GCSE Science podcasts - Homeostasis

Hello, I’m Dr Alex Lathbridge and this is Bitesize Biology.

This series is all about homeostasis.

You’re in luck because that means we’re covering everything from the nervous system and hormones to blood glucose levels and the menstrual cycle.

First things first in this episode – what actually is homeostasis?

You know how, when a computer or laptop or games console starts getting a bit too hot and its fans starts spinning up to cool it down? So, it doesn’t end up overheating and suddenly switching off, or worse, getting damaged. And the hotter the computer is, the faster the fans spin?

Well, the same thing goes on in your body, but on a wider scale.

And it’s not just temperature, it’s really important that your body keeps all of its internal conditions at the right levels.

So remember this definition of homeostasis:

Homeostasis is the maintenance of a constant internal environment in the body.

So in really simple terms, when external conditions change, homeostasis allows your body to get internal conditions back to where they should be.

So homeostasis isn't really just one "thing". It’s all the different collective processes that an organism carries out to maintain the internal conditions necessary for survival, and you and I are going to go through them in this series.

There are 3 main conditions that you need to know :

1. blood glucose concentration, how much sugar is in our blood

2. water levels in cells

3. our internal body temperature   So why is homeostasis so important?

Think back to our previous series on The Cell. I pretty much said that we’re just big containers of cells where chemical reactions are happening all the time. Well, those chemical reactions need to happen.

The best example are probably enzymes.

They need really specific conditions to do their important job of speeding up (or catalysing) chemical reactions in the body.

If internal conditions are too hot or too acidic, or too alkaline, enzymes will be denatured.

This means they stop functioning, and so the chemical reactions slow down or just stop. If you want a recap of enzymes, go back and listen to our episode on them in our series on the Organisation of Plants and Animals on BBC Sounds.

There are so many things in the external environment that constantly change, and your body responds to them without you thinking thanks to homeostasis.

We call these changes in the environment stimuli (or if it’s one, it’s a stimulus).

Examples of stimuli that change all the time include temperature, light, sound, pressure, pain, a chemical change, or you move about, so a change in position.

What does that look like in the real world? Well, one day I was making dinner, and I was removing a hot tray filled with chips from the oven. And I only put one oven glove on, because I’m an idiot, but I got distracted by someone talking to me and picked up the pan with both hands, burning my ungloved hand and dropping the pan (and my chips just everywhere.)

That is homeostasis in action, I’ll explain more in a bit, after you’ve jotted down these key terms.

So grab a pen and make a note:

In order to maintain a constant internal environment at the right levels, our body has automatic control systems.

These control systems involve nervous responses, regulated by our nervous system, or chemical responses, regulated by the endocrine system.

Don’t worry, we’ll going to more in-depth on those two automatic control systems later in this series.

But for now, you need to know that these automatic control systems have three main components that all work together to maintain constant internal conditions.

You need to know the names of these and how they work:

1. Receptors - these detect changes in the environment, or stimuli

2. The coordination centre – these are things like the brain, which processes information from receptors around the body

3. Effectors – these are things like muscles or glands, which the coordinator generates a response in

The receptors of an automatic control system detect a stimulus when something in our internal conditions might be too high or low, like temperature.

They then pass that information to the coordination centre, which has a think, (or if you want to pass your exams) processes the information and generate a response in the effectors.

The effectors work to regulate internal conditions back to their optimum level.

So remember the order is receptors, the coordination Centre and effectors.

This process where optimum levels are restored is called a negative feedback loop.

Constantly maintaining the optimum internal conditions is a bit like always making sure everything is perfectly balanced. What the body doesn’t want is an imbalance of anything, for example where our internal conditions are either too hot or too cold.

And at optimal conditions, the temperature in the body is usually around 37 degrees Celsius, which enables enzymes and cells to work at their best.

So with what you’ve just learnt about the negative feedback loop in mind, let’s go back to my terrible cooking.
 An external stimulus (the hot pan) was detected by receptors on the palm of my hand, they detected the pain and the change in temperature.

The coordination centre, my brain in this instance, received and processed that information from the receptors.

Signals were sent to muscles in my arm and hand (the effectors) to respond by dropping the pan, thus preventing even more damage to my body, and attempting to bring that part of my body back to optimal conditions.

This is an extreme example. The same thing can be said for being in a hot environment, you have heat receptors detecting that change, it gets processed by the coordination centre (the brain), and the effectors cause you to do something like sweating.

So basically, a negative feedback loop responds to any changes away from the optimum level, in order to bring the levels back to this optimum state. It’s a continuous, looping process.

Just like a fan on a computer, laptop or gaming console.

In terms of temperature, if the scales are tipped too far the other way, and the internal body temperature gets too cold, receptors will detect this too, and send information to the coordination centre, which will generate a response in the effector.

This negative feedback loop happens without you even thinking about it, you don’t have to do anything consciously. It’s all automatic.

