Biology Note

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Prepared by: Nur Syakireen Bt. Ishak

•Definition and importance of homeostasis •Homeostatic organ •Negative and positive feedback mechanisms •Blood-glucose regulation •Thermoregulation •Osmoregulation


• Homeostasis:
• is the steady state of physiological condition of the body. • it is the physiological processes by which organisms maintain a constant & balanced internal environment.

• In achieving homeostasis, animal maintain a relatively constant internal environment even when the external environment changes significantly.

• In multicellular organisms, the internal environment refers to the tissue fluid that baths the cells.

• Homeostatic mechanisms maintains optimal physical & chemical conditions in the body to allow normal enzymatic & other cellular activities to be carried out.

• This enable the organisms to live in different habitats with a wider

range of environmental conditions.

• Humans exhibit homeostasis for a range of physical & chemical properties. Examples: • The body maintains a fairly constant,
• body temperature  ~ 37ºC • pH of the blood & interstitial fluid  pH 7.4 • level of glucose concentration in the bloodstream  ~90 mg of glucose per 100 mL of blood.

• Example of homeostatic organs: skin, liver & kidney.

Mechanisms Homeostasis
• Mechanisms of homeostasis moderate changes in the internal environment. • An animal achieves homeostasis by maintaining a variable (e.g. body temperature or solute concentration) at or near a particular value or set point.

• Fluctuation in the variable above or below the set point serves as the stimulus.

• A receptor, or sensor, detects the stimulus & triggers a response. • A response is the physiological activity that help return the variable to the set point

Mechanisms Homeostasis
(a) INPUT (Stimulus) (b) Receptor (Sensor) (c) Coordinator (Control) (d) Effector (Output)

(e) Feedback loop
b) c)

A stimulus (input) is the detectable change in the level of the factor being regulated. A receptor (sensor) detects a stimulus. A coordinator (control) receives the information & triggers the action that will correct the change.


An effector (output) carries out the action (corrective mechanisms  response) that restores condition to its optimum.


The feedback loop enables the sensor to monitor any changes brought about by the effector.

Feedback Loops in Homeostasis
• All homeostatic control processes include regulatory mechanisms. • Regulatory mechanisms use the principle of: • Negative feedback • Positive feedback

• Homeostasis in animal relies largely on negative feedback, • a response that reduces the stimulus, which helps to return a variable to either a normal range or set point. • Example: when you exercise vigorously, heat is produced, which increases body temperature. • The nervous system detect this increase & triggers sweating. • As you sweats  the evaporation of moisture from skin cools your body & helps return the body temperature to its set point.

Feedback Loops in Homeostasis
• Positive feedback also occurs in animals,
• but this mechanism do not usually contribute to homeostasis.

• Positive feedback triggers mechanisms that amplify rather than diminish the stimulus.

• Example:
• During childbirth, the pressure of the baby’s head against receptors near the opening of the uterus  stimulates the uterus to contract. Contractions occurs until the baby is born.

A General Homeostatic Control System
Factor value increase accelerated
Positive Feedback
Transmission of nerve impulse or secration of hormone


Control centre

Effector/ target organ

Correction mechanism
decrease Negative Feedback

Normal condition
Transmission of nerve impulse or secration of hormone Control

Normal condition restored
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