4.5 Homeostasis and response

Why does the body require control systems that monitor and adjust the composition of the blood and tissues?

Because cells in the body can only survive within narrow physical and chemical limits; they require a constant temperature and pH as a well as a constant supply of dissolved food and water.

What are the human body’s two communication systems?

Endocrine system

Nervous system

Why is hormonal coordination important for reproduction?

Because it controls the menstrual cycle and gamete production

What are hormones and why are they important?

Hormones are chemical messengers produced by endocrine glands

They are carried in the blood to target organs where they cause specific responses

• They coordinate and control body functions

• Maintain homeostasis

• Important in reproduction

In simple terms, what are receptors and effectors?

Receptors: sense changes

Effectors: bring about changes

Why do the conditions inside the body need to be kept steady, even when the external environment changes?

Cells need the right conditions in order to function properly, including the right conditions for enzyme action

What is homeostasis?

Homeostasis maintains a constant body environment; it involves regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes.

It makes sure that all of the systems and reactions in the body are working optimally.

What does homeostasis do?

Maintain the optimum conditions for all cell functions and enzyme action by maintaining a constant internal environment. It does this by resisting changes in the internal and external environment.

Give examples of homeostasis action in the human body

Control of:

• blood glucose concentration

• body temperature

• water levels

Give one example each for how and why homeostasis will resist changes in the internal and external environment

Internal: if blood glucose concentration rises, the body will work to reduce glucose levels. It does this because having too much (hyperglycemia) or too little (hypoglycemia) glucose in the blood has negative impacts on the body.

External: if air temperature rises, the body will work to activate cooling mechanisms. It does this because if body temperature is too high it may denature enzymes, but if it is too low it may cause the enzymes to work too slowly.

What are the two types of responses that automatic control systems may involve?

• Nervous responses

• Chemical responses

Give three examples of what homeostasis would control in the human body

• Body temperature

• Blood glucose concentration

• Water level

Explain why homeostasis must control body temperature, blood glucose concentration and water levels

Homeostasis controls body temperature, blood glucose, and water levels to maintain optimum conditions for a cell functions and enzyme action (to regulate the internal environment)

1. Body temperature

• The body must be kept around 37°C, which is the optimum temperature for enzymes.

• If the temperature is too high, enzymes become denatured (the shape of their active site changes) so they can no longer catalyse reactions

• If the temperature is too low, enzyme activity slows down, and chemical reactions happen too slowly to keep you alive.
  ➡ Therefore, temperature control ensures that metabolic reactions occur at the right rate.

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2. Blood glucose concentration

• Cells need a constant supply of glucose for respiration, to release energy.

• If blood glucose levels are too high, water moves out of cells by osmosis, which can damage cells.

• If blood glucose levels are too low, there isn’t enough glucose for respiration, so cells can’t release enough energy.
  ➡ Therefore, glucose concentration must be controlled to provide energy for cells while preventing damage.

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3. Water levels

• The body must keep the water levels stable

• If the body loses or gains too much water, the concentration of the blood changes.

• If blood becomes too concentrated, water moves out of cells by osmosis, causing them to shrink.

• If blood becomes too dilute, water moves into cells, causing them to swell and burst.

• Therefore, water levels are controlled to protect cells from damage and maintain proper cell function.

Describe nervous and hormonal responses

Nervous: consists of many neurons (nerve cells) which produce electrical impulses that travel along their axons to transmit signals to respond quickly to short term changes. These responses target specific regions and can be voluntary or involuntary

Hormonal: chemical response which involves hormones (chemical messengers) which are released by glands; it takes a long time for hormones to travel around the body through the bloodstream. These responses target the body as a whole and are always involuntary.

What is in the human body to regulate your internal environment?

Automatic control systems; these include both nervous and hormonal control systems

What are the components of automatic control systems that work together to maintain a steady condition?

