Topic 5 - Homeostasis and Response

Homeostasis means to maintain a stable internal environment.
The conditions inside the body need to be kept steady, even when the external environment changes.
This is because cells need the right conditions in order to function properly, including the right conditions for enzyme action.
Homeostasis involves the regulation of the conditions inside your body (and cells) to maintain a stable internal environment, i’m response to changes in both internal and external conditions.
There are lots of automatic control systems in the body that regulate the internal environment, including both nervous and hormonal communication systems.
For example, there are control systems that maintain the body’s temperature, your blood glucose and water content.
All the automatic control systems are made up of three main components which work together to maintain a steady condition:

• Cells called receptors
• Coordination centres (including the brain, spinal chord and pancreas)
• Effectors.

The nervous system means that humans can react to their surroundings and coordinate their behaviour.
Organisms need to respond to stimuli (changes in the environment) in order to survive.
An single-celled organism can just respond to its environment, but the cells of multicellular organisms need to communicate with each other first.
So multicellular organisms have developed nervous and hormonal communication systems.
Different parts of the nervous system:

• Central nervous system (CNS) - in vertebrates (animals with backbones) this consists of the brain and spinal cord only. In mammals, the CNS is connected to the body by sensory neurones and motor neurones
• Sensory neurones - the neurones that carry information as electrical impulses from the receptors to the CNS
• Motor neurones - the neurones that carry electrical impulses from the CNS to effectors
• Effectors - all your muscles and glands, which respond to nervous impulses.

The CNS is a coordination centre; it receives information from the receptors and then coordinates a response which is carried out by effectors.
Example - A bird is eating some seed and spots a cat approaching:
Stimulus (the bird’s eye sees the cat)
⬇️
Receptors (the receptors in the eye are stimulated)
⬇️
Sensory neurones (carry the information from the receptors to the CNS)
⬇️
CNS (decides what to do about it)
⬇️
Motor neurones (the CNS sends information to the wings across motor neurones)
⬇️
Effector (the effector being the muscle contracts and the bird flies to safety).
The connection between two neurones is called a synapse.
The nerve signal is a chemical impulse which diffuses across the gap.
These chemicals then set off a new electrical signal in the next neurone.

Neurones transmit information very quickly to and from the brain, and your brain quickly decides how to respond to a stimulus; but reflexes are even quicker.
Reflexes are rapid, automatic responses to certain stimuli that don’t involve the conscious part of the brain and can reduce the chance of being injured.
For example: if someone shines a bright light in your eye, your pupils automatically get smaller so that less light gets into the eye and this stops it getting damaged. Similarly, if you get a shock your body releases the hormones adrenaline automatically and doesn’t wait for you to decide that you’re shocked.
For example - You receive a bee sting:
Stimuli (the finger the bee has stung)
⬇️
Receptors (pain receptors are stimulated)
⬇️
Sensory neurone (impulses travel along)
⬇️
Relay neurone (impulses are passed along, via a synapse)
⬇️
Motor neurone (impulses travel along, via synapse)
⬇️
Effector (impulses reach muscle and it contracts).

Reaction time is the time it takes to respond to a stimulus (often less than second), but it can be affected by factors such as age, gender or drugs.
This practical tests the affects of caffeine on a person’s reaction time.
Method:
1 - The person being tested should sit with their arm resting on the edge of the table (to stop them moving their arm up or down during the test)
2 - You should then hold a ruler vertically between their thumb and forefinger and make sure that the zero end is level with their thumb and finger. Then let go without any warning
3 - The person being tested should catch the ruler as quickly as they can; as soon as they see it fall
4 - Reaction time is measured by the number on the ruler when it’s caught and the further down it’s caught (e.g. the higher the number), the slower the reaction time
5 - Repeat the test several times and then calculate the mean distance that the ruler fell
6 - The person being tested should then have a caffeinated drink and after 10 minutes should repeat the experiment.
Reaction time can also be tested and measured on a computer.
This gives more accurate recordings in milliseconds and also removes the possibility of a person predicting when to respond due to the tester’s body language.

