Negative feedback

Most of the body's compensatory homeostatic mechanisms function by way of negative feedback. This is a response that causes the level of a variable to change in a direction opposite to that of the initial change. For example, when blood pressure increases, the arterial baroreceptors are stimulated and an increased number of nerve impulses are transmitted to the CNS through afferent pathways. The region of the brain regulating the cardiovascular system responds to this sensory input by altering efferent nerve activity to the heart. The result is a decrease in heart rate and therefore a decrease in blood pressure back to its baseline value (see Figure 1.2). In general, when a physiological variable becomes too high or too low, a control system elicits a negative feedback response consisting of one or a series of changes that returns the variable to within its normal physiological range. These compensatory mechanisms operating via negative feedback allow the body to maintain homeostasis effectively.

Interestingly, one of the greatest stressors on the body, and therefore challenges to the maintenance of homeostasis, is increased physical activity or exercise. During intense exercise, glucose utilization can be increased up to 20-fold; skeletal muscle pH drops dramatically; several liters of water can be lost in the form of sweat; and core body temperature can increase to as high as 106°F. These profound disturbances must be compensated for in order to ensure cell survival. An important focus throughout this textbook will be how tissue and organ system function is regulated under various normal physiological conditions and, where appropriate, under abnormal pathophysiological conditions. Furthermore, discussions of how basic physiological principles may be applied to the practice of pharmacy are included.

CENTRAL NERVOUS SYSTEM

SENSORY DIVISION

Baroreceptors (monitor blood pressure)

SENSORY DIVISION

Afferent neuron

CENTRAL NERVOUS SYSTEM

Baroreceptors (monitor blood pressure)

Afferent neuron

Efferent neuron

MOTOR DIVISION

Efferent neuron

MOTOR DIVISION

Heart

(changes in heart rate help regulate blood pressure)

Heart

(changes in heart rate help regulate blood pressure)

Figure 1.2 Negative feedback. These types of responses are employed throughout the body in order to maintain homeostasis. In this example, any change in blood pressure, which is monitored within the circulatory system and processed within the CNS, will cause reflex changes in heart rate. The change in heart rate will be in the opposite direction of the change in blood pressure: if blood pressure increases, then heart rate decreases; if blood pressure decreases, then heart rate increases. In this way, blood pressure is adjusted back to its normal value.

Pharmacy application: homeostatic functions of drugs

Diseases are generally divided into two categories: those in which the pathophysiology involves internal failure of some normal physiological process and those that originate from some external source such as bacterial or viral infection. In either case, one or more variables in the internal environment will be disrupted. Therefore, many of the medications currently in use are designed to assist the body in the maintenance of homeostasis when its own regulatory mechanisms fail to do so. For example, angiotensin-converting enzyme (ACE) inhibitors, such as enalo-pril, and beta-blockers, such as propranolol, lower blood pressure in patients with idiopathic (unexplained) hypertension (elevated blood pressure). Glibenclamide, which increases cellular sensitivity to insulin and decreases hepatic glucose production, maintains blood glucose within the normal range in patients with type II diabetes mellitus. Diuretics such as furosemide decrease blood volume and therefore reduce cardiac workload in patients with congestive heart failure. In each of these disorders, pharmacological intervention is necessary for the given organ system to function efficiently and effectively in order to maintain the health of the patient.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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