• Receptors

• Coordination centres

• Effectors

Describe what receptors, coordination centres and effectors are

Receptors are cells which detect stimuli (changes in the environment)

Coordination centres (eg. brain, spinal chord, pancreas) receive and process information from the receptors

Effectors (eg. muscles, glands) bring about responses which restore optimum levels

Give examples of a receptor

Skin receptors detect a change in the surface of the skin (external)

Osmoreceptors detect changes in the water levels in the blood and tissues (internal)

Give examples of coordinators

Brain and spinal chord process information from receptors to initiate a response

The pancreas is important in homeostasis of glucose levels in the blood

Give examples of effectors

Muscles contraction can bring about a response (eg. if it is cold hair erector muscles in the skin will contract)

What is the nervous system?

Complex network of cells, tissues and organs that are responsible for transmitting and processing signals (electrical impulses) in the body.

The main structures include the brain, the spinal cord and the nerves.

What is the purpose of the nervous system?

It enables humans to react to their surroundings and coordinate their behaviour

What are the two parts of the nervous system?

• CNS (central nervous system)

• Peripheral nervous system (nerves that send impulses to and away from the CNS)

What is the CNS?

In vertebrates, this consists of only the brain and the spinal cord.

In mammals, the CNS is connected to the body by sensory neurones and motor neurones.

What is a neurone?

A nerve cell

What is the coordination centre of the nervous system?

The CNS (central nervous system)

What do receptors do?

These cells detect a stimulus (change in internal or external environment) and send electrical impulses down sensory neurones to the central nervous system (consisting of the brain and the spinal cord).

How does the nervous system send information?

In the form of electrical signals called nerve impulses; these are passed along neurones

What are nerves made up of?

Bundles of neurones

What are the three types of neurone that form a circuit and what do they do?

• Sensory neurone: carries information as electrical impulses from the receptors to the CNS (coordination centre)

• Relay neurone: forms a link between the sensory and motor neurones within the CNS to enable information to be passed from a point of sensory reception to motor output. RELAY NEURONES ARE ONLY USED IN AUTOMATIC, UNCONSCIOUS RESPONSES (eg. reflex actions).

• Motor neurone: carry information as electrical impulses from the CNS (coordination centre) to effectors

They carry information from the receptors to the coordination centres to the effectors.

What does the coordinator do?

It coordinates the response (decides what to do in response) to the electrical impulses from the receptors that have been carried by the sensory neurones. It then sends the information in the form of electrical impulses along motor neurones to the effectors.

What do effectors do?

Respond to the electrical impulses from the coordinators, carried by the motor neurones and bring about a change. They are usually either a muscle which contracts, or a gland which secretes hormones in response to a nervous impulse.

Describe how the nervous system will detect and react to stimuli

• Receptors in organs detect stimuli (changes in environment)

• The receptors send information in the form of electrical impulses along sensory neurones to the coordination centre (the CNS (central nervous system), made up of the brain and spinal cord)

• The CNS will then process and coordinate a response to the stimuli (decide what to do)

• It will send information in the form of electrical impulses along motor neurones to effectors

• The effectors are usually either muscles or glands; muscles respond to electrical impulses by contracting, and glands respond by secreting hormones

What are the properties of neurones?

Neurones are nerve cells.

Nerves are made up of bundles of neurones.

Properties:

• Long axon: carries information away from the cell body across long distances across the body to other neurones, coordinators or effectors (depending on the type of neurone)

• Myelin sheath: an insulating layer of fat which increases the speed and efficiency of electrical impulses and prevents short circuits

• Cell body: the nerve cell body is at the end of the axon; it has the nucleus and lots of dentrites extending from it

• Branching dentrites: receive signals from other neurones and transmits them towards the neurone’s cell body

• Synapses are the places where two neurones connect (tiny gap) (or where a neuron connects to an effector or coodinator). Electrical impulses reach the synapse and trigger the release of chemicals called neurotransmitters. The neurotransmitters diffuse across the synapse and bind to the receptors on the next neurone which causes a new electrical impulse to be generated in the next neurone

What are synapses and how do they work?

A synapse is a tiny gap between two neurones (nerve cells). It allows an electrical impulse to pass from one neurone to the next.

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How a synapse works:

1. When an electrical impulse reaches the end of a neurone, it triggers chemicals called neurotransmitters to be released.

2. These chemicals diffuse across the synapse (the gap).

3. The neurotransmitters then bind to receptors on the next neurone.

4. This causes a new electrical impulse to be generated in the next neurone.

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Why synapses are important:

• They ensure impulses travel in one direction only.

• They allow communication between neurones.

• They make it possible to coordinate complex responses and control the speed and strength of signals.

How do axons use neurotransmitters to conduct impulses across a synapse?

• An impulse is carried along the axon of the first neurone

• When it reaches the end of the neurone, it triggers a neurotransmitter to be released

• The neurotransmitter diffuses across the synapse (the place where two neurones connect) 

• The neurotransmitter binds to the receptor on the next neurone, causing a new electrical impulse to be generator in the next neurone

What are reflexes?

Rapid, automatic responses to stimuli that don’t involve the conscious part of the brain - they can reduce the chances of being injured

What is a reflex arc?

The passage of information (from a receptor to effector) in a reflex

Describe the reflex arc pathway

A reflex arc is the pathway taken by nervous impulses to produce a rapid, automatic response to a stimulus without conscious thought.

• The receptor picks up a stimulus (change in environment)

• The sensory neurone transmits an electrical impulse to the coordinator centre (CNS)

• When the sensory neurone carrying the electrical impulse reaches a relay neurone, the electrical impulse will reach the end of the sensory neurone and trigger the release of neurotransmitters (chemicals)

• The neurotransmitters will diffuse between the synapse between the neurones

• The neurotransmitters will then bind with the receptor of the relay neurone, causing new electrical impulses to be generated in the relay neurone

• The relay neurone will then carry electrical impulses to the motor neurone, connecting the sensory and motor neurones, and when the electrical impulses reach the synapse between the relay and motor neurone, it will again trigger neurotransmitters to be released, which will diffuse between the synapse, bind to the receptors on the motor neurone, causing a new electrical impulse to be generated in the motor neurone

• The electrical impulses will then travel along the motor neurone to an effector

• The effector will then cause a protective response against a stimulus

Because you don’t have to consciously think about the response (which takes time) it is quicker than normal responses.

What is the brain and its role?

Along with the spinal cord, the brain is a part of the central nervous system (a coordination centre) that works to control complex behaviours.

It is made up of billions of interconnected neurones and has different regions that carry out different functions.

Identify the different parts of the brain

Top right: Cerebral cortex (consciousness, memory, intelligence, language)

Middle right: Cerebellum (muscle coordination)

Bottom right: spinal cord

Bottom left: Medulla 

Cerebral cortex - main one so big one

Cerebellum - sounds flowery so is the flowery one

Medulla - medusa, looks at people, lump at front of spinal cord

Spinal cord - cord at back

Name the three areas of the brain and their functions

Cerebral cortex: Higher level functions and consciousness such as: emotions, thoughts, personality, intelligence, memory and language

Cerebellum: Controls movement, balance and muscle coordination. Without this, movements would be erratic

Medulla: Unconscious activities such as breathing and heart rate.

What is one of the main aims of research of the brain?

Mapping regions of the brain to particular functions.

What are methods of studying the brain?

• Studying patients with brain damage

• Electrically stimulating different parts of the brain

• Using MRI scanning techniques

How do scientists study the brain by studying patients with brain damage?

Scientists study patients with brain damage to understand the importance and function of different parts of the brain; it helps to map out parts of the brain to specific functions.

If a small part of the brain has been damaged, the effect this has on the patient can reveal a lot about what the damaged part of the brain does.

How do scientists study the brain by electrically stimulating the brain?

Electrical stimulation allows scientists to map out parts of the brain to particular functions.

The brain can be stimulated electrically by pushing an electrode into the tissue and giving it a small shock of electricity. By observing what electrically stimulating parts of the brain does can signal what those parts do.

Eg. stimulating some parts may control muscle contraction and movement

How do scientists study the brain using MRI scanning techniques?

Uses magnetic fields and electromagnetic waves to produce a detailed picture of the brain’s structure. It can help to show what parts of the brain is active when people are doing things (eg. speaking or remembering something).

