Homeostasis

Homeostasis is

the relative constancy of the body’s internal environment.

Because of homeostasis,

even though external conditions may change dramatically,

internal conditions stay within a narrow range.

For example,

regardless of how cold or hot it gets,

the temperature of the body stays around 37°C (97° to 99°F).

No matter how acidic your meal,

the pH of your blood is usually about 7.4,

and even if

you eat a candy bar,

the amount of sugar in your blood is just about 0.1%.

It is important to realize that

internal conditions are not absolutely constant;

they tend to fluctuate above and below a particular value.

Therefore, the internal state of the body is
often described as one of dynamic equilibrium.

If internal conditions change to any great degree, illness results.

This makes the study of homeostatic mechanisms medically  important.

Negative Feedback

Negative feedback is the primary homeostatic mechanism
that keeps a variable close to a particular value, or set point.

A homeostatic mechanism has three components:

a sensor,

a regulatory center, and

an effector.

The sensor

detects a change in the internal environment;

the regulatory center
activates the effector;

the effector reverses the change and
brings conditions back to normal again.

Now, the sensor is no longer activated.

Mechanical Example

A home heating system illustrates

how a negative feedback mechanism works .

You set the thermostat at, say,68°F.

This is the set point.

The thermostat contains a thermometer,
a sensor that detects when the room temperature
falls below the set point.

The thermostat is also the regulatory center;

it turns the furnace on.

The furnace plays the role of the effector.

The heat given off by the furnace raises
the temperature of the room to 70°F.

Now, the furnace turns off.

Notice that a negative feedback mechanism prevents change
in the same direction;

 the room does not get warmer and warmer

 because warmth inactivates the system.

Human Example: Regulation of Blood Pressure

Negative feedback mechanisms in the body function

similarly to the mechanical model.

For example,

when blood pressure falls,

sensory receptors signal a regulatory center in the brain

This center sends out

nerve impulses to the arterial walls

so that they constrict.

Once the blood pressure rises,

the system is inactivated.

Human Example: Regulation of Body Temperature

The thermostat for body temperature is located

in a part of the brain called the hypothalamus.

When the body temperature falls below normal,

the regulatory center directs (via nerve impulses)

the blood vessels of the skin to constrict

This conserves heat.

If body temperature falls even lower,

the regulatory center sends nerve impulses

to the skeletal muscles, and shivering occurs.

Shivering generates heat, and

gradually body temperature rises to 37°C.

When the temperature rises to normal,

the regulatory center is inactivated.

When the body temperature is higher than normal,

the regulatory center directs the blood vessels of the skin to dilate.

This allows more blood to flow near the surface of the body,
where heat can be lost to the environment. In addition, the
nervous system activates the sweat glands, and the evaporation
of sweat helps lower body temperature. Gradually, body
temperature decreases to 37°C.

[ 출처 : Human Anatomy & Physiology 5Th Edition ]