Homeostasis
• The term is derived from the Greek word meaning ‘ to stay the same ’
• It is used to describe the remarkable ability of organisms to maintain relatively constant internal conditions in the face of very different and often changeable external conditions.

Homeostasis
• In everyday life, an organism must regulate respiratory gases, maintain fluid and salt balance, regulate nutrient supply and energy, maintain body temperature, protect itself against disease and repair damaged tissues.
• All these responses must be coordinated.

Principles of Homeostasis
• Organisms need to maintain a cellular environment that is optimal for cell function.
• Homeostatic control systems have 3 functional components:
– A receptor – to detect change
– A control center
– An effector – to direct the appropriate response

• For homeostasis the body must be able to detect changes in the environment and bring about a response. stimulus response
Receptor
Communication
System
Effector

The Communication System
• There are 2 kinds of communication system:
– Nervous
– Endocrine

The Communication System
Nervous System Endocrine System
Route Travelled Specific pathways
(nerve axons)
Milliseconds
Via blood
Time to reach target cell
Seconds
Cells reached Only those at the end of axons
Entire body (only some cells respond)
Strength of the signal indicated by
Frequency of impulses Concentration of hormone
Duration of Signal Millisecond for each impulse
Minutes to hours or days

Principles of Homeostasis
• There are 2 feedback systems in homeostasis:
– Negative Feedback Mechanisms
• More common. A movement away from the steady state triggers a mechanism to counteract the change.
– Positive Feedback Mechanisms
• Rare. Leads to a response escalating in the same direction. Used in labour, lactation, fever and blood clotting.

• When a receptor detects a change from the set point , this is communicated to effectors , which respond by bringing about changes tending to correct the disturbance.
• The greater the disturbance from normal, the greater the tendency to correct it.
• This is negative feedback .

• E.g. the maintenance of O
2 and CO in the tissue fluid during exercise.
2 levels
• NB the more the blood CO
2 concentration rises above the normal, the more strongly the breathing muscles are stimulated to respond, thus the greater the rate of CO
2 excretion.

Increased rate of CO
2 excretion
Normal blood CO
2
(set point)
Increased breathing
Rise in CO
2 in blood
Increased muscular activity

• NB homeostatic systems cannot prevent any change in the internal environment.
• What happens is that conditions fluctuate above and below the set point .
• This is because there must be a delay between the detection of a change and the response that corrects it.
• The longer the delay, the greater the departure from the set point and the stronger the corrective response, leading to “ overshoot ”.

• Positive feedback is described as a selfamplifying cycle in which one change, leads to even more significant changes in the same direction.
• Positive feedback is not a corrective mechanism activated by the body when it strays from the set point, but rather, it is a way for the body to produce significant changes in a relatively short period of time.

• E.g. a woman in labor.
• The baby's head pushes on the cervix, sending signals to the brain, which then produces oxytocin , which stimulates contractions, which then push the baby's head even more tightly against the cervix, and the loop then repeats and becomes more and more intense until the baby has been born.

Summary of Key Ideas
• Living organisms have the ability to keep their internal conditions different from their external environment.
• The regulation of conditions is called homeostasis.
• In more complex organisms cells are bathed in a liquid that forms an internal environment.

Summary of Key Ideas
• In animals with closed blood systems (e.g. vertebrates) the internal environment is the tissue fluid, which is separated from the blood by the capillary walls.
• Tissue fluid and blood are collectively called extracellular fluid.
• Disturbances in the internal environment are detected by receptors, and corrections are made by effectors.

Summary of Key Ideas
• In most animals there are 2 communication systems linking the receptors and effectors: nervous and endocrine.
• Homeostasis involves negative feedback, in which the greater the disturbance from the set point the greater the tendency to correct it.
• Positive feedback is where one change, leads to even more significant changes in the same direction.