Biology 12

182 MHR • Unit 2 Homeostasis

regular pattern. Some of these biological cycles

correlate with the seasons. Other processes follow

a regular 24-hour cycle. Such daily cycles are

referred to as circadian rhythms. Cortisol is one

hormone that fluctuates in a circadian rhythm.

Cortisol levels tend to increase at night, peaking

just before a person awakes. These levels then

decrease sharply during the daytime. Another

hormone that is subject to circadian rhythm is

melatonin, a non-steroid hormone composed of a

modified amino acid. Melatonin is produced by the

pineal gland, a small, pine cone-shaped structure

located deep in the centre of the brain. Melatonin

production is highest during nighttime hours and

diminishes considerably during the day.

As daylight fades, melatonin levels rise, producing

the familiar feeling of sleepiness. Some studies

indicate a connection between abnormal melatonin

production and sleep disorders. In addition,

medical research suggests that abnormal melatonin

production could be a factor in the development of

mood disorders and depression.

One example is seasonal affective disorder (SAD),

a condition that typically produces symptoms of

depression and an overwhelming desire for sleep.

These symptoms generally appear at the onset of

winter and are believed to affect about 20 percent

of residents of northern countries. This disorder

is much less prevalent in southern regions. For

example, SAD appears in only about 2.5 percent

of the population of Florida.

Above-normal levels of melatonin enhance SAD

symptoms. Some researchers suggest that exposure

to bright lights for two to three hours each day can

diminish the symptoms of this disorder.

The secretion of thyroxine is another process

that follows a seasonal pattern. Thyroxine levels

tend to increase during winter months, boosting

metabolic rate. This may explain why a 15°C day

may feel cool in the fall but warm in the spring.

Research efforts continue to refine our

understanding of the role of these cycles in

maintaining homeostasis in the body. The many

rhythmic patterns discussed in this section are a

daily reminder that despite our technologically

advanced way of life, our close association with

the natural environment continues to be strong

and pervasive.

The Thymus Gland

The thymus glandis located between the lobes of

the lungs in the upper chest cavity. This part of the

immune system is especially active in young

CONCEPT ORGANIZER The Internal Communication System of the Body

Receptor sites

(Chapter 5, section 5.2

Chapter 6, section 6.1)

Neurotransmitters

(Chapter 5, section 5.2)

Endocrine hormones

(Chapter 6, section 6.1)

Intracellular

communication

Figure 6.17Internal communication is vital to maintaining the body’s homeostasis.

Homeostasis depends on an effective communication

system within the organism. The interactions between the

receptor, modulator, and effector are primarily dependent

on the actions of the nervous system and endocrine

hormone system. There are many kinds of hormones and

neurotransmitters, and these molecules trigger a wide

variety of responses. However, their mode of action is

remarkably similar. The chemical signals (hormone or

neurotransmitter) bind to specific receptors that are usually

on the surface of the cell. (An exception is the steroid

hormones that interact with receptor sites within the cell.)

These receptors are usually specific protein structures that

combine with a specific signal molecule to trigger a particular

intracellular response. The specificity of the receptor and its

response allows for control of the body’s systems.