Three structures in the brain are responsible for maintaining the body’s homeostasis:

♦ medulla oblongata, the part of the brain stem associated with vital functions, such as respiration and circulation

♦ pituitary gland, which regulates the function of other glands and thereby a person’s growth, maturation, and reproduction

♦ reticular formation, a network of nerve cells (nuclei) and fibers in the brain stem and spinal cord that help control vital reflexes, such as cardiovascular function and respiration.

Homeostasis is maintained by self-regulating feedback mechanisms. These mechanisms have three components:

♦ a sensor that detects disruptions in homeostasis (caused by nerve impulses or changes in hormone levels)

♦ a central nervous system control center that receives signals from the sensor and regulates the body’s response to those disruptions (by initiating the effector mechanism)

♦ an effector that acts to restore homeostasis.

Feedback mechanisms exist in two varieties:

♦ A positive feedback mechanism moves the system away from homeostasis. It takes the original response to the sensed change and exaggerates it. This amplified response proceeds in the same direction as the initial disturbance, causing a further deviation from homeostasis. For example, a positive feedback mechanism is responsible for intensifying labor contractions during childbirth.

♦ A negative feedback mechanism works to restore homeostasis by correcting a deficit in the system.

An effective negative feedback mechanism must sense a change in the body, such as a high blood glucose level, and attempt to return body functions to normal. In the case of a high blood glucose level, the effector mechanism triggers increased insulin production by the pancreas, returning the blood glucose level to normal and restoring homeostasis.

The cause of disease may be intrinsic or extrinsic. Genetic factors, age, sex, infectious agents, or behaviors (such as being inactive, smoking, or abusing illegal drugs) can cause disease. Diseases that have no known cause are called idiopathic.

A disease’s development is called its pathogenesis. Unless identified and successfully treated, most diseases progress according to a typical pattern of symptoms. Some diseases are self-limiting or resolve quickly with limited or no intervention; others are chronic and are never resolved. Patients with chronic diseases may undergo periodic remissions and exacerbations.

A disease is usually detected when it causes a change in metabolism or cell division that causes signs and symptoms. Manifestations of disease may include hypofunction (such as constipation), hyperfunction (such as increased mucus production), or increased mechanical function (such as a seizure).

How the cells respond to disease depends on the causative agent and the affected cells, tissues, and organs. The resolution of disease depends on many factors functioning over time, such as extent of disease and the presence of other diseases.

Typically, diseases progress through these stages:

♦ Exposure or injury—Target tissue is exposed to a causative agent or is injured.

♦ Latent phase or incubation —No signs or symptoms are evident.

♦ Prodromal period—Signs and symptoms are usually mild and nonspecific.

♦ Acute phase—The disease reaches its full intensity, possibly resulting in complications. This phase is called the subclinical acute phase if the patient can still function as though the disease weren’t present.

♦ Remission—This second latent phase occurs in some diseases and is usually followed by another acute phase.

♦ Convalescence—The patient progresses toward recovery after the disease is terminated.

♦ Recovery—The patient regains health or normal functioning. No signs or symptoms of disease remain.

When a stressor, such as a life change, occurs, a person can respond in one of two ways: by adapting successfully or by failing to adapt. A maladaptive response to stress may result in disease.

Stressors may be physical or psychological. Physical stressors, such as exposure to a toxin, may elicit a harmful response leading to an identifiable illness or set of signs and symptoms. Psychological stressors, such as the death of a loved one, may also cause a maladaptive response.

Hans Selye, a pioneer in the study of stress and disease, describes the following stages of adaptation to a stressful event: alarm, resistance, and recovery or exhaustion. (See Physical response to stress.) In the alarm stage, the body senses stress and arouses the central nervous system. The body releases chemicals to mobilize the fight-or-flight response. In this dual effort, the sympathoadrenal medullary response causes the release of epinephrine and the hypothalamic-pituitary adrenal axis causes the release of glucocorticoids. These systems work together to enable the body to respond to stressors. This release is the adrenaline rush associated with panic or aggression. In the resistance stage, the body either adapts and achieves homeostasis or it fails to adapt and enters the exhaustion stage, resulting in disease.

The stress response is controlled by actions that take place in the cells of the nervous and endocrine systems. These actions try to redirect energy to the organ that’s most affected by stress, such as the heart, lungs, or brain.

Stressful events can exacerbate some chronic diseases, such as diabetes or multiple sclerosis. Effective coping strategies can prevent or reduce the harmful effects of stress.

Like organisms, cells are complex units of specialized components, each component having its own function. A normal cell’s largest components are the cytoplasm, the nucleus, and the cell membrane, which surrounds the internal components and holds the cell together. (See Cell components and structures, page 4.)