Explain some of the difficulties of investigating brain function and treating brain damage and disease

The brain is very complex and delicate, making it difficult to investigate and treat safely.

Complexity: controls many different functions, so can be hard to link a specific area to a specific activity. This means that scientists don’t fully understand how all parts of the brain works so treatments can be uncertain;  it may also mean that damage to one area can also significantly affect others

Delicacy: the brain is soft and surrounded by the skull, so it is easily damaged and also very difficult to access without causing damage

Treatment risks: the potential side effects and risks of surgeries and treatments can sometimes be worse than the conditions

Therefore, research into the brain may be much slower than other organs

What is the eye?

The eye is a sense organ that contains many receptors; some components of the eye are a part of the CNS (the retina and optic nerve).

What are the eye’s key functions?

• Accommodation (the process by which the eye changes the shape of its lens to focus on objects of different distances) to focus on near or distance objects

• Refracting light to make light rays converge together onto one point on the retina (as for a clear image to be formed they must converge on a single point on the retina

• Respond to light intensity by changing pupil size

• Maintenance and protection

What are the receptors in the eye sensitive to?

• Light intensity

• Colour

Name the four main components of the eye

• Retina

• Optic nerve

• Sclera

• Pupil

How does the structure of the retina relate to its function?

The retina is the light sensitive layer at the back of the eye.

Function: to detect and absorb light energy to convert it into electrical impulses that can be interpreted by the brain into images.

• Has photo-receptor cells (rods and cones) which are light-sensitive cells that detect light and convert light energy into electrical impulses

Rods are a light sensitive cells: they are highly sensitive to light, even small amounts of light, enabling vision in low-light conditions. They do not distinguish different wavelengths of light, so only process information in black and white.

Cones are also light sensitive cells; there are three different types of cones and each type of cone is sensitive to a different wavelength of light (blue, red and green), allowing us to detect differences in the colour of light, enabling colour vision. They have a lower light sensitivity than rods, so only function in bright light.

• Connected to the optic nerve so that it can send the electrical impulses to the optic nerve where it will be carried along to the brain where the signals can be interpreted into images.

Explain the similarities and differences between rods and cones

Rods and cones are both types of photo-receptor cells which are light sensitive.

Rods:

Highly sensitive to light, so can detect even very low amounts of light, enabling vision in low light.

These cells do not distinguish different wavelengths of light, so only allow enable monochrome vision.

Cones:

Less sensitive to light, so can only function in bright light.

There are three types of cones which each detect different wavelengths of light (red, blue, green), allowing us to detect differences in the colours of light, enabling colour vision.

How does the structure of the optic nerve relate to its function?

It carries electrical impulses (that have been converted from light energy by the photo-receptor cells in the retina) from the retina to the brain as it is connected directly to the retina.

The nerve cells have a fatty myelin sheath, which provides an insulating layer to allow electrical impulses to travel more quickly and efficiently.

They also have a long axon to allow electrical impulses to be transported quickly over long distances from the retina to the brain.

How does the structure of the sclera relate to its function?

The sclera is the tough, fibrous (white) outer layer of the eye.

Function: protect the eye and help keep its shape. Also provides attachment points for muscles that move the eyeball

It is opaque to prevent the light from entering the eye from anywhere except through the cornea, helping to focus light correctly.

It is continuous with the cornea at the front, maintaining the shape of the eyeball and providing attachment points for eye muscles that move the eyeball

How does the structure of the pupil relate to its function?

The pupil is the hole in the eye where light enters.

It is a hole in the centre of the iris, allowing light to pass into the eye and reach the retina

It can change size, controlled by (the muscles in) the iris, to control the amount of light that can be allowed in to protect the retina.

What are the other five components of the eye?

• Iris

• Cornea

• Lens

• Ciliary muscles

• Suspensory ligaments

How is light focused?

Light is focused by refracting (bending) light rays so that they can coverge onto a single point on the retina to provide a clear image.

