Biology 12

4.1 Feedback Systems

108 MHR • Unit 2 Homeostasis

EXPECTATIONS

Describe and explain homeostatic processes at work in the body and the

systems that govern them.

Begin to develop a model that illustrates the essential components of the

homeostatic process.

The cells of the human body are surrounded by a

fluid having a salinity, or salt concentration, nearly

equal to that of the Earth’s oceans from which life

began. The bearer of this evidence of our

evolutionary past is called interstitial fluid. The

salinity and pH of the interstitial fluid, or fluid in

the pores between our cells, must be relatively

constant in order for the cells of the body to remain

healthy. This maintenance of constant extra-

cellular conditions is a function of homeostasis,

which is an example of dynamic equilibrium.

As used here, dynamic equilibriumis a state of

balance achieved within an environment as the result

Figure 4.1Cells are surrounded by interstitial fluid, which

is continually refreshed. As shown, oxygen and nutrient

molecules constantly exit the bloodstream, and waste

molecules continually enter the bloodstream.

arteriole

venule

red

blood

cell

oxygen and

nutrients

wastes

capillary

blood flow

blood flow tissue cell interstitial fluid

of internal control mechanisms that continuously

oppose outside forces that tend to change that

environment. For example, consider a house

equipped with a furnace, an air conditioner, and

a thermostat that controls both (see Figure 4.2). The

furnace comes on when the temperature drops.

When the temperature increases, however, the air

conditioner turns on instead. The thermostat

measures the temperature in the house and turns

either appliance on or off as needed. Although the

temperature in the house remains relatively

constant, this constancy is achieved through a

series of small temperature changes. Equilibrium

is thus maintained as long as the system is active

(dynamic). While the system functions to maintain

a desired temperature, large temperature changes

inside and outside the house might cause minor

fluctuations above and below that level.

Figure 4.2While a house may have a thermostat, furnace,

and air conditioner to keep its internal temperature constant,

the human body has a considerably more complex system

to maintain homeostasis.

furnace

floor register

floor register

temperature

of house air

conditioner

thermostat

feedback

desired

temperature