The brain is part of the central nervous system.
It’s made up of billions of interconnected neurones and is in charge of our complex behaviours.
Different regions of the brain include:

• Cerebral cortex - Outer part; responsible for things like consciousness, intelligence, memory and language
• Medulla - Down by the spinal cord; controls unconscious activities like breathing or heartbeat
• Hypothalamus - In the middle, monitors temp and water levels inside the body
• Cerebellum - Bottom circular part; responsible for muscle coordination.
  Studying a patient with brain damage tell neuroscientists a lot about which part of the brain is responsible for what function.
  Brain damage can be monitored by an EEG, MRI or by electrically stimulating the brain.
  An MRI is a magnetic resonance image and produces very detailed images of the brain’s structure which scientists can then use to find out which areas of the brain are active when people are carrying out different tasks.
  Electrically stimulating the brain involves pushing a tiny electrode into the tissue and giving it a small zap of electricity and then stimulating different parts of the brain can give an idea of what each part does.

Parts of the eye include:

• The sclera - located right at the bottom on the outer rim, this is the tough, supporting wall of the eye
• The cornea -located at the very front of the eye, this is the transparent outer layer that refracts light into they eye
• The iris - located inside the front part of the eye, these long parts contains muscles which allow it to control the diameter of the pupil and therefore how much light enters the eye
• The lens - the oval shape at the front of the eye, this focuses the light onto the retina (the inter layer at the back of the eye) which contains receptor cells called cones and rods sensitive to light intensity and colour
• The shape of the lens is controlled by the ciliary muscles (the small sections behind the iris) and suspensory ligaments (the parts either side of the lens)
• The optic nerve - located at the end of the eye, coming off of it, this carries impulses from the receptors on the retina to the brain.

Very bright light can damage the retina so there is a reflex to protect it.
When light receptors in the eye detect very bright light:

• Pupils get SMALLER
• Circular muscles CONTRACT
• Radial muscles RELAX
  However, when light receptors in the eye detect very dim light:
• Pupils get WIDER
• Circular muscles RELAX
• Radial muscles CONTRACT.

The eye focuses light on the retina by changing the shape of the lens which is also known as accommodation.
When looking at near objects:

• Ciliary muscles CONTRACT
• Suspensory ligaments RELAX
• Lens FATTENS and CURVES
  All of this increases the amount by which it refracts light.
  Older people often use reading glasses because their lenses lose flexibility and so they can’t easily spring back to create a round shape.
  When looking at distant objects:
• Ciliary musicales RELAX
• Suspensory ligaments CONTRACT
• Lens THINS and CURVES LESS.
  All of this allows light to refract by a smaller amount.

If the lens cannot refract the light by the right amount (so that it focuses on the retina), the person will be short-sighted or long-sighted.
Long-sighted people are unable to focus on near objects:

• This occurs when the lens is the wrong shape and doesn’t reflect the light enough or the eyeball is too short
• The images of near objects are brought into focus behind the retina
• You can use glasses with convex lenses (a lens that curves outwards) to correct it
• The lens refracts the light rays so they focus on the retina
• The medical term for long-sightedness is hyperopia.
  Short-sighted people are unable to focus on distant objects:
• This occurs when the lens is the wrong shape and refracts the light too much or the eyeball is too long
• The images of distant objects are brought into focus in front of the retina
• You can use glasses with concave lenses (a lens that curves inwards) to correct it
• The lens refracts light rays less so they focus on the retina
• The medical term for short-sightedness is myopia.

Contact Lenses:

• Thin lenses that sit on the surface of the eye that are shaped to compensate for the fault of focusing
• They are light weight and almost strong and are more convenient than glasses for sports, for example
• The two main types of contact lenses are hard lenses and soft lenses, with soft lenses being more comfortable but carrying a greater risk of eye infections than hard lenses.
  Laser Eye Surgery:
• A laser can be used to vaporise tissue, changing the shape of the cornea (and in turn how strongly it refracts light into they eye)
• Slimming it down makes it less powerful and can improve short sight, whilst changing the shape so that it’s more powerful will improve long sight
• The surgeon can precisely control how much tissue the laser takes off, completely correcting the vision
• However, there are risks of complications such as infection or the eye reacting in a way that makes your vision worse than before.
  Replacement Lens Surgery:
• Sometimes long-sightedness may be treated by replacing the lens of the eye
• In replacement lens surgery, the natural lens of the eye is removed and replaced with an artificial lens, made of clear plastic
• Replacing a lens can cause damage to the retina (could lead to loss of sight).

