Chapter 5: The Digestive System

Learning Objectives

By the end of this chapter, you will be able to:

• Describe the different levels of organization in the human body.
• Describe the processes of digestion, absorption, and elimination.
• Describe the specialized functions of each region of the digestive tract.
• Explain the role of the accessory organs in processing food.
• Describe how organs work together to digest food and absorb nutrients.

Humans, like all living things, need nutrients to survive. We obtain nutrients – carbohydrates, lipids, proteins, vitamins, minerals, and water – from foods and beverages. Getting nutrients into the body where they can be used is accomplished by the multistep processes of digestion and absorption. Materials that are not pass out of the body through excretion. During digestion, food particles are broken down to smaller components. This process includes mechanical processes such as chewing, and chemical processes involving action by stomach acid and enzymes. Once broken down into small enough pieces, food components are absorbed through the lining of the digestive system and into the circulatory system.

Cells, Tissues, Organs, Organ Systems, and Organisms

To understand the digestive system, we must understand the levels of organization in the human body (see Figure 5.1). All living things, or organisms, are composed of cells. Cells are the building blocks, both structurally and functionally, of all living things. Cells can differ from each other in size and other characteristics, bacterial may be tiny compared to a long, large nerve cell of an animal. Yet all cells have common features including genetic information in the form of deoxyribonucleic acid (DNA), proteins that do the work of the cells, and a boundary (cell membrane or cell wall) that separates the cell interior from the environment or adjacent cells. Eukaryotic cells that make up plants, fungi and animals, have internal structures called organelles. An important organelle is the mitochondrion which processes sugars into cellular energy in the form of ATP.

Tissues are groups of cells that share a common structure and function and work together. There are four basic types of human tissues: connective, which connects tissues; epithelial, which lines and protects organs; muscle, which contracts for movement and support; and nerve, which responds and reacts to signals in the environment. Groups of tissues arranged in a specific manner to support a common physiological function are called organs. Examples include the brain, liver, and heart. Groups of organs are organized into organ systems that support a specific function; for example, the digestive system breaks down food and absorbs its nutrients. An organism is the complete living system capable of conducting all of life’s biological processes.

Overview of Digestion

The process of digestion begins even before you put food into your mouth. When you feel hungry, your body sends a message to your brain that it is time to eat. Sights and smells influence your body’s preparedness for food. Smelling food sends a message to your brain. Your brain then tells the mouth to get ready, and you start to salivate in preparation for a meal.

Once you have eaten, your digestive system starts the process that breaks down the components of food into smaller molecules that can be absorbed into the body. Two types of processes are essential to digestion: mechanical processes shred, mix, and move ingested food while chemical processes break large structures into pieces small enough to be absorbed (see figure 5.2). Once absorbed, nutrients can be used by cells throughout the body for energy or as building blocks for new molecules and cells.

The digestive system is one of eleven organ systems of the human body (see Figure 5.3). It is composed of hollow tube-shaped organs including the mouth, pharynx, esophagus, stomach, small intestine, large intestine (colon), and rectum. This continuous tube is called the digestive tract, gastrointestinal (GI) tract, or the alimentary canal. It is lined with mucosal tissue that secretes digestive juices (which aid in the breakdown of food) and mucus (which facilitates the movement of food through the tract). Smooth muscle tissue surrounds the digestive tract and contracts in waves that propel food along the tract. This wave-like contraction is called peristalsis. As food passes through the digestive tract, nutrients as well as some non-nutrients are absorbed. Substances which are not absorbed, such as fiber, may be digested by microorganisms living in the intestines. Substances that are not absorbed are passed out of the body as feces in the process of excretion.

In addition to the digestive tract, the digestive process requires accessory organs. In the mouth, the teeth and tongue move and process the food into a bolus that can be swallowed. Salivary glands are accessory organs that produce saliva, an important liquid containing water, mucus, and enzymes to soften the bolus and begin digestion. Accessory organs we will discuss below also include the liver, which produces bile, the gallbladder where bile is stored, and the pancreas.