Just because it's called negative that doesn’t mean it refers to not having enough of something, it's both if something is too high or too low.  I’m Dr Alex Lathbridge and this is Bitesize Biology – all episodes available now on BBC sounds.

Hello, I’m Dr Alex Lathbridge and this is Bitesize Biology.This is episode two in our eight-part series about homeostasis. Today, we’re going to focus on the nervous system.

In the last episode, I introduced the concept of homeostasis, the processes that your body uses to maintain constant internal conditions.

The conditions inside your body must be carefully controlled so that enzymes and cells can functioneffectively.

Our bodies have two types of automatic control systems that regulate our internal conditions: nervous responses and chemical responses.

Automatic control systems have three important components:

Receptors, that detect stimuli (or changes in the environment).

A coordination centre, such as the brain, which processes information from receptors, and generates a response in the…

Effectors, which restore internal conditions to optimum levels.

The nervous system is one of these automatic control systems, and allows us to react to oursurroundings and coordinate our behaviour.

And it does all of this without you even realising. For instance, right now – I want you to start thinking about breathing. Also think about that weird feeling your tongue makes in your mouth. If you’re anything like me, you’ve just been forced into the driver’s seat of activities that your nervous system manages without you having to consciously do anything.

Like all control systems, the nervous system has receptors, a coordination centre (the brain) and effectors.

I’m going to break the nervous system down into three easy sections:

1. Getting information
2. Transmitting information
3. Using information

First up, getting information:

This involves stimuli being detected by receptors. These are found in all of your sensory organs:

Receptors on your skin detect changes temperature, touch and pain.

Receptors on your tongue can detect chemicals in food, or taste.

Receptors in your nose detect chemicals around us, in the air, which is smell.

Receptors on your eyes detect changes in light, which is sight.

Receptors on your ears detect changes in sound, which is hearing.

These are just the main receptors that each of those sensory organs are known for. Of course, you have pain receptors in your tongue.

Next, transmitting information.

Any information from the receptors, travel as electrical impulses along nerve cells (these are calledneurones) to the your brain.

It’s easy to shorthand it and say “receptors send signals to your brain” in your exam.

I know what you mean but I want you to get marks. So try to get in the habit of saying that these are “electrical impulses that travel in the nervous system.”

So where do those electrical impulses go?

Well, the brain and the spinal cord make up the central nervous system.

It’s called that because it collects information from across your whole body and manages activity across your whole body.

Information from receptors passes along neurones as electrical impulses to the central nervous system.

Using the information.

The central nervous system processes the information and generates a response in the effectors, sending an impulse along neurones to them, to create a response.

Examples of effectors include a muscle contracting to move an arm away from a hot oven or youreyelids shutting when you’re staring at something too bright, like the sun.

You need to remember the journey of an electrical impulse in response to a stimulus:

Stimulus – receptor – Coordinator: central nervous system – effector – response

Let’s take a look at the specialised cells of this system: the neurones. These all come together to formnervous tissue.

If you’re not sure about neurones, you can head back to our series on The Cell and have a listen toEpisode 3.

Quick recap: the two F’s; function and form.

What’s the function of neurones? To rapidly carrying electrical impulses from one place to another.

And how are they adapted, what’s their form? Basically, they’ve got a long fibre called an axon, and it's covered in a fatty, myelin sheath, so they can easily carry electrical impulses.

The neurones have lots of branches at one end (these are known as dendrites) to connect with other neurones and form a network.

They pass electrical impulses from one neurone to the next, like a baton being passed between runners in a relay race.

And like runners in a relay race, the neurones don’t actually touch each other. There are teeny tiny spaces between the neurons called synapses and this is where most of the activity occurs.

Remember those dendrites and axons? Axons send out chemical messengers, known asNeurotransmitters, across synapses to the next neurones.

So, these chemicals, these neurotransmitters, are able to hop across the gap, and carry the electrical impulse onto the next neurone, going from the end of one axon, across the synapse, and into a dendrite of the next neurone.

You might have two questions here:

1. How can electrical impulses become neurotransmitters, which are chemicals, and then become electrical again?

Great question, that’s for A-Levels so I shan’t waste your time (but it’s cool).

2. How do neurotransmitters, which are chemicals, hop across the gap to the next neuron?

This is where our good friend diffusion comes back. These neurotransmitters diffuse across the synapse, and they bind with receptor molecules on the membrane of the second neurone.

These are at the dendrite end of the neurone. The receptors only bind to specific neurotransmitters, released from the first neurone. This then stimulates the second neurone to carry the electrical impulse on to the next one.

And to make things even more hectic, neurones have many branching dendrites for multiple connections with axons, but usually only one long axon.

However, the end of the axon can branch off, to connect to multiple neurones. Now, all these neurones working together can process some complicated natural computations that when all working together are what allows your nervous system to function.

Earlier we talked about the journey of an electrical impulse in response to stimulus:

Stimulus – receptor – Coordination centre – effector – response

There are three types of neurones you need to know for your exams:

1. Sensory neurones. These are specialized to gather and carry sensory information towards the central nervous system. So, think about the cells in your eyes which detect light from the world around you.

