MCAT Biology Review

Chapter 9: The Digestive System

Introduction

As we continue our survey of organ systems, we come to the digestive system. As with our previous reviews of other organ systems, we will start with a basic anatomical overview of the organs of digestion (including the accessory organs) and then move on to discuss how these organs function to provide nutrition to the individual. The food we eat is complex, composed of meats, grains, vegetables and fruits, dairy products, and sugars. The job of the digestive system is to take these complex foods—composed of polysaccharides, fats, and proteins—and turn these large macromolecules into smaller, simpler monosaccharides, fatty acids, and amino acids. In order to cleave all of these bonds, the body requires a complex system of mechanical and chemical agents. These compounds can then be absorbed from the gut, transported to the tissues by the circulatory system, and used by cells. In this chapter, we will consider the organs that make up the digestive system as well as the processes by which the foods we eat become the fuel we need for energy, growth, development, and maintenance of other essential activities.

9.1 Anatomy of the Digestive System

There are two types of digestion that occur. First, intracellular digestion, as a part of metabolism, involves the oxidation of glucose and fatty acids for energy. However, our diets do not consist of pure glucose and fatty acids; rather, these substances must be extracted from our food. The process by which these nutrients are obtained from food occurs within the lumen of the alimentary canal and is known as extracellular digestion. This is technically “outside” the body, as it is outside the cell borders. The alimentary canal runs from the mouth to the anus and is sectioned off by sphincters, or circular smooth muscles around the canal that can contract to allow compartmentalization of function.

The human digestive tract has specialized sections with different functional roles. The most basic functional distinction is between digestion and absorption. Digestion involves the breakdown of food into its constituent organic molecules: lipids (fats) into free fatty acids and glycerol, starches and other carbohydrates into monosaccharides, and proteins into amino acids. Digestion can be subdivided into mechanical and chemical processes. Mechanical digestion is the physical breakdown of large food particles into smaller food particles, but does not involve breaking chemical bonds. Chemical digestion is the enzymatic cleavage of chemical bonds, such as the peptide bonds of proteins or the glycosidic bonds of starches. Absorption involves the transport of products of digestion from the digestive tract into the circulatory system for distribution to the body’s tissues and cells.

The digestive tract, shown in Figure 9.1, begins with the oral cavity (mouth) followed by the pharynx, a shared pathway for both food entering the digestive system and air entering the respiratory system. From the pharynx, food enters the esophagus, which transports food to the stomach. From the stomach, food travels to the small intestine, and then the large intestine. Finally, waste products of digestion enter the rectum, where feces are stored until the appropriate time of release. In addition to the actual organs of the digestive tract, the salivary glands, pancreas, liver, and gallbladder help to provide the enzymes and lubrication necessary to aid the digestion of food.

Figure 9.1. Anatomy of the Digestive System

The enteric nervous system is a collection of one hundred million neurons that governs the function of the gastrointestinal system. These neurons are present in the walls of the digestive tract and trigger peristalsis, or rhythmic contractions of the gut tube, in order to move materials through the system. This system can function independently of the brain and spinal cord, although it is heavily regulated by the autonomic nervous system. The parasympathetic division is involved in stimulation of digestive, activities, increasing secretions from exocrine glands and promoting peristalsis. The sympathetic division is involved in inhibition of these activities. The fact that so often we feel sleepy and lethargic after eating a big meal (often called a food coma colloquially) is due, in part, to parasympathetic activity. On the other hand, during periods of high sympathetic activity, blood flow is decreased to the digestive tract, and gut motility slows significantly.

KEY CONCEPT

All of the glands of the body (except sweat glands) are innervated by the parasympathetic nervous system.

MCAT Concept Check 9.1:

Before you move on, assess your understanding of the material with these questions.

1. What is the difference between mechanical and chemical digestion?

· Mechanical digestion:

· Chemical digestion:

2. Trace the path of food through the body, starting with ingestion and ending with excretion of feces:

3. What effect does the parasympathetic nervous system have on the digestive system? What effect does the sympathetic nervous system have?

· Parasympathetic nervous system:

· Sympathetic nervous system: