic diarrhea, do not digest fructose well. This is an important finding, because fructose is a common additive to a large number of food products in the United States. Treatment options for gas and bloating are reviewed in Chapters 15 and 19.

Summary

• The GI tract extends from the mouth to the anus and is 25 to 30 feet long.

• The process of digestion begins in the mouth, accelerates in the stomach via mixing and grinding and the addition of various enzymes, and continues in the small intestine with additional enzymes secreted by the pancreas.

• Constipation and diarrhea are symptoms rather than diseases. There are many different medical conditions that can produce symptoms of constipation, diarrhea, or both.

• The role of pelvic floor dysfunction is often overlooked during the evaluation of a person who has IBS and constipation. This disorder can be identified by history, a careful physical examination, and anorectal manometry. Pelvic floor retraining is the best therapy for this disorder; medications are rarely effective.

• Intestinal gas is a problem for many people, and people who have IBS are frequently more sensitive to the effects of intestinal gas than others. Dietary factors (lactose, fructose, artificial sugars, fiber) are often the culprits.