2. Motor neurones. These terminate on muscles and they’re how you move your body. These carry signals away from your central nervous system. They’re also responsible for the fight or flight response working, like slowing your intestines.

3. Relay neurones. Like the name suggests, these help the sensory neurones and motor neurones communicate. So, they’re found mainly in the brain and spine.

Easy to remember: sensory neurons deal with sensory information. Motor neurons deal with putting a response in effect. And relay neurons are in-between.

So now that we know the names of our neurones, let’s finish up today putting them in our nervoussystem pathway:

A stimulus is detected by the receptor, in sensory organs.

Then, sensory neurones carry electrical impulses from the receptors to the central nervous system.

Then the central nervous system, which is filled with relay neurones, processes the information.

And sends electrical impulses along the motor neurones to the effectors, which generate a response.

And finally, the response happens like a muscle contracting.

I’m Dr Alex Lathbridge and this is Bitesize Biology – subscribe to the podcast on BBC Sounds

Hello. I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the third episode in our eight-part series about Homeostasis and Response.

Today, we’re going to talk about the endocrine system, the system that secretes hormones into the bloodstream to regulate processes in our bodies.

You’re going to need to know what hormones are, so grab a pen to write this definition down:

A hormone is a chemical messenger, produced in glands and carried by the blood to specific organs in the body.

As I said in our series on The Cell, we are made up of trillions of cells where reactions are happening all the time and there are lots of chemicals that cells use to send messages to one another, usually in relatively close proximity, like within an organ.

Remember how we said in the last episode that the nervous system transmits signals very quickly?

Well, hormones are a lot slower, but the key thing about them is that they can send messages from one part of the body to all of the other parts of the body.

Remember, your blood circulates all around your body like water flowing in a river, so it’s a good way to send a message.

So if our message is a chemical in the blood, it goes where the blood goes, and the blood goes everywhere in the body.

When one body part, a gland, wants to send a message to the whole body, it just tips a certainhormone into the blood, and that gets the message to the rest of the body.

I already know your next question: if hormones are circulating in the blood, won’t they affect everysingle organ?

No, because cells in different organs have different receptors, so they can only pick up themessages from specific hormones.

So what we say is that hormones travel in the blood to specific target organs.

Hormones are produced and secreted by glands, which altogether make up the endocrine system.

You need to know about six different glands, what their names are, where they’re based, and what they do:

1. The Pituitary Gland. It’s situated at the base of the brain and its generally known as the ‘master gland.’ It produces many hormones, one of which is ADH (anti-diuretic hormone) which affects thekidneys and controls the water levels in blood.

2. The Thyroid Gland. It’s in the neck and produces thyroxine, which regulates temperature, heartrate and the rate of metabolism.

3. The Pancreas. This is an organ in the abdomen, it’s near to the liver and close to the stomach. It produces Insulin, which targets the liver, and regulates blood glucose levels.

4. The Adrenal Glands. There are two of them. They are located close to your kidneys. They produceAdrenaline, which is released in response to scary or stressful situations. The body gets prepared for the “fight or flight” response, increasing things like heart rate, breathing rate and blood flow to your muscles.

5. The Ovaries. They’re part of the female reproductive system, producing oestrogen, which plays an important part in the menstrual cycle.

6. The Testes. These are part of the male reproductive system, they produce testosterone, which effects puberty and the production of sperm.

In the last episode, we spoke about sending messages with the nervous system and now we’ve got the endocrine system. They do similar jobs but how they do it are very different. Let’s compare:

The nervous system carries electrical signals, whereas hormones are chemicals.

The nervous system transmits electrical impulses by nerve cells (called neurones), whereashormones are carried in the bloodstream.

The nervous system uses effectors such as muscles or glands to generate a response, whereashormones target cells in particular organs and tissues.

The nervous system generates a rapid response (it's really quick), whereas most hormones are slow to react.

The electrical impulses in the nervous system act for a very short time, whereas hormones actfor a long time, until the hormone is broken down.

You might be asked in your exam to identify if a response is from hormones or nerve impulses.

If a response is really quick, it's likely to be a nervous one. If your body needs to make a really quick change, like moving hands away from a hot stove, you need a message sent to the effectors as quickly as possible. Hormones would just take too long.

However, if a response lasts a long time, it likely to be a hormonal one. The effects of hormones aremuch slower than the nervous system, but they last for longer.

When adrenaline is released into your bloodstream as part of that fight or flight response, your heart rate and breathing rate goes up over time. It’s not just an instantaneous push. You also feel a bit wobbly afterwards, which is when the hormonal response still ongoing.

I’m Dr Alex Lathbridge and this is Bitesize Biology -the key things you need for your biology GCSE, all available now on BBC sounds.

Hello, I’m Dr Alex Lathbridge, and this is Bitesize Biology.

This is the fourth episode in an eight-part series on Homeostasis.
In this episode we’re going to talk about blood glucose levels and how this is regulated in the body by hormones. Plus, what happens if it’s too high, or too low. And we’re going to talk about the two types of diabetes.

