Homeostasis

This maintenance of relatively constant or steady-state internal conditions is referred to as homeostasis. It is important because the cells and tissues of the body will survive and function efficiently only when these internal conditions are properly maintained. This is not to say that the internal

Table 1.1 Contribution of Organ Systems to the Maintenance of Homeostasis

Organ System

Function

Nervous system

Endocrine system

Muscular system Circulatory system

Respiratory system

Gastrointestinal tract Renal system

Regulates muscular activity and glandular secretion; responsible for all activities associated with the mind Regulates metabolic processes through secretion of hormones

Allows for body movement; contributes to thermoregulation Transports nutrients, O2, waste, CO2, electrolytes, and hormones throughout the body Obtains oxygen and eliminates carbon dioxide; regulates acid-base balance (pH) Digests food to provide nutrients to the body

Eliminates waste products from the body; regulates blood volume and blood pressure; regulates acid-base balance (pH)

environment is fixed or unchanging. The body is constantly faced with a changing external environment as well as with events and activities occurring within it that may alter the balance of important variables. For example, most metabolic reactions within cells consume oxygen and glucose. These substances must then be replaced. In addition, these reactions produce metabolic wastes including carbon dioxide and urea, which must then be eliminated. Therefore, it is more accurate to say that the internal environment is in a dynamic steady state — one that is constantly changing, but in which optimal conditions are physiologically maintained.

All of the organ systems in the body, except the reproductive system, contribute to the maintenance of homeostasis (see Table 1.1). For example, the gastrointestinal tract digests foods to provide nutrients to the body. The respiratory system obtains oxygen and eliminates carbon dioxide. The circulatory system transports all of these materials and others from one part of the body to another. The renal system eliminates wastes and plays a role in regulating blood volume and blood pressure.

The study of physiology includes study not only of how each of these systems carries out its functions, but also of the mechanisms involved that regulate these activities in order to maintain homeostasis under a variety of conditions. For example, the body's needs are very different during a resting state compared to that of exercise. How do organ systems adjust their activities in response to varied levels of physical exertion or when confronted with altered internal and external environments? In order to maintain homeostasis, the body must be able to monitor and sense changes in the internal environment. It must also be able to compensate, or make adjustments, for these changes.

Two regulatory systems in the body influence the activity of all the other organ systems so that homeostasis is ultimately maintained:

• Nervous system

• Endocrine system

The nervous system has three functional components (see Figure 1.1):

• Sensory division of the peripheral nervous system

• Central nervous system

• Motor division of the peripheral nervous system

Many different types of sensory receptors are located throughout the body. These receptors monitor the status of the internal environment or that of the surroundings. Sensory receptors are sensitive to specific types of stimuli and measure the value of a physiological variable. For example, arterial baroreceptors measure blood pressure and chemoreceptors measure the oxygen and carbon dioxide content of the blood. The information detected by these sensors then travels by way of afferent neuronal pathways to the central nervous system (CNS). The CNS is the integrative portion of the nervous system and consists of the (1) brain and the (2) spinal cord.

The brain receives, processes, and stores sensory input; generates thoughts; and determines the reactions that the body should perform in

CENTRAL NERVOUS SYSTEM

CENTRAL NERVOUS SYSTEM

Figure 1.1 Functional components of the nervous system. The sensory division of the peripheral nervous system is sensitive to changes in the internal and external environment. The information gathered by this component is transmitted to the CNS where it is processed, integrated, and interpreted. The CNS then determines the appropriate response to this input. This response is carried out by the transmission of nerve impulses in the motor division of the peripheral nervous system to the effector tissues.

Figure 1.1 Functional components of the nervous system. The sensory division of the peripheral nervous system is sensitive to changes in the internal and external environment. The information gathered by this component is transmitted to the CNS where it is processed, integrated, and interpreted. The CNS then determines the appropriate response to this input. This response is carried out by the transmission of nerve impulses in the motor division of the peripheral nervous system to the effector tissues.

response to this input. The spinal cord is important in processing reflexes. It is within this integration area of the nervous system that the actual value of a physiological variable as measured by a sensory receptor is compared to its set point or optimal value. One or more compensatory responses are then determined.

The third component of the nervous system is the motor division. Appropriate signals are transmitted from the CNS to various body parts or effector tissues by way of efferent neuronal pathways. These effector tissues, which include organs, muscles, and glands, carry out the appropriate physiological responses to bring the variable back to within its normal limits.

The other regulatory system in the body contributing to the maintenance of homeostasis is the endocrine system, which carries out its effects by secreting hormones. These hormones are transported in the blood to the specific tissues upon which they exert their effects. In general, the nervous system primarily regulates muscular activity and glandular secretion and the endocrine system primarily regulates metabolic activity in the body's cells. However, these two systems may work together in the regulation of many organs, as well as influence each other's activity.

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