Homeostasis and Response



Homeostasis means to maintain a stable internal environment, these include loads of automatic control systems. 3 internal conditions that need to be controlled are:
1. Body temperature
2. The water content in body or salt regulation
3. Blood glucose concentration.
The control system has 3 features:
1. Receptors - cells that detect internal and external environment.
2. Co-ordination - area that receives and process the information from the receptors.
3. Effectors - muscles or glands that bring about responses to the stimulus that has been received.
Enzymes are sensitive to their environment so that they can still respond to the change to ensure it can still work. The internal conditions which are most important are body temperature and pH.

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Temperature regulation

Keeping warm
If core body temperature starts to fall, impulses sent from the thermoregulatory centre prevent you from cooling down. This reduces the energy transfer to the environment. The blood vessels that supply your skin capillaries close up to reduce the flow of blood. This reduces the energy transfer via radiation through the skin. Sweat production is reduced, less water from the sweat evaporates so less energy is transferred. Skeletal muscles contract causing you to shiver. Energy from this raises your body temperature until the shivering stops.

Cooling the body
More energy is transferred to cool down. Blood vessels for your skin capillaries open wider so more energy is transferred by radiation to your surroundings. You produce more sweat to cool the body down.

Basic temperature control
The thermoregulatory centre in the hypothalamus contains receptors that are sensitive to temperature change. These receptors can sense a 0.5C change

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The Nervous System

The Nervous System detects and reacts to stimuli. It is made up of the following:
1. The Central Nervous System - Consists of the brain and spinal cord only. It is connected to the body by sensory neurones and motor neurones.
2. Sensory Neurones - These carry info as electrical impulses from the receptors to the CNS
3. Motor Neurones - These carry electrical impulses from the CNS to the effectors
4. Effectors - All your muscles and glands, which respond to nervous impulses

Receptors detect stimuli.
Effectors respond to nerve impulses and bring about a change - muscles contract, glands secrete hormones.

CNS is a coordination centre - it receives info from receptors and the coordinates a response which is then carried out by the effectors.

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Conscious response - The Nervous System

Ned to remember this order.

1. Stimulus (Dropping a ruler)
2. Receptor (eye)
3. Sensory neuron
4. Brain
5. Spinal cord
6. Motor neuron
7. Effector (hand)
8. Response (hand closing to catch)

The brain, spinal cord and motor neurone will always stay the same, the rest can change depending on what happens.

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Reflex Arc (list)

Need to remember this:
1. Stimulus
2. Skin receptor
3. Sensory neurone
4. Spinal cord
5. Motor neurone
6. Effector

This is an unconscious response as it is automatic (don't have to think about it). They need to be fast to help survive anything dangerous.

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The connection between two neurones is called a synapse. The nerve signal cannot jump the gap so is transferred by chemicals that diffuse across the gap. These chemicals are called Neurotransmitters. These chemicals then set off a new electrical signal in the next neurone.

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Required Practical - Investigating reaction time

Reaction time is the time it takes to respond to a stimulus - it is often less than a second.
Caffeine is a drug that can speed a person's reaction time. Its effect can be measured in the following test:
Dropping a ruler.
Have the person rest their arm on the table.
Hold the ruler vertically between their thumb and forefinger.
The zero line must be lined up with their thumb and finger.
Then let go without warning.
The person being tested should try to catch the ruler as quickly as they can.
Repeat the test so you can get a mean reaction time.
The person should then drink a caffeinated drink, after 10 mins, repeat the experiment.

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The Brain

Regions of the brain and their functions I need to know:
1. Cerebral cortex - responsible for consciousness, intelligence, memory and language
2. Cerebellum - responsible for muscle coordination
3. Medulla - responsible for unconscious activities e.g. breathing and heartbeat

Ways of studying the brain:
Patients with brain damage - if a small area is damaged then its effect on the patient can tell you a lot about the damaged part.
Electrically stimulating the brain - by observing what stimulating different parts of the brain does, it's possible to get an idea of what those parts do.
MRI scans - Produces a very detailed picture of brains structure. Scientists use this to find out what areas of the brain are active when people are doing certain things.
Knowledge of how the brain works have led to the development of treatments for disorders of the nervous system
Con: The brain is incredibly complex and delicate - the investigation of brain function and any treatment of brain damage or disease is difficult.