In the last episode we looked at how the endocrine system uses glands to secrete hormones thatregulate our bodies.

We spoke about what a hormone is, make sure you remember the definition:

A hormone is a chemical messenger produced in glands and carried by the blood to specific organs in the body.

Today, we’re going to take a look specifically at how the hormone insulin controls blood glucose levels.

Glucose is a simple sugar that our body uses for respiration (remember episode nine of our series on The Cell, go back if you need a recap.)

It is very important that the concentration of glucose in the blood is carefully controlled.

This is the job of insulin, a hormone produced by the pancreas, an organ that monitors your blood glucose level.

When you eat food, your intestine starts absorbing nutrients. Carbohydrates especially are converted into glucose, which goes into the blood, causing your blood glucose levels to rise.

The pancreas detects this rise and starts producing more insulin than normal.

The reason why this is so important, is that glucose can’t go straight into your cells, even though they need it. Imagine that your cells are locked, insulin is the key to open that lock and allow glucose in.

So if blood glucose levels are too high:

An increase in blood glucose is detected by the pancreas.

The pancreas produces insulin and releases it into the bloodstream.

The insulin causes glucose to move out of the bloodstream and into body cells.

So obviously, glucose going from blood into cells reduces the glucose levels in the blood.

And cells in the liver and muscles can take up excess glucose and convert it into glycogen, where itcan be stored. Remember glycogen is insoluble.

So that’s how insulin reduces blood glucose levels.

If blood glucose levels are too low:

A decrease in blood glucose is detected by the pancreas.

The pancreas produces another hormone, glucagon, and releases it into the bloodstream. You can think of glucagon as the anti-insulin.

The glucagon causes the glycogen stored in the liver and muscles to be converted back into glucose.

This increases the glucose levels in the bloodstream, so glucagon increases blood glucose levels.

So insulin and glucagon are our hormones.

Insulin decreases blood glucose levels by moving glucose into cells.

Glucagon increases blood glucose levels by converting stored glycogen into glucose.

Glucagon and glycogen are very similar words, so let’s not get them mixed up.

Glycogen is what excess glucose is converted into and stored in the liver and muscle tissues.

Glycogen, think “generated for later.”

Glucagon is a hormone produced by the pancreas when blood glucose levels are too low. This in turn causes the glycogen stored in the liver to be turned back into glucose.

Glucagon, my glucose is gone, please use the stuff I have in storage.

It’s really important that your blood glucose level is controlled.

Because having lots of glucose in your blood for an extended period of time is not good.

At a cellular level, It can damage the small blood vessels that provide oxygen and essential nutrients from reaching your cells, literally starving them.

Over the long term, this increases the risk of eye diseases, circulation issues, damage to the feet, the gums, the kidney and more. Not to mention, damage to nerve cells, which can make it harder for them to carry messages between the brain the rest of the body, so it can affect how you see, hear, feel and move.

A condition where the blood glucose levels remain too high is known as diabetes.

It’s actually really common, you might have heard of it or even have friends or family members that have it.Depending on the type of diabetes, it can be controlled by injecting the hormone insulin, which we now know causes the liver and the muscles to convert excess glucose into glycogen, lowering the blood glucose levels.

There are two types of diabetes, Type 1 and Type 2, and you need to know the differences between them.

The symptoms are similar, but the causes are different:

Type 1 diabetes is when blood glucose levels are too high because the pancreas doesn’t produceenough, or any, insulin.

Type 1 diabetes is detected from an early age and is controlled by injecting insulin into the bloodstream, replacing the insulin that your own body can’t make.

People with type 1 diabetes have to monitor the blood glucose levels throughout the day. If left untreated, blood glucose levels can rise to a fatal amount.

Insulin injections normally happen throughout the day after mealtimes, to stop glucose levels getting too high.

Insulin injections are the crucial method of treating Type 1 diabetes.

Patients with Type 1 diabetes can also carefully monitor the amount of simple sugars found in carbohydrates they eat and exercise regularly.

Type 2 diabetes is when the cells in someone’s body no longer respond to their own insulin produced by the pancreas, as the result of an unhealthy lifestyle.

They still make enough of their insulin, but the cells don’t respond as they should, so if insulin is a key, it’s like the cells have changed their locks and the glucose stays in the blood.

Type 2 diabetes is more common in older and overweight people. Injecting more insulin doesn’t have any effect so it's not a method of treatment.

Type 2 diabetes is controlled by eating a diet low in in carbohydrates and doing regular exercise, to lower the blood glucose levels.

Being obese and not exercising enough are big risk factors that can lead to diabetes. But a good diet and regular exercise can usually fully reverse Type 2 diabetes.

It’s really important that people avoid getting Type 2 diabetes because diabetes leads toalmost 9,600 leg, toe or foot amputations every year in the UK. That's 185 a week.

I’m Dr Alex Lathbridge and this is Bitesize Biology. You can find more Bitesize podcasts by searching Bitesize on the BBC Sounds app.

