Unit 1 – Homeostasis Introduction

Homeostasis Defined: The body’s ability to maintain a relatively stable internal environment Homeo = same Stasis = stand, stay

Control of Homeostasis ALL body systems work together to maintain homeostasis! The majority of homeostatic mechanisms are controlled by 2 systems: NERVOUS SYSTEM Sends nerve impulses to organs; usually messages from the brain * FASTER changes ENDOCRINE SYSTEM Secretes molecules called hormones into the blood *when released work more slowly

Homeostatic Feedback Mechanisms cycle of events through which the body’s internal conditions are monitored, reevaluated, and changed STIMULUS  any disruption in the body that causes a response (change in the human body condition) Stimuli may be external and come from outside the body or they are internal changes that occur from within

3 Basic Components of a Feedback Mechanism 1) RECEPTOR  body structure that monitors changes in the body and sends INPUT to the control center The INPUT are messages either in the form of nerve impulses or chemical signals

3 Basic Components of a Feedback Mechanism 2) CONTROL CENTER  body structure that evaluates the INPUT that it receives from receptors and generates OUTPUT as needed to be sent to the effector(s) The OUTPUT are messages either in the form of nerve impulses or chemical signals

3 Basic Components of a Feedback Mechanism 3) EFFECTOR  body structure that receives OUTPUT from the control center and produces a response (effect) that changes the condition This leads to a return to HOMEOSTASIS when the response brings the condition back to normal 

NEGATIVE VS. POSITIVE FEEDBACK Negative Feedback  REVERSES the condition to bring the body back to its normal state Positive Feedback  REINFORCES the condition *Eventually negative feedback will kick in; if the positive feedback mechanism cannot be stopped it can produce life threatening changes in the body

NEGATIVE VS. POSITIVE FEEDBACK Negative Feedback Examples  regulation of body temperature or blood pressure changes * NEGATIVE feedback is the mechanism in action in MOST instances!! Positive Feedback Examples  childbirth, blood clotting Many diseases produce positive feedback loops that overwhelm the normal negative feedback loops that maintain homeostasis

HOMEOSTASIS EXAMPLES

BODY TEMPERATURE as an example of HOMEOSTASIS Cold air is blowing in the classroom and your body temperature drops quickly, the cold air is the stimulus Receptors in the skin sense the decreased body temperature (these receptors are called thermoreceptors because they are temperature-sensitive) and they… Send nerve impulses called input to the brain… The brain acts as a control center which evaluates the input and sends… Output in the form of nerve impulses to your skeletal muscles… Your skeletal muscles are the effectors which cause you to shiver which generates heat as your muscles contract and… Reverses the original stimulus and brings your body temperature back to normal to maintain HOMEOSTASIS!

BLOOD PRESSURE as an example of HOMEOSTASIS Some stimulus disrupts homeostasis and your blood pressure changes (increases or decreases) Receptors in the blood vessels sense the altered blood pressure (these receptors are called baroreceptors because they are pressure-sensitive) and they… Send nerve impulses called input to the brain… The brain acts as a control center which evaluates the input and sends… Output in the form of nerve impulses to your heart and blood vessels… Your heart and blood vessels are the effectors which cause a change in your heart rate and blood vessels which… Reverses the original stimulus and brings your blood pressure back to normal to maintain HOMEOSTASIS!