Chapter 6

Getting on Track with the Digestive System

IN THIS CHAPTER

Understanding what the digestive system does

Following the path of the digestive tract

Looking at the role of the liver, pancreas, and digestive fluids in digestion

Just how does that meal of steak, potatoes, and salad become the tissues of your body? The digestive organ system gets it halfway there. (The circulatory system, which we cover in Chapter 4, does most of the rest.)

Living systems constantly exchange energy. Physiological processes — anabolic, catabolic, and homeostatic — require energy. Ultimately, that energy comes from the light energy that plants use to transform carbon in the atmosphere (as CO₂) into biological matter (as carbohydrate) in the process of photosynthesis. Humans get their energy by consuming this biological matter, either directly or by consuming other organisms that do. The digestive system takes apart this biological matter step by step and transforms it into a form that human cells can use. In this chapter, we explain the ins and outs of the process and the organs responsible for the chore.

Your Digestive System in a Nutshell

Digestion itself is a middle step. Other important functions of this organ system come before and after:

• Ingesting: Although all animals ingest — take something into the body through the mouth — only humans, and possibly some of the great apes, appear to enjoy food as they ingest it.

  The perception of subtle flavors is more closely connected to olfaction than digestion. The perception of the five basic flavors is also considered neurosensory. Perception in the mouth is more about texture and is closely related to food’s protein and fat content. This concept is captured by the food industry term mouth feel. These sensory perceptions guide you in the selection of foods.

  Sometimes, though, ingestion isn’t a feast for the senses — nothing delicious is available. The body still needs calories, though, so whatever’s available is chewed and swallowed just the same.

• Digesting: Eating is fun, and ingestion is bearable, but neither provides biological molecules that your cells can use. That task is accomplished by the interaction of physical and chemical forces. The digestive tract is a muscular tube lined with chemical factories that operate under the direction of their own dedicated neural structures and under hormonal control.

  The digestive system processes everything down the same track, extracting fuel, biological molecules and monomers, and micronutrients from whatever you eat. (See the section “Importing and exporting: The intestines,” later in this chapter.)

• Exporting nutrients to the body: The end products of digestion are biological molecules, such as glucose, that are absorbed across the digestive membrane into the blood and then distributed in the body.
• Eliminating: The elimination of digestive waste is part of digestion. Other organ systems have evolved to make use of the digestive system’s structures to eliminate metabolic wastes of other kinds.

Moving through the Structures of the Digestive Tract

The digestive tract, also called the alimentary canal (alimentary means food), or the gastrointestinal (GI) tract, is a tube through which ingested substances are pushed along for physical and chemical processing. The tube walls are made up of an outer fibrous layer, a muscular layer, a supportive connective tissue layer, and an inner layer (containing an epithelial lining), called the digestive mucosa. All the layers vary in thickness from one place to another along the digestive tract. The space inside the tube is the lumen, and its size varies, too.

The digestive system’s gross anatomy (no pun here) is comparable to that of an industrial smelter. Some structures bring in raw materials; other structures extract, process, and ship out specific substances; and still other structures export the unused part of the raw materials back into the environment. The body uses both mechanical and chemical mechanisms to break down the raw materials and export products to the larger system (the economy in the case of the smelter; the organism in the case of the intestine). These efficient systems are organized linearly — things keep moving along in one direction at a steady pace.

In the following sections, we describe in detail each structure of the digestive tract and what it does to keep things moving through the digestive process.

Allowing entry: The mouth

Your mouth is the starting point of your digestive system, the gateway to your other digestive organs. Besides making eating a fun experience, your mouth (or oral cavity, for the technical term) serves some important digestive functions, as described in the next sections.

Teeth and gums for tearing and grinding

Humans have 32 teeth — 16 on the top and 16 on the bottom. Your teeth tear and grind food into pieces that are small enough to swallow, and they come in four basic types: incisors for biting, canines for tearing, and premolars and molars for grinding.

The gingiva (gums) hold teeth in position, and a binding material called cementum embeds your teeth’s roots in your jawbone. Blood vessels that run through the jawbone and up into the pulp of the tooth supply the teeth with blood. Dentin, a bonelike material, covers the pulp, and an extremely hard protective enamel covers the dentin.