Hello, I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the fifth episode in our eight-part series on homeostasis. Today, we’re going to learn about hormones during puberty and the menstrual cycle.

In Episode 3, we went through hormones and the fact that they’re chemical messengers produced in glands, carried by the bloodstream, to target specific organs. You need to know the names of some hormones like insulin and adrenaline, plus where they’re released. So go back and listen to episode 3 if you need to.

Puberty is the stage in life when a child’s body develops into an adult body.

Puberty takes place gradually, usually occurring between the ages of 10 and 16. But, everyone is different and there are no rules as to when puberty starts.

During puberty, reproductive hormones are released in the body, that cause changes known as secondary sex characteristics. Some of these changes happen across both biological sexes, whereas some only happen in either females or males.

In females, oestrogen is the main reproductive hormone. It is produced by the ovaries, where its function is to release eggs and begin the menstrual cycle.

In males, testosterone is the main reproductive hormone, and it is produced by the testes. Its function is to stimulate sperm production.

Both females and males undergo changes such as developing pubic hair, underarm hair, and the development and growth of sexual organs.

The menstrual cycle is a recurring, monthly process in females. On average, it takes around 28 days.

But it’s completely normal to have a cycle that’s longer or shorter, but it’s really simple to understand, let me explain.

Imagine you’re preparing a bed for a guest that you might be expecting to come over.

You’re going to throw down a new bedsheet on the bed. You wait a couple of days to see if they’re turning up.

And if they don’t show up, you’re going to rip off those sheets and get rid of them.

And that happens again and again every 28 days or so, on the off chance that that guest does turn up. It’s basically that, but with more bodily fluids.

The important thing is that it’s a cycle, and it’s a really interesting one.

It involves the female releasing an egg, the lining of the uterus (the womb) being prepared andthickening, in case it receives a fertilised egg (that’s the guest).

If the released egg is fertilised by sperm, then the female becomes pregnant.

If the egg is unfertilised, it doesn’t implant into the prepared uterus lining, so this lining is no longer needed and so it gets shed, like layers of wallpaper coming off a wall.

This is known as menstruation, or a period, containing all of that shed lining and blood.

You’re going to need to know the four stages of a menstrual cycle. We’ll say it’s over 28 days:

Stage 1. The first day of your period. The lining of the uterus breaks down and menstruation starts, and that bleeding and shedding goes on for about four days.

Stage 2. The lining of the uterus builds back up again, over about ten days, and becomes a thick, spongy, lining of blood vessels ready to support a fertilised egg.

Stage 3. In the middle of the cycle, on day 14, an egg is released from the ovary in a process called ovulation.

Stage 4. The lining of the uterus is then maintained up until day 28. If no fertilised egg arrives by day 28, the lining begins to break down again, and the whole cycle starts again with the first day of menstruation, or the first day of the period.

Those are the stages. Now we need to know the names of hormones involved, and where they are made in the body, plus what they do.

Because this cycle is driven by a constant change of hormones being released into the blood and sensed by various different organs, it can get a little bit tricky.

It might help you right now if you pause it here and pull up the Bitesize website. You can find a diagram illustrating what I’m about to explain to you.

There are four hormones that you need to know about:

1. Follicle Stimulating Hormone (we call that FSH)

2. Oestrogen

3. Luteinising Hormone (we call that LH)

4. Progesterone

First, Follicle Stimulating Hormone (or FSH) is produced in the pituitary gland (in the brain).

Easy one to remember, it stimulates the ovaries to mature an egg.

The egg matures inside a fluid filled sac on the ovaries known as a follicle.

Oestrogen is released by the ovaries, and it does a couple of things.

It stops, or inhibits, the Pituitary Gland from producing more FSH (we don’t need more than one mature egg per cycle).

Oestrogen also repairs and thickens the uterus lining (basically telling the uterus “Hey, we might get a fertile egg soon, so be ready”).

Finally, it stimulates the release of Luteinising Hormone (also known as LH).

The Luteinising Hormone (LH) is produced by the pituitary gland in the brain, same as FSH.

It stimulates the release of a mature egg on day 14, that’s around the middle of the cycle, this is ovulation.

So once that egg is released, the follicle is now empty, and it gets broken down by the ovaries to produce progesterone alongside oestrogen.

Progesterone is the last hormone involved. I call it the menstrual maintenance hormone.

It maintains the lining of the uterus in the second half of the cycle, so days 14 to 28.

And inhibits the release of LH and FSH.

Both of those things basically tell the pituitary gland and womb “okay chill – theremight be a fertilized egg, just wait to see.”

If a woman becomes pregnant, the fertilized egg sticks to the uterus and the placenta produces more progesterone. This maintains the lining of the uterus during pregnancy and means that menstruation does not happen.

If pregnancy doesn’t occur, progesterone levels decrease, and the lining of the uterus breaks away again starting the period, or menstruation, so the cycle continues.

