Biology – Key Topic 5
4.5 Homeostasis and response
4.5.1 Homeostasis
Homeostasis is the regulation of internal conditions of a cell or organism to maintain
optimal conditions for function in response to internal and external changes.
Homeostasis maintains optimal conditions for enzyme action and all cell functions
- Maintaining a stable internal environment is crucial because cells need the right
conditions in order to function properly
In the human body, these include control of:
- Blood glucose concentration – will affect the water balance of the cell. Low blood
glucose levels will affect respiration and brain function. High blood glucose levels
will affect respiration and brain function. High blood glucose will lead to
circulatory problems. Relies on hormones/chemicals
- Body temperature – high temperature will damage enzymes, low temperature will
slow down chemical reactions. Relies on nerves
- Water levels – water levels can affect the osmotic balance in cells, which might
affect the state of the cell. Relies on hormones/chemicals
These automatic control systems may involve nervous responses or chemical
responses
We have special receptors which can detect certain types of stimuli:
- Eyes – sense light
- Ears – senses sound and orientation
- Nose – senses chemicals in the air
- Tongue – senses chemicals in food
- Skin – senses touch, pressure, pain and temperature
All control systems include:
- Cells called receptors which detect stimuli (changes in the environment)
- Coordination centres (such as the brain, spinal cord and pancreas) that receive and
process information from receptors
- Effectors, muscles or glands, which bring about responses which restore optimum
levels
- Stimulus (changes in the environment) receptor (cells that detect a change in
the environment) coordination centre (areas that receive and process
information, they coordinate a response) effector (muscles or glands that bring
about a response) response
Internal Stimulus receptor Coordination Effector
condition centre
Body Change in Skin Brain Skin + muscles
temperature body (hypothalamus)
, temperature
Water Change in Brain Pituitary gland Kidney
content of water
body content of
blood
Blood Change in Pancreas Pancreas Muscles/cells/liver
glucose blood
glucose
levels
4.5.2 The human nervous system
4.5.2.1 Structure and function
The nervous system enables humans to react to their surroundings and to coordinate
their behaviour
Information from receptors passes along cells (neurones – single specialised cells) as
electrical impulses to the central nervous system. The CNS is the brain and spinal cord
which is connected to the body by sensory neurones and motor neurones in
mammals. The CNS coordinates the response of effectors which may be muscles
contracting or glands secreting hormones
Stimulus receptor coordinator effector response
Reflex actions are automatic and rapid, they do not involve the conscious part of the
brain. The passage of information in a reflex is called a reflex arc
- vital body functions e.g. breathing need to occur without conscious thought.
Reflexes avoid damage or danger, prevent injury
- it is important some actions don’t go to the conscious part of the brain, because the
response would be too slow, and be less effective at preventing harm
- an electrical impulse is passed from the sensory receptor along the sensory neurone
to the spinal cord
- the impulse then passes to a relay neurone and straight back along a motor neurone
to the effector organ
- the time between the stimulus and the reflex action is as short as possible. It allows
you to react to danger without thinking about it
1. finger placed in flame
2. receptor sends a nervous impulse along a sensory neurone
3. sensory neurone enters the CNS via the dorsal root
4. nervous impulse arrives at synapse and causes neurotransmitter to diffuse across
5. neurotransmitter attaches to membrane of relay neurone and causes a new
nervous impulse in the next neurone
6. relay neurone carries impulse within the CNS to the next synapse
4.5 Homeostasis and response
4.5.1 Homeostasis
Homeostasis is the regulation of internal conditions of a cell or organism to maintain
optimal conditions for function in response to internal and external changes.
Homeostasis maintains optimal conditions for enzyme action and all cell functions
- Maintaining a stable internal environment is crucial because cells need the right
conditions in order to function properly
In the human body, these include control of:
- Blood glucose concentration – will affect the water balance of the cell. Low blood
glucose levels will affect respiration and brain function. High blood glucose levels
will affect respiration and brain function. High blood glucose will lead to
circulatory problems. Relies on hormones/chemicals
- Body temperature – high temperature will damage enzymes, low temperature will
slow down chemical reactions. Relies on nerves
- Water levels – water levels can affect the osmotic balance in cells, which might
affect the state of the cell. Relies on hormones/chemicals
These automatic control systems may involve nervous responses or chemical
responses
We have special receptors which can detect certain types of stimuli:
- Eyes – sense light
- Ears – senses sound and orientation
- Nose – senses chemicals in the air
- Tongue – senses chemicals in food
- Skin – senses touch, pressure, pain and temperature
All control systems include:
- Cells called receptors which detect stimuli (changes in the environment)
- Coordination centres (such as the brain, spinal cord and pancreas) that receive and
process information from receptors
- Effectors, muscles or glands, which bring about responses which restore optimum
levels
- Stimulus (changes in the environment) receptor (cells that detect a change in
the environment) coordination centre (areas that receive and process
information, they coordinate a response) effector (muscles or glands that bring
about a response) response
Internal Stimulus receptor Coordination Effector
condition centre
Body Change in Skin Brain Skin + muscles
temperature body (hypothalamus)
, temperature
Water Change in Brain Pituitary gland Kidney
content of water
body content of
blood
Blood Change in Pancreas Pancreas Muscles/cells/liver
glucose blood
glucose
levels
4.5.2 The human nervous system
4.5.2.1 Structure and function
The nervous system enables humans to react to their surroundings and to coordinate
their behaviour
Information from receptors passes along cells (neurones – single specialised cells) as
electrical impulses to the central nervous system. The CNS is the brain and spinal cord
which is connected to the body by sensory neurones and motor neurones in
mammals. The CNS coordinates the response of effectors which may be muscles
contracting or glands secreting hormones
Stimulus receptor coordinator effector response
Reflex actions are automatic and rapid, they do not involve the conscious part of the
brain. The passage of information in a reflex is called a reflex arc
- vital body functions e.g. breathing need to occur without conscious thought.
Reflexes avoid damage or danger, prevent injury
- it is important some actions don’t go to the conscious part of the brain, because the
response would be too slow, and be less effective at preventing harm
- an electrical impulse is passed from the sensory receptor along the sensory neurone
to the spinal cord
- the impulse then passes to a relay neurone and straight back along a motor neurone
to the effector organ
- the time between the stimulus and the reflex action is as short as possible. It allows
you to react to danger without thinking about it
1. finger placed in flame
2. receptor sends a nervous impulse along a sensory neurone
3. sensory neurone enters the CNS via the dorsal root
4. nervous impulse arrives at synapse and causes neurotransmitter to diffuse across
5. neurotransmitter attaches to membrane of relay neurone and causes a new
nervous impulse in the next neurone
6. relay neurone carries impulse within the CNS to the next synapse