A tongue to help with chewing

Your tongue is mainly skeletal muscle tissue. The muscle is covered on the upper surface by a mucous membrane, in which are embedded taste buds. The tongue muscles move the food around in your mouth to assist chewing. The mucus moistens and lubricates the bolus, the technical term for a mouthful of food in the process of being chewed.

Muscles attach your tongue to your skull bones, and a mucous membrane on the tongue’s underside attaches your tongue to the oral cavity floor. That stringy piece of membrane that you see when you touch your tongue’s tip to the roof of your mouth is the lingual frenulum.

The buccal membrane to kick-start digestion

The buccal membrane is that portion of the digestive mucosa that lines the inside of the mouth. Several salivary glands have ducts that course through the buccal membrane and secrete mucus and salivary amylase, a digestive enzyme, into the oral cavity. These glands often go into action before you take the first bite of your meal. A delicious aroma or even just the anticipation of eating something you enjoy can get those juices flowing.

Sending food to the stomach: The pharynx and esophagus

The pharynx, better known as your throat, leads to the esophagus, the tube that extends from the mouth to the stomach. When you swallow, the bolus bounces off a piece of cartilage called the epiglottis and is diverted from the trachea (which we tell you about in Chapter 5) and into the esophagus (see Figure 6-1).

FIGURE 6-1: Structures of the mouth and pharynx.

The esophagus has two sphincters — one at the top and one at the bottom — that control the movement of the bolus into and out of the esophagus. The pharyngoesophageal sphincter, composed of skeletal muscle, takes part in the muscular actions involved in swallowing. Peristalsis propels the bolus along the esophagus. The lower esophageal sphincter surrounds the esophagus just as it enters the stomach.

Lining it up: The walls

The upper third of the esophagus is made of skeletal muscle. Beginning in the middle third of the esophagus and extending to the anal sphincter, layers of smooth muscle line the digestive tract. This smooth muscle contracts in pulsating waves, pushing the lumen’s contents along in a single direction. This constant wave-like contraction is called peristalsis.

A mucous membrane lines the digestive system, running continuously from the mouth all the way through to the rectum. This membrane protects your digestive organs from the strong acids and powerful enzymes secreted in the digestive system. The membrane’s innermost cells (next to the lumen) are among those cells that are continuously replaced.

The digestive mucosa’s secretions keep everything in the digestive tract moist, soft, and slippery, protecting the membrane and its underlying structures from abrasion and corrosion. The digestive mucosa contains tissues and cells that secrete other substances as well, including gastric acid, hormones, neurotransmitters, and enzymes. The digestive mucosa also contains an extensive network of lymphatic tissue.

Breaking it all down: The stomach

After passing through the bottom sphincter of the esophagus, the bolus drops into the stomach, the widest and most flexible part of the alimentary canal. Food remains in the stomach about two to six hours, during which it’s churned in an acidic substance that the stomach secretes called gastric juice, ground up by thousands of strong muscle contractions, and offered in tiny pieces to protein-digesting enzymes.

The outside of the stomach is a tough connective tissue layer called the serosa. Beneath the serosa is the muscular coat, which has three layers of smooth muscle fibers — oblique, circular, and longitudinal — that contract in different directions. Stretch receptors in this layer send nerve impulses to the brain when the stomach is full. These two layers support the structure of the stomach as a hollow organ.

Beneath these connective tissue layers are two mucosal layers, the submucosal coat and the mucous coat, also called the stomach lining. Gastric glands in the mucosa secrete the components of gastric juice. The mucosal coat is corrugated, increasing the surface area inside the hollow. The folds are called rugae. As the stomach fills, the rugae smooth out, allowing the stomach to expand.

The stomach’s muscular action is part of physical digestion, like chewing, swallowing, and peristalsis. But the stomach’s contribution to chemical digestion is what really helps break down the food you eat.

As the stomach churns the bolus in the gastric acid, the material turns into an oatmeal-like paste called chyme. The chyme squirts into the small intestine through the pyloric sphincter, between the lower part of the stomach, called the pylorus, and the top of the small intestine, called the duodenum.

Importing and exporting: The intestines

The intestine is a long muscular tube (up to about 20 feet, or 6 meters) that extends from the pyloric sphincter to the anal sphincter. How does 20 feet of tubing fit into a relatively small space that’s also crowded with other organs? It becomes narrow and convoluted. The intestines are classified as small and large based on their width, not their length (like hoses). The lumen of the small intestine is about 1 inch (2.5 centimeters) in diameter; the large intestine is about 2.5 inches (6.4 centimeters).

Overall, the intestine specializes in the import and export of biological substances of many kinds. As is usual for organs with import-export functions, the intestine has structures that maximize the surface area available for the exchange.

The intestine’s muscular outer walls lie coiled closely together within the abdominal cavity, held in place by the fibrous sheets of the peritoneum. With two layers of smooth muscle tissue, longitudinal and circular, the intestine specializes in strong, sustained peristalsis.

The intestinal mucosa is continuous with the rest of the digestive mucosa. It’s studded with specialized “work areas” that produce hormones, neurotransmitters, enzymes, and other substances integral to the digestive process.

The capillary beds that line the intestine define the interface of the digestive and circulatory systems. These capillaries are arrayed more or less continuously along the intestine’s lumen.

The lumen is lined by villi (singular, villus), a structure that’s specialized for import and export processes and that’s characteristic of tissues in body locations where substances are exchanged. Villi are fingerlike projections of the mucosa that multiply the surface area available for exchange, much like wharves and piers extending into a harbor increase the area for harbor activities.

Villi line the entire length of the small intestine, projecting out into the lumen. Each villus has its own assigned capillary for absorbing materials from the intestine into the blood (flip to Chapter 4 for more on the circulatory system). Microvilli are even smaller projections on the epithelial cells of the mucosa.

Some of these processes require active transport — the expenditure of some energy in the form of ATP (see Chapter 2).

The following sections focus in on the specific jobs of both the small and large intestines.

Investigating the small intestine

The small intestine does a lot of the physical work of the digestive system, beginning with peristalsis. It’s also majorly involved in digestive chemistry.

The small intestine is an endocrine gland as well as a digestive organ, producing and secreting hormones that control digestion. Cells in the small intestine’s walls secrete the hormones secretin and cholecystokinin (CCK), which stimulate the release of digestive fluids, such as bile, from the gallbladder and pancreatic juice from the pancreas.

The small intestine’s walls are lined with secretory tissue that functions in chemical digestion. Cells in the duodenum’s walls secrete digestive enzymes. Brunner’s glands in the small intestine’s lining secrete mucus and bicarbonate directly into the lumen to help neutralize the gastric juice in the chyme. (Most enzymes require a near-neutral pH.)

The small intestine is divided into three structures along its 10-to-20-foot (3-to-6-meter) length: the duodenum (about 1 foot long, or 0.3 meters), the jejunum (about 3 to 6 feet, or 1 to 2 meters), and the ileum (about 6 to 12 feet, or 2 to 4 meters). The small intestine is approximately 1 to 2 inches (2.5 to 5 centimeters) in diameter.

Understanding the work of the large intestine

Chyme oozes from the small intestine to the large intestine (also called the colon), passing out of the ileum through the ileocecal valve into the cecum, the first portion of the large intestine. The material is now called feces.

The large intestine is about 6 feet long (almost 2 meters) and is positioned anatomically like a “frame” around the small intestine. Beyond the cecum, the large intestine moves upward as the ascending colon, across as the transverse colon, and downward as the descending colon and finally into the sigmoid colon.

In the large intestine, water is reabsorbed from the feces by diffusion across the intestinal wall into the capillaries. The removal of water compacts the indigestible material in the colon, forming the characteristic texture of the feces.

In addition to undigested food, the feces contain other bodily wastes to be excreted. The brown color of feces comes from the combination of greenish-yellow bile pigments, broken down hemoglobin, and bacteria.