But here’s the absolute beauty of biology. By understanding the role of hormones in preparing for pregnancy, we can manipulate that to prevent pregnancy.

Yes, let’s talk about contraceptives, preventing pregnancy with science.

All contraception does is prevent sperm and egg from meeting to fertilise, either biologically (hormonal methods) or physically (barrier methods).

These are things like male and female condoms, diaphragms and caps. It’s exactly what it says on the tin, they create a physical barrier to stop the sperm from reaching the egg.

Now onto hormonal methods of contraception.

We have the oral combined contraceptive, also known as “the pill”.

It contains a combination of artificial forms of the hormones oestrogen and progesterone and is taken daily.

How does this work? It basically tricks the body.

Oestrogen taken every day inhibits FSH production. No FSH means that no eggs are stimulated to mature.

Progesterone stimulates cervical mucus to become super thick, so it’s like the sperm are swimming through dense porridge instead of water.

The pill makes the uterus lining thin, so even if there is fertilisation, there’s less chance of the egg being able to grow, because it won’t be able to stick to the lining of the uterus.

The pill can also inhibit egg maturation.

Besides pills, there are injections, implants and skin patches that contain slow release of artificial progesterone to prevent the maturation and release of eggs.

I’m Dr Alex Lathbridge and this is Bitesize Biology. Listen now on BBC Sounds.

Hello, I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the sixth episode in our series on homeostasis. Today, we’re going to talk about the brain and the eyes.

I mentioned the brain in our episode on the Nervous System. It is part of our central nervous system along with the spinal cord.

It is made of billions interconnected nerve cells, called neurones, that carry electrical impulses.

The brain is responsible for complex behaviour, and stupid behaviour too.

Scientists have mapped different regions of the brain and have been able to find out what they do, in terms of controlling our functions and behaviours.

You will need to learn the names of three parts and what they control:

The cerebral cortex. That’s the outer layer of the brain and is divided into two hemispheres. It is responsible for memory, consciousness and personality.

The cerebellum. This is underneath the cerebral cortex and controls balance, co-ordination of movement and muscular activity.

The medulla. This is a long, stem-like structure in the bottom part of the brainstem. It controls unconscious activities like heart rate and breathing rate

So that’s the cerebral cortex, cerebellum and medulla. Obviously there’s a lot more going on in the brain than that, but that’s all you need to know for now.

So now let’s focus on the eyes. What is the eye, biologically speaking?

The eye is a sensory organ, containing receptors.

The eye receptors are sensitive to changes in light intensity and colour.

The eye contains two types of receptors: rod cells which are sensitive to light intensity and cone cells which are sensitive to colour.

All of the structures of the eye function together, to allow light to hit an area called the retina, which sends signals to the brain, processing that as vision.

You need to know the names of these different structures of the eye and their functions so it might be a good idea to grab a pen and draw your own eye diagram.

The cornea. This is a transparent lens in front of the eye, it refracts light as it enters the eye. This just means it bends it.

The iris. This controls how much light enters the pupil.

The lens is transparent disc that further refracts light to focus it onto the retina.

The retina contains receptor cells. These are the rod cells to detect light intensity and the cone cells to detect colour.

The optic nerve is a bundle of neurones that carries impulses from the eye to the brain.

The sclera is a tough white, outer layer of the eye, it helps protect your eye from injury.

You can check your own eye diagram against the ones found on the Bitesize website.

Unlike a digital camera where you can control the amount of light entering through the lens, the amount of light entering the eye is controlled by reflex action. You don’t control it by thinking.

This happens by the size of pupil changing automatically in response to either bright or dim light.

This is controlled by the muscles of the iris.

When there is bright light, this reflex action prevents the retina from damage.

When there is dim light (when its dark), this reflex action protects us from not being able to see anything.

Go in front of a mirror, turn the light off for about 10 to 15 seconds, and then turn it back on. If you’re fast, you’ll be able to catch your pupil going from very dilated to very constricted.

This reflex action is controlled by two groups of muscles in the iris:

The radial muscles and the circular muscles.

They do different things in dim light or bright light

In dim light the pupil dilates (gets bigger) to allow as much light into the eye as possible.

If it’s dark, you want to make sure you can really capture all of the light so in dim light, the receptors detect changes in the environment and the radial muscles contract.

The circular muscles relax and the pupil dilates so that more light can enter the eye, making it easier for us to see in dark environments.

So in bright light, receptors detect changes in the environment and the radial muscles relax, the circular muscles contract, and the pupil constricts so that less light can enter the eye, protecting the retina from damage.

You really don’t want your retina to burn out, this why you don’t stare directly into the sun or the direct path of a laser.

Accommodation is the process of the lens changing shape in order to focus on near or faraway objects.

The lens is elastic and the suspensory ligaments attached to it become tighter or looser, and this changes the shape of the lens in the process.

The ciliary muscles either relax or contract depending on how near or faraway an object is.

In order to focus on a near object :

The ciliary muscles contract

The suspensory ligaments loosen

The lens becomes thicker and refracts light rays more intensely.

