Homeostasis

Life Sciences-HHMI Outreach. Copyright 2009 President and Fellows of Harvard College.

What is homeostasis? Process that occurs in all

living things

All organ systems work together to achieve homeostasis

Ability of an organism to maintain its internal environment, despite changes to its internal or external environment

http://en.wikipedia.org/wiki/File:Tightrope_artist_Cologne_1.jpg

Definition of Homeostasis:

The maintenance of a constant internal environment in the

body is called Homeostasis

What is Homeostasis?

What do animals need to keep constant?

Body cells work best if they have the correct Temperature Osmoregulation- Water balance Blood Pressure Blood pH level Glucose levels Oxygen and Carbon Dioxide Levels

Your body has mechanisms to keep the cells in a constant environment.

Life Sciences-HHMI Outreach. Copyright 2009 President and Fellows of Harvard College.

How does homeostasis work? Feedback pathways

A cellular relay race!

Specific organs and structures must communicate with each other in response to changes in the body

Keeps levels of certain processes within a normal rangehttp://en.wikipedia.org/wiki/File:Southern_12_stage-02_1988.jpg

Pathways Negative feedback – a stimulus changes some

condition and it triggers a response that reverses the change. (stops the change) Necessary for homeostasis An example of negative feedback is your home’s

thermostat

Positive feedback - the original stimulus initiates a chain of events that intensify change from an original condition. (increases change) Positive feedback usually moves away from

homeostasis. An example of positive feedback is blood clotting.

NEGATIVE FEEDBACK Temperature regulation, and water and sunlight regulation in

plants are all examples of negative feedback mechanisms.

Processes in which an initial change will bring about an additional change in the opposite direction.

POSITIVE FEEDBACK

A positive feedback is a process in which an initial change will bring about an additional change in the same direction.

An example of a simple positive feedback in everyday life is the growth of an interest-earning savings account. As interest is accrued the principal will begin to grow (assuming money is not withdrawn). As the principal grows, even more interest will be accrued, quickening the rate of principal growth.

A good example of a positive feedback system is child birth. During labor, a hormone called oxytocin is released that intensifies and speeds up contractions. The increase in contractions causes more oxytocin to be released and the cycle goes on until the baby is born. The birth ends the release of oxytocin and ends the positive feedback mechanism.

 

Negative Feedback Loop ExampleNegative Feedback Loop Example::

Holding Holding breath, breath, COCO2 2 levels levels riserise

Control system Control system forces exhale, forces exhale, inhaleinhale

OO2 2 // COCO2 2 level level returns to returns to normalnormal

Positive feedback increases change

Example: Torn blood vessel stimulates release of clotting factors. Example: Torn blood vessel stimulates release of clotting factors. Once a vessel is damaged, platelets start to cling to the injured site and release chemicals that attract more platelets. The platelets continue to pile up and release chemicals until a clot is formed.  

– growth hormones stimulate cell divisiongrowth hormones stimulate cell division

platelets

fibrin

white blood cellred blood cell

blood vessel

clot

Controlling body temperature

All mammals maintain a constant body temperature.

Human beings have a body temperature of about 37ºC. E.g. If your body is in a hot environment your body

temperature is 37ºC If your body is in a cold environment your body

temperature is still 37ºC

What mechanisms are there to cool the body down?

1. Sweating When your body is hot, sweat glands are

stimulated to release sweat.

The liquid sweat turns into a gas (it evaporates)

To do this, it needs heat.

It gets that heat from your skin.

As your skin loses heat, it cools down.

Sweating

The skin

2. Vasodilation Your blood carries most of the heat energy

around your body. There are capillaries underneath your skin

that can be filled with blood if you get too hot.

This brings the blood closer to the surface of the skin so more heat can be lost.

This is why you look red when you are hot!

What mechanisms are there to cool the body down?

If the temperature rises, the blood vessel dilates (gets bigger).

This means more heat is lost from the surface of the skin

What mechanisms are there to warm the body up?

1. Vasoconstriction This is the opposite of vasodilation The capillaries underneath your skin

get constricted (shut off). This takes the blood away from the

surface of the skin so less heat can be lost.

If the temperature falls, the blood vessel constricts (gets shut off).

This means less heat is lost from the surface of the skin

What mechanisms are there to warm the body up?

2. Piloerection This is when the hairs on your skin

“stand up” . It is sometimes called “goose

bumps” or “chicken skin”! The hairs trap a layer of air next to

the skin which is then warmed by the body heat

The air becomes an insulating layer.

Temperature Regulation in Humans

To keep cool: Sweating Vasodilation

To keep warm: Vasoconstriction Piloerection Involuntary Muscle Contraction (Shivering)

Homeostasis in Plants

What do you think plants need to keep constant?

Water

Sunlight

CO2

Control of water levels in the plant is important to the survival of the plant.

This is achieved by a number of methods: Waxy cuticle on leaves Storage of water Opening and closing of stomates

Stoma Open Stoma Closed

Guard Cells

Stoma

Function of Stomata

CO2

O2 H2O

What goes in?

What goes out?

•What process involves using CO2 and H2O releasing O2 as a

waste product?

•Photosynthesis

•What is the plant using this process to make?

•Carbohydrates-glucose

•If the plant needs water for photosynthesis, why is water coming out of the stoma?

Stoma Open Stoma Closed

Guard CellsFunction of Guard Cells

Guard Cells•These stomata (leaf openings) allow water and O2 out of the leaf.

•Why would the plant close stomata with guard cells?

•Prevent excess water loss through transpiration. (conserve water)

•So what is the point of having stomata?

•Allow gas exchange for photosynthesis and control water loss

Homeostasis in Plants

The opening and closing of the stomata maintains water balance in the plant and thus maintains homeostasis.

Stems bend towards sunlight to maintain the amount of photosynthesis.