The body’s temperature has to stay at about 37 degrees celsius inside as this is the optimum temperature for enzymes in the body.
Core temperature is the temperature inside the body where the internal organs are.
The body has to balance the amount of energy gained (through respiration) to the amount lost in order to keep the core body temperature constant.
The thermoregulatory centre inside the hypothalamus in the brain contains receptors that are sensitive to the temperature of the blood flowing through the brain.
The thermoregulatory centre also receives impulses from temperature receptors in the skin to give information about skin temperature.
From there it can decide on actions or take to either cool down or warm up.
Negative feedback loop for body temperature:
Temperature receptors detect that the core body temperature is too high/low
⬇️
The thermoregulatory centre acts as a coordination centre and receives information from the temperature receptors and triggers the effectors automatically
⬇️
Effectors (e.g. dilation or construction of arterioles) produce a response and counteract the change
⬇️
Body warms up/cools down and corrective mechanisms are switched off

Antagonistic effectors work antagonistically, with one effector heating and the other cooling, to achieve a very precise temperature and allowing for a more sensitive response.
Responses produced by effectors when too hot:

• Hairs lie flat (hair erector muscle relaxes) - When standing, hairs trap a layer of air which insulates the body so this reduces the insulating layer and increases heat loss.
• Sweating (sweat gland produces sweat) - Sweat evaporates from the skin which transfers heat energy to the environment and removes it from the skin.
• Vasodilation (more blood supplied to surface of the skin) - The blood vessels supplying the skin (arterioles) dilate so more blood can flow close to the surface of the skin. This helps transfer energy from the skin to the environment by radiation.

Responses produced by effectors when too cold:

• Hair stand up (hairs erect and hair erector muscle contracts) - This traps an insulating layer of air to heat up the body.
• No sweat is produced (sweat glands don’t produce sweat) - So less heat energy is lost from the skin to the environment through evaporation.
• Vasoconstriction (blood supply shut off) - Arterioles supplying skin capillaries constrict to close off the skin’s blood supply. Less blood can flow to the skin and so less heat energy is lost through radiation.
• Shivering (a series of rapid muscle contractions) - Muscles require fast respiration to automatically contract quickly and radiation is an exothermic reaction and releases heat energy which transfers warmth to the body.

This is another way to send information around the body (that is not along nerves) by using hormones.
Hormones are chemical messengers sent in the blood.
The chemical molecules are released directly into the blood and are then carried to other parts of the body, but only affect particular cells in particular organs (called target organs).
Hormones control things in organs and cells in organs and cells that need constant adjustment.
Hormones are produced and secreted by various glands called endocrine glands, which make up the endocrine system.
Hormones tend to have relatively long-lasting effects.

Endocrine glands locations and uses:

• Pituitary Gland - Found at the base of the brain, attached to the hypothalamus. It produces many hormones that regulate body temperature. It is sometimes called the ‘master gland’ because these hormones act on other glands, directing them to release hormones that bring about change.
• Thyroid - Located at the bottom of the throat. Produces thyroxine which is involved in regulating things like the rate of metabolism, heart rate and temperature.
• Adrenal gland - Attached to the top of the kidneys (two of them). Produces adrenaline, which is used to prepare the body for a ‘fight or flight’ response.
• Pancreas - Located around the kidneys. Produces insulin, which is used to regulate the blood glucose level.
• Ovaries - Located in the hips (two of them). Produce oestrogen, which is involved in the menstrual cycle.
• Testes - Located either side of the penis (two of them). Produce testosterone, which controls puberty and sperm production in males.

Nerves:

• Very fast action
• Act for a very short time
• Act on a very precise area.

Hormones:

• Slower action
• Act for a long time
• Act in a more general way.

To tell if a response is nervous or hormonal, think about the speed of the reaction and how long it lasts.

An example of a nervous reaction is when information needs to be passed to effectors really quickly (e.g. a pain signal) so hormones would be too slow.

An example of a hormonal reaction would be if you get a shock and adrenaline is released. You can tell it’s hormonal because it lasts for a long time and you still feel a bit wobbly afterwards.

Blood glucose levels are controlled as part of homeostasis, as well as body temperature.
Insulin and glucagon are hormones that control how much glucose there is in the blood.
Eating foods that contain carbohydrates puts glucose into the blood from the gut.
The normal metabolism of cells removes glucose from the gut.
Vigorous exercise removes much more glucose from the blood.
The level of glucose in the blood must be kept steady and changes are monitored and controlled by the pancreas, using the hormones insulin and glucagon in a negative feedback cycle.

If blood glucose level is too high, insulin is added:
Blood with too much glucose
⬇️
Insulin secreted by pancreas
⬇️
Insulin removes glucose from blood
⬇️
Glucose moves from blood into liver and muscle cells
⬇️
Insulin makes liver turn glucose into glycogen
⬇️
Blood glucose is reduced

If blood glucose level is too low, glucagon is added:
Blood with too little glucose
⬇️
Glucagon secreted by pancreas
⬇️
Glucagon adds glucose to the blood
⬇️
Glucose released into blood by the liver
⬇️
Glucagon makes liver turn glycogen into glucose
⬇️
Blood glucose levels increase

Diabetes is an example of homeostasis ‘going wrong’.
It is a condition that affects a person’s ability to control their blood sugar levels.
Type 1 Diabetes:

• The pancreas produces little or no insulin
• The person’s blood glucose level can rise to a level that can kill them
• People with Type 1 Diabetes need insulin therapy with involves several injections of insulin throughout the day, mostly at mealtimes. This makes sure that glucose is removed from the blood quickly once the food has been digested, to stop the level getting too high. It is a very effective treatment.
• The amount of insulin that needs to be injected depends on the person’s diet and how active they are
• As well as insulin therapy, people with Type 1 Diabetes are encouraged to limit their intake of food rich in simple carbohydrates (e.g. sugars because they cause blood glucose levels to rise rapidly) and to take regular exercise (because this helps to remove excess glucose from the blood).

Type 2 Diabetes is where a person becomes resistant to their own insulin:

• They still produce insulin, but their body’s cells don’t respond properly to the hormone
• This can cause a person’s blood sugar level to rise to a dangerous level
• Being overweight can increase your chance of developing Type 2 Diabetes, as obesity is a major risk factor in the development of the disease
• Type 2 Diabetes can be controlled by eating a carbohydrate-controlled diet and getting regular exercise.

Kidneys are very important when it comes to homeostasis.
The kidneys make urine by taking waste and unwanted products and substances out of the blood.
The substances are filtered out of the blood as it passes through the kidneys (filtration).
Useful substances like glucose, some ions and the right amount of water are then absorbed back into the blood (selective reabsorption).
Substances removed from the body in urine include:
Urea -

• Proteins (and the amino acids they’re broken down into) can’t be stored by the body so any excess amino acids are converted into fats and carbohydrates, which can be stored. This occurs in the liver and involves a process called deamination
• Ammonia is produced as a waste product in this process
• Ammonia is toxic and so it’s converted to urea in the liver. Urea is them transported to the kidneys, where it is filtered out of the blood and excreted from the body in urine
• A small, unregulated amount of urea is also lost from the skin in sweat.

Ions -

• Ions such as sodium are taken into the body in food. and then absorbed into the blood
• If the ion (or water) content of the body is wrong, it could upset the balance between ions and water, meaning too much or too little water is drawn into the cells by osmosis, which can damage cells and mean they don’t work as well as normal
• Some ions are lost in sweat (which is why it tastes salty). This amount is not regulated, so the right balance of ions in the body must be maintained by the kidneys. The right amount of ions is reabsorbed into the blood after filtration and the rest is removed from the body in urine.

Water -

• The body has to constantly balance the water coming in against the water going out
• Water is lost from the skin in sweat and from the lungs when breathing out
• We can’t control how much we lose in these ways. so the amount of water is balanced by the amount we consume and the amount removed by the kidneys in urine.