But, to focus on a faraway, distant object:

The ciliary muscles relax

The suspensory ligaments tighten

The lens becomes thinner and refracts less light

Myopia is the scientific name for short-sightedness, and hyperopia is the name for long-sightedness.

In both myopia and hyperopia, rays of light do not focus properly on the retina, and that means that a person’s vision is not a clear image. So what do people do ?

People wear glasses, with lenses which refract the rays of light to compensate, so that the rays of light do focus on the retina properly.

Myopia, short-sightedness, can be corrected by using a concave lens (bending in.)

Hyperopia, long-sightedness, is corrected by glasses using a convex lens.

I’m Dr Alex Lathbridge and this is Bitesize Biology. All episodes available right now on BBC Sounds.

Hello, I’m Dr Alex Lathbridge and this is Bitesize Biology.

This the seventh episode in our eight-part series on homeostasis.

Today, we’re going to talk about water and nitrogen balance.

In our series on The Cell, we looked at osmosis, the movement of water across a selectively permeable membrane from a region of higher concentration to a region of lower concentration. If that doesn’t ring a bell, go back and listen because it’s going to come up today.

Water and mineral salt levels need to be controlled, so that the concentration of water and salts is the same inside and outside the cells.

If the water concentration is too low outside of the cell, water leaves the cell by osmosis, meaning it might shrivel up.

If the water concentration outside the cell is too high outside of the cell, too much water enters the cell by osmosis and they swell up, and they might burst.

So maintaining a constant level of water and mineral salts in the blood is known as osmoregulation.

There are a few different ways that water leaves the body :

When we sweat, water leaves through the skin.

When we breathe out, or exhale, water leaves from the lungs, like when you breathe out on a window or into the air on a cold day, you can see water vapour.

Surprisingly, the lungs and skin have no control over how much water is lost.

It’s down to the kidneys, they control water loss. They remove excess water, salt and urea from the blood to produce urine.

Urine is actually a really good marker for what’s going on in your body. Because it’s one of the routes by which the body gets rid of all the by-products that shouldn’t really stay in your bloodstream.

So the general rule of hydration is that the lighter and clearer your urine, the better. If you don’t drink enough water and get dehydrated, your urine can end up dark brown in colour. But this can also be affected by medication that you take, vitamins, alcohol, and diet.

Let’s have a deeper look at the urinary system now. You need to know the different parts and what they do, so grab a pen and write this down:

The urinary system is made up of the kidneys, the renal artery, the renal vein, and the bladder.

This is an example of an organ system.

The kidneys produce urine by filtration of the blood, selecting the useful substances that it wants tokeep, and reabsorb.

So blood is transported to the kidney by the renal artery.

Once at the kidney, blood is filtered at high pressure and the kidney reabsorbs anything usefullike glucose, salt ions, and water.

After the blood has been filtered in the kidneys, it returns to the circulatory system by the renalvein.

The kidneys produce urine which is taken from them to the bladder. The bladder storesurine until is expelled from the body (by peeing.)

Urine is made up of water, salts, and urea. Urea is the main waste product in urine, as it's notreabsorbed by the kidneys.

Urea is made by the liver when excess amino acids are broken down (don’t worry, more about that later!)

Let’s focus specifically on the kidneys now and look at the nephron.

You can think of nephrons like big, helpful sieves. They’re responsible for “cleaning” the blood by removing urea, excess water and mineral ions.

There are three stages of how the kidney works to maintain water levels and produce urine:

Stage 1 is filtration. Blood containing waste products arrives at the nephron inside the kidney, where there are networks of capillaries (these are really small blood vessels.)

This means the blood travels under high pressure, which helps with the filtration.

Small molecules including urea, water, ions and glucose are filtered, or forced under high pressure, out of the blood and pass into the nephron.

Large molecules such as proteins are too big to fit through the small capillary walls and remain in the blood.

Stage 2 is selective reabsorption. Now that all the small essential molecules such as water, ions, and glucose have been filtered out of the blood, the kidneys must reabsorb molecules that are needed, whilst letting the molecules that aren’t, to pass out in the urine.

The kidneys selectively reabsorb, back into the bloodstream, only the molecules that the body absolutely needs.

These reabsorbed molecules include glucose, just enough water to maintain the right water levels in the blood, and just enough ions to maintain the right levels of mineral ions in the blood. This filtered blood leaves the kidney via the renal vein.

Stage 3 is the formation of urine. The waste products, the molecules which are not selectively reabsorbed, travel along the nephron as urine. This then passes down to the bladder until it can be expelled.

Finally, we’re going to talk about what urea is and how the body controls nitrogen levels.

As well as blood glucose and water levels, the body also needs to carefully maintain and control thelevels of nitrogen.

When we eat, we digest lots of proteins. Long protein molecules are broken down into amino acids.This results in excess amino acids which need to be excreted safely.

Our bodies can’t store proteins because of the nitrogen molecules in them. The rest of the protein is very useful, but the body can’t deal with nitrogen so we have to get rid of it.