The cornea does most of the light refraction, and has a fixed focusing power, whereas the lens does the fine tuning of the focusing, and can change shape (due to the suspensory ligaments and ciliary muscles) to precisely increase and reduce its refractive power.

How does the structure of the iris relate to its function?

It is the coloured part of the eye surrounding the pupil that contains muscles to change the size of the pupil to therefore control the amount of light entering the eye.

Function: to control the size and diameter of the pupil to control the amount of light entering the eye, otherwise the photo-receptor cells in the retina may get damaged

It contains muscles which contract and relax to constrict or widen the pupil.

How does the structure of the cornea relate to its function?

It is the transparent outer layer of the eye that protects it and provides a fixed amount of the eye’s focusing power to focus light onto the lens.

It is transparent, allowing light to pass through onto the retina.

Has a tough outer layer to protect the eye from dust, pathogens and damage

It has a curved shape to help refract light’s rays so that they can converge together on one point on the retina - it does this by refracting light onto the lens; however it doesn’t change shape so its focusing power is fixed.

How does the structure of the lens relate to its function?

Function: the lens finely focuses the light rays onto the retina to provide a clear image - it is behind the pupil.

It is transparent to allow light to pass through onto the retina.

It is flexible to change shape to focus light from different distances.

It is curved to refract light rays precisely on the retina.

Made of elastic fibres so it can become thicker or thinner to focus light.

Held in place by suspensory ligaments, which attach the lens to ciliary muscles which contract and relax to allow its shape to be changed to focus light.

Describe the similarities and differences between the cornea and lens

Similarities:

• both transparent to allow light to pass through to reach the retina

• both are curved to refract light to focus it onto a point on the retina to provide a clear image

• both at the front of the eye to direct and focus incoming light

Differences:

• Cornea is at the front of the eye, whereas the lens is behind the iris and pupil

• The cornea has a fixed shape, whereas the shape of the lens is controlled by suspensory ligaments and ciliary muscles

• The cornea also acts as a tough outer layer to protect the eye from damage, pathogens and dust

• The cornea does most of the light refraction (focusing), whereas the lens fine tunes the focusing of the light

How does the structure of the ciliary muscles relate to its function?

Function: the ciliary muscles contract and relax to control the shape of the lens, allowing the eye to focus on near and distance objects in the process of accomodation.

It is connected to the lens by suspensory ligaments so that when the muscles contract or relax it can thicken or thin the lens to change its refractive power and focusing.

Made of involuntary muscle, so it works without conscious control to continuously adjust focus.

What is the function of the suspensory ligaments?

Function: they connect the lens to the ciliary muscles to allow the lens to change shape

Draw a diagram of the eye and label it

How to differentiate between cornea and iris?

Iris is the coloured part of the eye surrounding the pupil; it contains muscles to control the size and diameter of the pupil.

The cornea however is the transparent outer layer of the eye that refracts light to focus it and acts as a protective layer.

Describe what the iris reflex is and how it works

The iris reflex is a reflex that protects damage that may occur to the retina (photo-receptor cells in the retina) when exposed to bright light.

• When light receptors in the eye detect bright light, a reflex is triggered to make the pupil smaller.

• In bright light, the iris’ circular muscles will contract and radial muscles will relax, causing the size of the pupil to shrink and reducing the amount of light that can enter the eye.

• In dim light, the iris’ radial muscles will contract and circular muscles will relax, making the pupil wider to increase the amount of light that can enter the eye.

Describe what accommodation is and how the eye would focus on a near or distance object

Accommodation is the process of changing the shape of the lens to focus on near or distant objects.

The lens refracts the light entering the eye so that it can converge on one point of the retina. Accommodation changes the shape of the lens to adjust how much refraction is taking place; this is a reflex action as it does not involve conscious control. 

Near object:

• The ciliary muscles contract causing the suspensory ligaments to loosen. This causes the lens to become thicker (more curved), causing the rays to bend more sharply at a greater angle and therefore converge sooner; refract light rays strongly.

Distant object:

• The ciliary muscles relax, causing the suspensory ligaments to be pulled tight. This causes the lens to be pulled thin (less curved), causing the rays to bend less sharply at a lower angle and therefore converge less quickly; only slightly refracting the light rays.

What are two common defects of the eyes?

Myopia (short sighted-ness) and hyperopia (long sighted-ness); in which rays of light do not focus on the retina.

What does it mean if someone has myopia or hyperopia?

The rays of light passing through their eye do not focus on the retina

Describe why myopia may occur, how it can be fixed and draw diagram

Myopia is short sightedness, meaning that they are unable to focus on distant objects; the light rays from distant objects converge before they meet the retina so there is too much refraction.

This may occur if:

• the lens is the wrong shape and refracts light too much

• the eyeball is too long

You can use glasses or contact lenses with concave lenses (curve inwards) to correct it so that light rays focus on the retina.

The lenses will cause the light rays to refract in the opposite directions to the eye’s lens, so when the eye’s lens refract the newly refracted light rays, the overall refraction of the light rays will be less so they will converge on a single point on the retina.

Describe why hyperopia may occur, how it can be fixed and draw a diagram

Hyperopia is long sightedness, meaning that they are unable to focus on near objects; the light rays will not be refracted enough so they will not converge.

This may be caused if:

• the lens is the wrong shape and doesn’t refract the light enough

• the eyeball is too short

The light rays will converge behind the retina, so therefore do not converge on a single point at the retina.

You can use glasses with a convex lens (curves outwards) to correct it as the lens will refract the light so that it will focus on the retina.

Give alternative treatments to glasses for eye conditions

• Contact lenses

• Laser eye surgery (involves using a laser to change the shape of the cornea, and therefore how strongly it refracts light)

• Replacement lens surgery (involves surgery which requires a cut into the cornea and inserting a lens of the correct shape)

What controls body temperature?

Body temperature is monitored and controlled by the thermoregulatory centre in the brain.

What is the thermoregulatory centre?

The thermoregulatory centre is a part of the CNS and is a control centre located in the hypothalamus in the brain.

The thermoregulatory centre is a control centre that contains thermo-receptors that are sensitive to the temperature of the blood flowing through the brain. The skin also contains thermo-receptors sensitive to the temperature of the skin and sends nervous impulses to the thermoregulatory centre to give information about skin temperature. 

Why must the body maintain a constant temperature?

• for enzymes to function correctly (not to become denatured or work too slowly)

• allow cells to function optimally

Draw a brief flow chart diagram of how the thermoregulatory system will respond to a decrease or increase in temperature

Where is the thermoregulatory centre located?

In the hypothalamus in the brain.

The body also has thermoreceptors in the hypothalamus and in the skin.

How does the body respond if body temperature is too high?

• The thermoreceptors in the skin and hypothalamus in the brain will detect a rise in temperature

• The receptors will send nervous impulses to the brain along sensory neurones where this information will be interpreted

• The thermoregulatory centre in the hypothalamus will then process this information and coordinates a response

• The hypothalamus will automatically (without conscious control) send nervous impulses along motor neurones to the effectors

The effectors to cool the body down will be:

Sweat glands, blood vessels and hairs on the surface of the skin

• The sweat glands will respond to the electrical impulse by secreting sweat onto the skin; when the sweat evaporates off of the skin’s surface it will remove heat, cooling the body down

• The blood vessels will also be stimulated to dilate (vasodilation), allowing more blood to flow through the capillaries near the skin, causing more head to be lost from the body by radiation, cooling the body down

• The erector muscles on the base of each of the hairs on the surface of the skin will relax, causing the hairs to lie flat. This prevents a layer of insulating air being trapped next to the surface of the skin, so more heat is lost to the environment.

These mechanisms cause a transfer of energy (in the form of heat) from the skin to the environment.

How does the body respond if body temperature is too low?

