Life depends on five environmental factors

• water• food• oxygen• heat• pressure

Survival NeedsSurvival Needs

Water- most abundant substance in body- required for metabolic processes- required for transport of substances- regulates body temperature

Food- provides necessary nutrients- supplies energy- supplies raw materials

Survival NeedsSurvival Needs

Oxygen (Gas)- one-fifth of air- used to release energy from nutrients

Heat- form of energy - partly controls rate of metabolic reactions

Pressure - application of force on an object - atmospheric pressure – important for breathing - hydrostatic pressure – keeps blood flowing

Survival NeedsSurvival Needs

HomeostasisHomeostasisMaintenance of a stable internal environment

• Homeostatic imbalance – a disturbance in homeostasis resulting in disease

Temperature HydrationSugar level

Blood pressure

Feedback MechanismsFeedback Mechanisms

Negative feedback

• Includes most homeostatic control mechanisms

• Shuts off the original stimulus, or reduces its intensity

•Works like a household thermostat

Monitor aspects of the internal environment and corrects any changes

Homeostatic Mechanisms Homeostatic Mechanisms

•Receptors - provide information about stimuli

•Control center - tells what a particular value should be (includes a set point)

•Effectors - elicit responses that change conditions in the internal environment

The body communicates through neural and hormonal control systems

CONTROL CENTER

RECEPTOR

EFFECTOR

RESPONSE

STIMULUS: DECREASE IN EXTERNAL TEMPERATURE

Feedback MechanismsFeedback Mechanisms

Positive Feedback• Increases the original stimulus to

push the variable farther• In the body this occurs only in

blood clotting and child birth

Feedback MechanismsFeedback Mechanisms