 From the Mouth to the Stomach

There are four steps in the digestion process: ingestion, digestion, absorption and excretion. Ingestion is the intake of food into the digestive tract. It may seem a simple process, but ingestion can include the smelling food or even thinking about food, which can lead to the involuntary release of saliva in the mouth to prepare for food entry. The mouth is the site of the beginning of the second step, digestion, which is the mechanical and chemical breakdown of food. Important chemicals include enzymes, such as salivary amylase that starts the breakdown of starch, and lingual lipase, which breaks down lipids.

 

Mechanical breakdown starts with mastication (chewing) in the mouth. Teeth crush and grind large food particles, while saliva allows us to taste and provides lubrication. The slippery mass of partially broken-down food is called a bolus, which moves from the mouth through the pharynx and into the esophagus as you swallow. Swallowing may seem voluntary at first because it requires conscious effort to push the food with the tongue back toward the throat, but after this, swallowing proceeds involuntarily, meaning it cannot be stopped once it begins. As the bolus travels through the pharynx, a small flap called the epiglottis covers the top of the trachea to keep food out of the lungs and to prevent choking. Peristaltic contractions in the esophagus propel the food bolus down to the stomach (see Figure 5.4). Circular muscles in the esophageal wall contract, pushing the bolus down the esophagus. Longitudinal muscles contract, shortening the pathway for the bolus. The rhythmic contractions propel food down the esophagus. The bolus reaches the stomach from the wave of peristaltic contractions causing, the sphincter to relax and allow the bolus to enter the stomach. Solid food takes between four and eight seconds to travel down the esophagus, and liquids take about one second. Peristalsis of one sort or another occurs throughout most of the digestive tract.

At the junction between the esophagus and stomach, the gastrointestinal sphincter remains closed until the food bolus approaches. The pressure of the food bolus stimulates the sphincter to relax and open so food may enter the stomach. Sphincters are specialized muscular structures that, as parts of the digestive tract, keep the contents of the tract from “backing up” or moving too quickly from one section to the next.

From the Stomach to the Small Intestine

In the stomach, a highly muscular organ, powerful peristaltic contractions mash, pulverize, and churn food into chyme. Chyme is a semiliquid mass of partially digested food that contains liquids added in the digestive tract. Cells lining the stomach produce gastric juice which contains hydrochloric acid and the enzyme pepsin. Hydrochloric acid kills bacteria on the food and denatures the food structure that remains after chewing. Pepsin is an enzyme that begins the breakdown of protein.

The length of time food spends in the stomach varies by the macronutrient composition of the meal. A high-fat or high-protein meal takes longer to break down than one rich in carbohydrates. It usually takes a few hours after a meal to empty the stomach contents completely into the small intestine. Chyme passes from stomach to small intestine through the pyloric sphincter. Small amounts are passed at a time to protect the delicate lining of the small intestine from the acidic chyme; neutralizing bicarbonate is added to the chyme as it enters the small intestine.

The small intestine is divided into three structural regions: the duodenum, the jejunum, and the ileum. When chyme enters the duodenum (the first segment of the small intestine), the pancreas and gallbladder release juices that aid in digestion. The pancreas secretes up to 1.5 liters (.4 US gallons) of pancreatic juice per day through a duct into the duodenum. This fluid contains bicarbonate as well as enzymes that digest proteins, carbohydrates, and lipids.

The gallbladder secretes a much smaller amount of a fluid called bile that breaks up droplets of lipids so that they can be digested. Bile passes through a duct that joins the pancreatic ducts and is released into the duodenum. Bile is made in the liver and stored in the gall bladder. Bile’s components act like detergents by surrounding fats similar to the way dish soap removes grease from a frying pan. This allows for the movement and digestion of lipids in the watery environment of the small intestine. Two different types of muscular contraction, peristalsis and segmentation, move and mix chyme in various stages of digestion through the small intestine.