• The thermoreceptors in the skin and hypothalamus in the brain will detect a decrease in temperature

• The receptors will send nervous impulses to the brain along sensory neurones where this information will be interpreted

• The thermoregulatory centre in the hypothalamus will then process this information and coordinates a response

• The hypothalamus will automatically (without conscious control) send nervous impulses along motor neurones to the effectors

The effectors to cool the body down will be:

Sweat glands, blood vessels and hairs on the surface of the skin and muscles

• The sweat glands will stop producing sweat, so no heat will be lost from the skin by evaporation of water off of the surface of the skin

• The blood vessels will respond by constricting (vasoconstriction) to reduce the blood flow to the capillaries near the skin, to reduce the amount of heat lost by radiation from the blood

• The erector muscles on the surface of the skin will contract, causing the hair to stand up trapping an insulating layer of air on the surface of the skin, preventing heat loss

• The skeletal muscles will begin to contract rapidly; these muscle contractions require energy from respiration, which release heat, helping to raise body temperature

These mechanisms cause a transfer of energy (in the form of heat) from the skin to the environment.

What is ideal body temerature?

37 degrees

What are hormones?

Hormones are chemical molecules released directly into the blood

What do hormones do?

They coordinate and control the functions of the body by signalling to target organs what to do and when.

What are the key principles of hormonal coordination?

• Hormones are chemical messengers released by glands into the bloodstream.

• They are carried in the blood to target organs or tissues (effectors).

• Hormones produce specific responses in the target organs because only the target cells have receptors for that hormone.

• Hormonal responses are usually slower than nervous responses, but they can be long-lasting.

• The endocrine system works to maintain homeostasis and coordinate processes such as growth, metabolism, reproduction, and responses to stimuli.

What are hormones produced by?

Hormones are produced and secreted by endocrine glands

What are glands?

An organ in the body that secretes particular chemical substances for use in the body or for discharge into the surroundings

Describe the differences between nervous and hormonal responses

nervous:

very fast responses

act for a short time

responses are electrical impulses that travel along neurones

act on a very precise area

voluntary or involuntary

hormonal:

slower responses

act for a long time

responses are chemical messengers that are carried by plasma in the blood

target the whole body’s functions more generally

always involuntary

Why are hormonal responses slower than nervous responses?

• Hormones need to travel in the blood which takes time to circulate around the body

• Nervous impulses travel along neurones, which move very quickly

• The nervous system sends impulses directly to target cells, so the response happens immediately; this is because the transportation is more direct

What is the endocrine system composed of?

Hormones which acts as chemical messengers

Glands which secrete hormones directly into the bloodstream

The blood carries the hormones to the target organ where it produces an effect

What principle does homeostasis work by?

Negative feedback

What is meant by a negative feedback loop?

Negative feedback is a key principle in homeostasis.

It is an automatic control mechanism in which a change in the body’s internal environment (stimulus) is detected by receptors which trigger responses that counteract the change, bringing the internal environment back to its normal state.

Negative feedback reverses a change to maintain a balance (homeostasis)

Draw flowcharts for examples of negative feedback

Describe how negative feedback works

The receptor will detect a stimulus

The control centre will receive and process the information and then coordinate a response

The effector produces a response which counteracts the change and restores the optimum level

The effectors will continue on producing the responses for as long as they’re stimulated by the coordination centre. This may cause the opposite problem by making the level change too much, however the receptors will detect if the level becomes too different again, and the negative feedback mechanism will begin again.

This process is automatic.

What is the pituary gland and what is its function?

The pituary gland exists as a structure in the brain.

It secretes several primary hormones into the blood in response to body conditions (negative feedback).

It is referred to as the ‘master gland’ because the primary hormones secreted in turn act on other glands to stimulate other secondary hormones to be released to bring about effects.

Give four examples of what the pituary gland may control

• water levels

• fertility

• growth

• blood sugar

What hormone do the ovaries secrete?

Oestrogen, which is involved in the menstrual cycle and the development of female secondary sexual characteristics

What hormone do the testes secrete?

Testosterone, which controls puberty and sperm production in males

What hormone does the thyroid secrete?

Thyroxine, involved in regulating rate of metabolism, heart rate, temperature

What hormone does the adrenal gland secrete?

Adrenaline, used to prepare the body for a ‘flight or fight’ response

What hormones do the pancreas secrete?

Secretes insulin and glucagon, which are used to regulate blood glucose levels