Homeostasis, Reflexes, and Biological Rhythms

Homeostasis

The maintenance of a relatively stable internal environment essential for health and survival, achieved by coordinated regulatory systems and regulated exchange of matter and energy.

Afferent

Toward the central nervous system; carries sensory information from receptors to the brain or spinal cord.

Efferent

Away from the central nervous system; carries signals from the brain or spinal cord to effectors.

Negative feedback

A regulatory mechanism that counteracts deviations from a set point to restore balance.

Positive feedback

A regulatory mechanism that amplifies changes, driving the system further from the set point.

Feedforward regulation

An anticipatory adjustment that prepares a physiological variable before a change occurs.

Reflex

A specific, involuntary, unpremeditated response to a stimulus.

Reflex arc

The neural pathway mediating a reflex, typically including receptors, afferent pathway, integrating center, efferent pathway, and effectors.

Local homeostatic response

A stimulus–response sequence confined to the area of the stimulus, with no nerves or hormones involved.

Hormone

A chemical messenger released into the blood that acts on distant target cells.

Neurotransmitter

A chemical messenger released by neurons that acts on nearby neurons or other cells by diffusing through the extracellular fluid.

Paracrine substance

Secreted by a cell and travels through interstitial fluid to affect nearby cells; acts locally.

Autocrine substance

A substance that acts on the same cell that secreted it.

Circadian rhythm

A biological cycle that repeats roughly every 24 hours.

Suprachiasmatic nucleus (SCN)

The brain region that houses the internal circadian pacemaker setting biological rhythms.

Melatonin

A hormone produced by the pineal gland; levels are high at night and low during the day, influenced by day length.

Pineal gland

Brain gland that secretes melatonin to regulate circadian rhythms.

Adaptation

An inherited physiological characteristic that improves survival in a particular environment.

Acclimatization

A reversible, non-genetic adjustment to environmental conditions.

Interstitial fluid (IF)

The fluid surrounding cells, through which paracrine signals diffuse to nearby cells.

Local vasodilation

Widening of local blood vessels to increase blood flow in active tissues.

Integrating center

The neural or hormonal center that processes afferent information and initiates an efferent response in a reflex.

Structure–function relationship

The idea that the form of a body part determines its function, and evolution shapes both together.

Heat stroke

A life-threatening condition of extreme hyperthermia due to failure of thermoregulation.