Similar to what occurs in the esophagus and stomach, peristalsis is important in the small intestine to propel chyme in a generally-forward direction. Segmentation has a different function. Segmentation from circular muscle contraction slows movement in the small intestine by forming temporary “sausage link” type of segments that allows chyme to slosh food back and forth in small areas to promote mixing with enzymes and to enhance absorption (see Figure 5.5). Most food is broken down to its simplest units within the first 25 centimeters of the small intestine. Instead of proteins, carbohydrates, and lipids, the chyme now consists of amino acids, monosaccharides, and emulsified components of triglycerides.

Nutrient absorption, the third step of digestion, takes place mainly in the jejunum and ileum of the small intestine, which may be more than five meters in length. The small intestine is structured to maximize nutrient absorption. Its surface area in an adult is greater than 200 square meters, which is about the size of a tennis court. The large surface area is due to multiple levels of folding. The internal tissue of the small intestine is covered in villi, finger-like projections covered with even smaller projections, called microvilli (Figure 5.6). The digested nutrients pass through the absorptive cells of the intestine via diffusion or special transport proteins. Amino acids, short fatty acids, and monosaccharides (sugars) are transported from the intestinal cells into capillaries, while larger fatty acids, fat-soluble vitamins, and other lipids are transported first through lymphatic vessels, which soon meet up with blood vessels (Figure 5.7).

From the Small Intestine to the Large Intestine

The process of digestion is fairly efficient. Any food that is incompletely broken down (usually less than ten percent of food consumed) and indigestible fibers move from the small intestine to the large intestine (colon) through the ileocecal valve. A main task of the large intestine is to absorb much of the remaining water. Remember, water in chyme comes not only from food and beverages, but from the saliva, gastric juice, pancreatic juice, bile, and intestinal juice added to it along the way. For the body to conserve water, it is important that excessive water is not lost in fecal matter. In the large intestine, no further chemical or mechanical breakdown of food takes place except that accomplished by the bacteria and other microbes that are a normal, healthy part of the digestive tract. The great majority of bacteria in the intestines are harmless or beneficial.

From the Large Intestine to the Anus

After a few hours in the stomach, three to six hours in the small intestine, and sixteen hours in the large intestine, the digestion process enters step four, excretion. Undigestible food matter and live and dead bacteria (almost 50 percent of content) are compacted into feces, stored in the rectum, and expelled through the anus via defecation.

Nutrients Are Essential for Cell and Organ Function

When the digestive system has broken down food to its nutrient components, the body eagerly awaits delivery. Water soluble nutrients glucose, amino acids and water-soluble vitamins and minerals are absorbed into the blood and travel directly to the liver via a major blood vessel called the portal vein. Lipids are absorbed into the lymphatic system, which also contains immune system components such as white blood cells in a liquid called lymph. The lymphatic system slowly moves its contents through the lymphatic vessels and empties into blood vessels in the upper chest area. Now, the absorbed lipid soluble components are in the blood where they can be distributed throughout the body and utilized by cells (Figure 5.7).

The liver helps regulate distribution of the nutrients throughout the body to supply the needs of all the cells. One of the liver’s primary functions is to regulate metabolic homeostasis, which is achieved when the nutrients consumed match the needs of the body. The liver is capable of exporting nutrients for energy production to other tissues. Therefore, when a person is between meals (fasted state) the liver exports nutrients, and when a person has just eaten (fed state) the liver stores nutrients. Levels of nutrients and hormones in the blood signal the liver to store or to release glucose. Other signals lead to storage of lipids in adipose tissue or release of stored lipids for body cells to use as fuel to produce energy.

The human body requires nutrients to function. Breaking down food, removing nutrients from food, and absorbing nutrients into the body are the roles of the digestive system.

Attributions

Adapted from Human Nutrition, “The Digestive System,” by University of Hawai‘i at Mānoa Food Science and Human Nutrition Program which is licensed under a Creative Commons Attribution 4.0 International License.