During digestion, excess proteins are broken down into amino acids, and then comes deamination.

This is where amino acids are further broken down to form ammonia.

Ammonia is the substance containing nitrogen and its toxic, so it gets immediately converted into urea, so it can be excreted safely.

So urea is actually a waste product made of nitrogen, resulting from this breakdown of proteins.

In the liver, amino acids are converted into ammonia, but as ammonia cannot be allowed toaccumulate in the body, it is converted into urea.

Urea is released from the liver into the bloodstream and is carried to the kidneys where it can befiltered out, so the urea is passed out of the body in urine when we pee.

I’m Dr Alex Lathbridge and this is Bitesize Biology, listen now on BBC Sounds.

Hello. I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the final episode in our eight-part series on homeostasis.

In this episode we’re going to talk about plant hormones.

You might think plants are boring, I understand. They don’t speak. They generally don’t have strong views on important topics. They’ve never even been on holiday.

But on the inside plants are as complex as you and I, with many needs and drive to survive.

In order to survive, plants need light and water for photosynthesis. There's a whole episode about it if you want to check that out in our series on The Cell.

Of course, you know that plants don’t have the ability to walk on two legs like you and me.

So, in order to give themselves the best chance of survival, plants have developed responses called tropisms which help them grow towards sources of light and water: Plants grow in response to light, which is called phototropism.

Plants also grow in response to gravity, which is called geotropism. (Or gravitropism.) When a plant grows towards a change in the environment, the stimulus, it’s a positive tropism. When a plant grows away from the stimulus, it's a negative tropism.

Ok, there’s some key terms, let’s go over them again quickly:Phototropism – growing in response to light.Geotropism – growing in response to gravity.Positive tropism – growing towards a stimulus.Negative tropism – growing away from a stimulus.

Different parts of plants respond to light in different ways: In the plant shoot, responses to light are known as positive phototropism, because the shoot grows towards the light, so the leaves can absorb as much sunlight as possible.

In the plant root, responses to light are known as negative phototropism, because the root grows away from the light. Roots need to grow downwards into the soil, away from the light, in order to absorb water and minerals.

But plants don’t have muscles or legs or anything – so how do they control where to grow? Well, it's all thanks to a hormone called auxin. Auxins are mostly made in the tips of growing shoots and roots, and these hormones can diffuse to other parts of the shoots or roots. Auxin controls the growth of plants by promoting cell division and causing elongation of the plant cells.

Once again, different parts of plants use auxin in different ways:High concentrations of auxins in shoots promotes more growth, and the cells become longer.High concentrations of auxins in roots inhibits growth, so even though the concentration of auxin is high in the roots, its effect is to inhibit growth.

The key thing to remember here is that while both the shoots and the roots have high concentrations of auxins, they do different things.

This happens because of an unequal distribution of auxin in different parts of the plant.In shoots, the shaded side, away from the sun, contains more auxin and grows longer, which makes the shoot grow towards the light.

But auxins have the opposite effect on roots. In a root, the shaded side contains more auxin and grows less, causing the root to bend away from the light and down into the soil.

Now let’s look at how different parts of the plant respond to changes in gravity, geotropism. When the shoot grows upward, against the force of gravity, this is known as negative geotropism.When the roots grow downwards, so with the direction of gravity, this is known as positive geotropism.

This is also caused by an unequal distribution of auxin in different parts of the plant: In a root placed horizontally, the bottom side of the root contains more auxin (because more of the hormone accumulates in the lower, bottom half) and so it grows less, so the root grows downwards in the direction of the force of gravity. Auxin slows the growth in the root and it curves downwards because it stops growing.

The opposite happens in a shoot. When a shot is placed horizontally, the bottom side of the shoot contains more auxin (more of it accumulates in the bottom half) but grows more, so the shoot grows upwards, against the force of gravity, this is negative geotropism. Auxin stimulates growth in the shoot and it curves upwards.

It might be useful to check out the pages on the Bitesize website, because they have pictures of these processes going on.

I know you probably want me to shut up about auxins but the most important thing to remember is that they help plants grow, stimulating their cells to elongate.

Finally, let's look at how humans use auxins are used to help plants grow. They’re used in agriculture, or farming, and horticulture, as weed killers, rooting powders and for promoting growth in tissues.

Selective weedkillers are useful because they kill some plants, but not all of them. Selective weedkillers contain a growth hormone that causes the weeds to grow too quickly and die.

But selective weedkillers also kill plants that some animal species rely on for food, this can result in a reduction of biodiversity.

Rooting powder works with plant cuttings. These are little cut offs of plants that can be dipped in hormone rooting powder before they are planted. Rooting powder contains growth hormones that makes the roots of plant cuttings develop quickly.

Tissue culture is a technique used to grow whole new plants from small sections of a parent plant. Hormones are used to stimulate cell division and elongation.

I’m Dr Alex Lathbridge and this is bitesize biology.

To listen to the other episodes in this series, and the other bitesize podcasts available, search Bitesize on BBC Sounds.