Adenosine Triphosphate Functions as an “Energy Currency” in Metabolism Carbohydrates, fats, and proteins can all be used by cells to synthesize large quantities of adenosine triphosphate (ATP), which is used as an energy source for almost all other cellular functions. For this reason, ATP has been called an energy “currency” in cell metabolism. Indeed, the transfer of energy from foodstuffs to most functional systems of the…

Energy Intake And Output are Balanced Under Steady-State Conditions Proteins, carbohydrates, and fats in the diet provide energy for various body functions or for storage and later use. Stability of body weight and composition over long periods requires that a person’s energy intake and energy expenditure be balanced. When a person is overfed, and energy intake persistently exceeds expenditure, most of the excess energy is stored…

Although the liver is a discrete organ, it performs many different interrelating functions. The critical interrelated functions of the liver become especially evident when abnormalities of the liver occur. This chapter summarizes some of the major functions of the liver, including the following: (1) filtration and storage of blood; (2) metabolism of carbohydrates, proteins, fats, hormones, and foreign chemicals; (3) formation of bile; (4) storage of…

About three-quarters of the body solids are proteins. These proteins include structural proteins, enzymes, nucleoproteins, proteins that transport oxygen, proteins of the muscle that cause muscle contraction, and many other types that perform specific intracellular and extracellular functions throughout the body. The basic chemical properties that explain the diverse functions of proteins are so extensive that they constitute a major portion of the entire discipline of…

Several chemical compounds in food and in the body are classified as lipids, including the following: (1) neutral fat , also known as triglycerides; (2) phospholipids; (3) cholesterol; and (4) a few others of less importance. Chemically, the basic lipid moiety of triglycerides and phospholipids is fatty acids , which are long-chain hydrocarbon organic acids. A typical fatty acid, palmitic acid, is the following: CH 3…

The next few chapters deal with metabolism in the body—the chemical processes that make it possible for the cells to continue living. It is not the purpose of this text to present the chemical details of all the various cellular reactions, which lie in the discipline of biochemistry. Instead, these chapters are devoted to (1) a review of the principal chemical processes of the cell and…

Effective therapy for most gastrointestinal disorders depends on a basic knowledge of gastrointestinal physiology. The purpose of this chapter is to discuss a few representative types of gastrointestinal malfunction that have special physiological bases or consequences. Disorders of Swallowing and the Esophagus Paralysis of the Swallowing Mechanism Damage to the fifth, ninth, or tenth cerebral nerve can cause paralysis of significant portions of the swallowing mechanism.…

The major foods on which the body lives (with the exception of small quantities of substances such as vitamins and minerals) are carbohydrates, fats, and proteins. They generally cannot be absorbed in their natural forms through the gastrointestinal mucosa, and for this reason, they are useless as nutrients without preliminary digestion. This chapter discusses the processes by which carbohydrates, fats, and proteins are digested into small…

Throughout the gastrointestinal tract, secretory glands subserve two primary functions: (1) digestive enzymes are secreted in most areas of the alimentary tract, from the mouth to the distal end of the ileum; and (2) mucous glands located from the mouth to the anus provide mucus for lubrication and protection of all parts of the alimentary tract. Most digestive secretions are formed in response to the presence…

The time that food remains in each part of the alimentary tract is critical for optimal processing and absorption of nutrients. In addition, appropriate mixing must be provided. Because the requirements for mixing and propulsion are quite different at each stage of processing, multiple automatic nervous and hormonal mechanisms control the timing of each of these activities so they will occur optimally—not too rapidly and not…

The alimentary tract provides the body with a continual supply of water, electrolytes, vitamins, and nutrients, which requires the following: (1) movement of food through the alimentary tract; (2) secretion of digestive juices and digestion of the food; (3) absorption of water, various electrolytes, vitamins, and digestive products; (4) circulation of blood through the gastrointestinal organs to carry away the absorbed substances; and (5) control of…

Thus far, we have discussed the function of the brain as if it were independent of its blood flow, its metabolism, and its fluids. However, this is far from true because abnormalities of any of these aspects can profoundly affect brain function. For example, total cessation of blood flow to the brain causes unconsciousness within 5 to 10 seconds because lack of oxygen (O 2 )…

The autonomic nervous system is the portion of the nervous system that controls most visceral functions of the body . This system helps to control arterial pressure, gastrointestinal motility, gastrointestinal secretion, urinary bladder emptying, sweating, body temperature, and many other activities. Some of these activities are controlled almost entirely and some only partially by the autonomic nervous system. One of the most striking characteristics of the…

All of us are aware of the many different states of brain activity, including sleep, wakefulness, extreme excitement, and even different levels of mood such as exhilaration, depression, and fear. All these states result from different activating or inhibiting forces generated usually within the brain. In Chapter 59 , we began a partial discussion of this subject when we described different systems that are capable of…

Control of behavior is a function of the entire nervous system. Even the wakefulness and sleep cycle discussed in Chapter 60 is one of our most important behavioral patterns. In this chapter, we deal first with the mechanisms that control activity levels in different parts of the brain. Then we discuss the causes of motivational drives, especially motivational control of the learning process and feelings of…

It is ironic that of all the parts of the brain, we are the least certain about the functions of the cerebral cortex, even though it is by far the largest and perhaps the most studied portion of the nervous system. However, we do know the effects of damage or stimulation of various portions of the cerebral cortex. In the first part of this chapter, the…

In addition to areas in the cerebral cortex that stimulate muscle contraction, two other brain structures are essential for normal motor function—the cerebellum and the basal ganglia . Neither of these structures can control muscle function by itself. Instead, these structures always function in association with other systems of motor control. The cerebellum plays major roles in timing of motor activities and in rapid, smooth progression…

Most “voluntary” movements initiated by the cerebral cortex are achieved when the cortex activates “patterns” of function stored in lower brain areas—the cord, brain stem, basal ganglia, and cerebellum. These lower centers, in turn, send specific control signals to the muscles. For a few types of movements, however, the cortex has almost a direct pathway to the anterior motor neurons of the cord, bypassing some motor…

Sensory information is integrated at all levels of the nervous system and causes appropriate motor responses that begin in the spinal cord with relatively simple muscle reflexes, extend into the brain stem with more complicated responses, and finally extend to the cerebrum, where the most complicated muscle skills are controlled. In this chapter, we discuss spinal cord control of muscle function. Without the special neuronal circuits…

The senses of taste and smell allow us to separate undesirable or even lethal foods from those that are pleasant to eat and nutritious. They also elicit physiological responses involved in the digestion and utilization of foods. The sense of smell allows animals to recognize the proximity of other animals or even individual animals. Finally, both senses are strongly tied to primitive emotional and behavioral functions…