Your intestines are home to unimaginably large numbers of bacteria, including hundreds of species. Trillions of tiny (prokaryotic) cells ingest some of the undigested material in your feces, producing molecules that have a well-known odor. (It’s nothing to be embarrassed about and nothing to be proud of, either.) Some of these bacteria produce beneficial substances, such as vitamin K, which is necessary for blood clotting. These substances are absorbed through the intestinal wall and transported into the blood via the capillaries.

Passing through the colon and rectum

As the colon completes its work, peristalsis moves feces into the rectum, which is located at the bottom of the colon. Stretch receptors in the rectum signal to the brain the need to defecate (release feces) when the rectum contains about 5 to 8 ounces (142 to 227 grams). Pushed by peristalsis, the feces pass through the anal canal and exit the body through the anal sphincter.

Doing the Chemical Breakdown

The liver and pancreas are often referred to as the accessory organs of digestion. They’re not part of the digestive tract; they never come into contact with ingested material, and they take no part in the mechanical aspects of digestion. But they do produce and make available to the digestive tract’s organs some of the chemical and biological substances that assist in digestion’s chemical aspects.

We explore these organs and their function in the digestive process in the following sections.

The liver

The liver is one of the most important organs, not just in digestion but in many other functions. The liver’s digestive function is the production and transport of bile, one of the digestive chemicals.

Many of the terms related to the liver’s structures and functions contain the prefix hepato-, meaning “liver.”

Liver anatomy

Your liver is both the largest internal organ and the largest gland in the human body. A healthy adult human liver weighs about 3 to 3.5 pounds (1.4 to 1.6 kilograms). It’s located under your diaphragm and above your stomach on the right side of your abdomen (see Figure 6-2). The liver is soft, pinkish-brown, and triangular, with four lobes of unequal size and shape: the right lobe, left lobe, quadrate lobe, and caudate lobe. The liver is covered by a connective tissue capsule that branches and extends throughout its insides, providing a scaffolding of support for the afferent blood vessels, lymphatic vessels, and bile ducts that traverse it.

FIGURE 6-2: A closer look at the liver.

The liver receives oxygenated blood through the hepatic artery, which comes from the aorta. It receives nutrient-rich blood through the portal vein, which carries blood from the capillaries of the small intestine and the descending colon (see Chapter 4 for more on the circulatory system). Three hepatic veins drain deoxygenated blood from the liver, exiting the liver at the top of the right lobe and draining into the inferior vena cava.

Each of the four lobes is made up of tiny lobules, about 100,000 of them in all. The hepatic lobule is the liver’s functional unit. Each lobule is made up of millions of hepatic cells and bile canals and is supported and separated by branches of the capsule. At the lobule’s vertices are regularly distributed portal triads that contain a bile duct, a terminal branch of the hepatic artery, and a terminal branch of the portal vein. The hepatocytes are in a roughly hexagonal arrangement, with a vein in the center that carries the lobule’s products out into the blood. On the surface of the lobules are ducts, veins, and arteries that carry fluids to and from them.

Bile production and transport

The liver produces bile, a major factor in the digestion of fats and lipids of all kinds. The bile that some of the lobules produce is collected in bile canaliculi, which merge to form bile ducts. The intrahepatic (within the liver) bile ducts eventually drain directly into the duodenum through the common hepatic duct.

Bile can also be transported for storage into the gallbladder via the extrahepatic bile ducts. Your gallbladder is a pear-shaped sac tucked into the curve of your liver whose only function is to store bile and deliver it on demand to the duodenum. The bile flows through the common bile duct into the duodenum, near the entry point of the pancreatic duct.

Other functions of the liver

The liver functions in many other ways, affecting other organ systems. Here’s a brief overview of its many functions:

• It processes and eliminates toxins. Toxic byproducts of some drugs, including alcohol, and other substances arrive from the digestive organs through the portal vein.
• It processes and eliminates metabolic waste. The liver removes dying red blood cells from the blood and converts the hemoglobin to bilirubin and other byproducts. These are delivered to the intestine and excreted with the feces. (The iron is recycled.)
• It stores glucose in the form of glycogen and reconverts it when blood glucose levels get low. This function is mediated by insulin and glucagon.
• It stores vitamins and minerals.
• It produces many kinds of protein, including protein hormones, the plasma proteins, and the proteins of the clotting cascade and the complement system, as well as the production of alpha and beta globulin.

The pancreas

The pancreas sits in the abdominal cavity next to the duodenum and behind the stomach. It produces pancreatic juice, which is full of pancreatic enzymes that are important in digestion. Refer to Table 6-1 for information about pancreatic enzymes. Nearly every cell of the pancreas secretes pancreatic juice and passes it through the pancreatic duct into the duodenum.

TABLE 6-1 Pancreatic Enzymes

Enzyme	Targeted Nutrient	Result of Breakdown
Trypsin	Proteins	Peptides (chains of amino acids)
Peptidase	Peptides	Individual amino acids
Lipase	Fats	Fatty acids and glycerol
Nuclease	Nucleic acids (DNA, RNA)	Nucleotides
Amylase	Carbohydrates	Glucose and fructose

The pancreas also produces the hormones insulin and glucagon. These hormones work to control the amount of glucose in the blood.

Insulin is released when the blood’s glucose level rises. It acts by stimulating glucose uptake in cells. With plenty of glucose available, the cell’s metabolic rate increases, and it produces more of its specialized anabolic product. Glucose is burned to fuel physiologically productive reactions. Insulin also stimulates glucose uptake activity in the energy storage cells in the liver, muscle cells, and fatty tissue.

A lower blood glucose level stimulates the pancreas to secrete insulin’s partner glucagon, which pulls glucose from cells where it’s stored and releases it into the blood. Glucagon keeps the metabolic fires burning at a steady level.

Digestive fluids, enzymes, and hormones

Each part of the digestive system has its characteristic fluid, each a complex mixture of water, electrolytes, and biological substances with a specific role in digestion.

• Mucus: Every inch of the digestive tract has mucus glands whose secretions keep everything in the digestive tract moist, soft, and slippery, protecting the digestive membrane and its underlying structures from abrasion and corrosion.

• Saliva: Saliva (or spit) is a clear, watery solution that the salivary glands in your mouth produce constantly. You produce about 2 to 4 pints (1 to 2 liters) of spit every day. Saliva moistens food and makes it easier to swallow. It’s also a component of the sense of taste — a food substance must be dissolved in the watery solution for its chemical signals to act on your taste buds.

  Enzymes in saliva start starch digestion even before you swallow food. The combination of chewing food and coating it with saliva makes the tongue’s job a bit easier — it can push wet, chewed food toward the throat more easily.

  Saliva cleans the inside of your mouth and your teeth. The enzymes in saliva also help to fight off infections in the mouth.

• Enzymes: Thousands of enzymes are involved in digestion. Enzymes are specialized in their function — a given enzyme typically catalyzes one or only a few specific reactions. Digestive enzymes specialize in reactions that break specific molecules apart into component chemical entities. They can be broadly classified as proteinases and peptidases, lipidases, and various kinds of carbohydrate-active enzymes. Enzymes are part of the digestive fluids’ gastric juice and pancreatic juice.

  The suffix -ase indicates an enzyme that breaks a molecule apart.

• Gastric juice: Gastric juice is secreted from millions of tiny gastric glands in the gastric mucosa and enters the hollow of the stomach through gastric pits on the mucosa’s inner surface. Gastric juice contains hydrochloric acid (HCl), which is extremely acidic and kills bacteria that may have entered the body with food. It also contains the powerful proteinase pepsin, which can work only in this highly acidic environment.
• Pancreatic juice: Pancreatic juice contains many types of digestive enzymes. Refer to Table 6-1 for details.

• Bile: Bile, also called gall, is a very alkaline, bitter-tasting, dark-green to yellowish-brown fluid produced by the liver. Bile may remain in the liver or be transported to the gallbladder for storage before being expelled into the duodenum.

  The physiological function of bile is to emulsify fats — that is, to create an environment in which lipid-based substances can be mixed in a watery matrix for transportation and to make them available for chemical reactions to break them down. Bile’s high alkalinity helps neutralize the strongly acidic chyme that comes into the duodenum from the stomach. Another purpose of bile is to help absorb the fat-soluble vitamins K, D, and A into the blood.