The kidneys’ work is controlled by a hormone, and so is the concentration of urine.
The hormone that controls the concentration of urine is called anti-diuretic hormone (ADH).
This hormone is released into the bloodstream by the pituitary gland.
The brain monitors the water content of the blood and instructs the pituitary gland to release ADH into the blood according to how much is needed.
The process of water content regulation is controlled by negative feedback.
This means if the water levels get too high or low a mechanism will be triggered to bring it back to normal.

If the water level is too low:
A receptor in the brain detects that the water content is too low
⬇️
The coordination centre in the brain receives the information and coordinates a response
⬇️
The pituitary gland releases more ADH, so more water is reabsorbed from the kidney tubules
⬇️
Water content increases

If the water level is too high:
A receptor in the brain detects that the water content is too high
⬇️
The coordination centre in the brain receives the information and coordinates a response
⬇️
The pituitary gland releases less ADH. so less water is reabsorbed into the kidney tubes
⬇️
Water content decreases

If someone’s kidneys stop working, there are two treatments: regular dialysis or a transplant.
Dialysis:

• Filters the blood and has to be done regularly to keep they concentration of dissolved substances in the blood at normal levels, and to remove waste substances
• In a dialysis machine the person’s blood flows between partially permeable membranes, surrounded in dialysis fluid. The membranes are permeable to things like ions and waste substances, but not big molecules like proteins (just like the membranes in the kidney)
• The dialysis fluid has the same concentration of dissolved ions and glucose as healthy blood
• This means that useful dissolved ions and glucose won’t be lost from the blood during dialysis
• Only waste substances (such as urea) and excess ions and water diffuse across the barrier
• Many patients with kidney failure have a dialysis session three times a week, with each session taking 3-4 hours
• Dialysis can cause blood clots and infections and is very expensive for the NHS to run
• However, it buys a patient valuable time until a donor organ is found.

A kidney transplant is the only known cure for kidney failure.
Healthy kidneys are usually transplanted from people who have died suddenly.
One kidney can also be transplanted from people who are still alive but it carries a small risk to the donor.
However, there is a risk that the donor kidney can be rejected by the patient’s immune system and the patient is treated with drugs to attempt to prevent this.
Donor kidneys are also ideally matched to the patient’s blood type to prevent this, too.
Transplants are cheaper in the long run than dialysis and can put and end to the hours spent on dialysis.
However there is a very long transplant waiting list.

Plant hormones make sure plants grow in the right direction.
Auxin is a plant hormone that controls growth near the tips of shoots and roots.
It controls growth in response to light (phototropism) and gravity (geotropism).
Auxin is produced in the tips and moves backwards to stimulate the cell elongation (enlargement) process which occurs in the cells just behind the tips.
If the tip of a shoot is removed, no auxin is available and the shoot may stop growing.
Extra auxin promotes growth in the shoot but inhibits growth in the root, producing the desired result.
SHOOTS GROW TOWARDS LIGHT:

• When a shoot tip is exposed to light, move auxin accumulates on the side that’s in the shade than the side that’s in the light
• This makes the cells grow (elongate) faster on the shaded side so the shoot bends towards the light.
  SHOOTS GROW AWAY FROM GRAVITY:
• When a shoot is growing sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side
• This cause the lower side to grow faster, bending the shoot upwards.

ROOTS GROW TOWARDS GRAVITY:

• A root growing sideways will also have more auxin on its lower side
• However, in a root extra auxin inhibits growth and so the cells on top elongate faster, and the root bends downwards.
  By responding to stimuli in their environment, plants increases their chances of survival (e.g. in this and photosynthesis).
  By growing towards the light, plants increase the amount of light they receive for photosynthesis.
  You can do a required practical using 10 cress seeds in three different Petri dishes, each with moist filter paper and shining a light on each but in different directions, to see how the seedlings grow towards light.
  You can do the same with gravity, placing four seedlings on damp cotton wool in a Petri dish, each with their roots facing in a different directions, to see that the roots all grow downwards.
  The control variables include - Number of seeds, type of seed, temperature, water and light intensity.
  To record this experiment it is good to use labelled, scientific diagrams.