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The Endocrine System

Hormones are produced in various glands called endocrine glands, these then make up the endocrine system. Hormones tend to have long-lasting effects.

Some Examples that you need to know:
1. The pituitary gland - known as the 'master gland', directs other glands to release hormones.
2. Thyroid - produces thyroxin which regulates things like the rate of metabolism, heart rate, and temperature.
3. Adrenal gland - produces adrenaline, preparing for fight or flight
4. The Pancreas - produces insulin, regulate blood glucose level
5. Ovaries - produces oestrogen, involved in the menstrual cycle
6. Testes - produce testosterone, which controls puberty and sperm production.

Nerves are very fast, act in a very precise area but act for a very short time.
Hormones are a slower action, act in a more general way but act for a long time.

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Controlling Blood Glucose

Insulin and glucagon control blood glucose levels.
Normal metabolism of cells removes glucose from the blood, Vigorous exercise removes much more glucose and excess glucose is stored as glycogen in the liver and muscles.

If the Blood Glucose level is too high, insulin is secreted from the pancreas
If the Blood Glucose level is too low, glucagon is secreted from the pancreas.

With diabetes, you can't control your blood sugar level:
Type 1 is where the pancreas produces little or no insulin. Can be treated with several injections throughout the day at mealtimes. However, it limits the amount of food rich in simple carbohydrates they can eat.
Type 2 is where a person becomes resistant to their own insulin. This can cause the person's blood sugar level to rise to a dangerous level. Can be controlled by getting regular exercise and eating a carbohydrate-controlled diet.

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In puberty, the body starts to release sex hormones that trigger off secondary sexual characteristics (facial hair in men, breasts in women) and causes eggs to mature in women.
In men, the main reproductive hormone is testosterone. It is produced in the testes and stimulates sperm production.
In women, the main reproductive hormone is oestrogen. It is produced by the ovaries. As well as bringing about physical changes, oestrogen is also involved in the menstrual cycle.

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The menstrual cycle

The menstrual cycle has 4 stages:
Stage 1: day 1 - menstruation starts. The uterus lining breaks down for about four days.
Stage 2: The uterus lining builds up again, from days 4 to 14, into a thick spongy layer full of blood vessels, ready to receive a fertilized egg.
Stage 3: An egg develops and is released from the ovary at day 14 - this is called ovulation
Stage 4: The wall is then maintained for about 14 days until day 28. If no fertilized egg has landed on the uterus wall by day 28, the spongy lining starts to break down and the whole cycle starts again.

This process is controlled by 4 hormones:
1: FSH - produced in the pituitary gland, causes the egg to mature, stimulates the ovaries to produce oestrogen
2: Oestrogen - produced in the ovaries, causes the lining in the uterus to grow, stimulates the release of LH but inhibits release of FSH
3: LH - produced by the pituitary gland, stimulates the release of an egg at day 14
4. Progesterone - produced in the ovaries by the remains of the follicle after ovulation, maintains the lining of the uterus when levels fall the lining breaks down, inhibits the release of LH and FSH

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Controlling Fertility - Reducing Fertility with Ho

Oestrogen - Can be used to prevent the release of an egg. If used every day to keep the level permanently high, it inhibits production of FSH and after a while egg development and production stop and stays stopped.
Progesterone - reduces fertility e.g. by stimulating the production of a thick mucus which prevents any sperm getting through.
The pill - contains both oestrogen and progesterone. It is 99% effective but does have some side effects. Doesn't protect from STI's
Also a progesterone-only pill - fewer side effects but just as effective
Other methods:
1. Contraceptive patch - releases oestrogen and progesterone. each patch lasts a week
2. Contraceptive implant - releases a continuous amount of progesterone, an implant can last 3 years
3. Contraceptive injection - contains progesterone, each dose lasts 2-3 months
4. Intrauterine device (IUD)- a T-shaped device which is inserted into the uterus and kills any sperm and prevents implantation of a fertilised egg. Two main types:

  • Plastic IUD - releases progesterone
  • Copper IUD - prevents sperm surviving in the uterus
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Controlling Fertility - Barrier methods

Non-hormonal forms are designed to stop the sperm from getting to the egg.
Condoms - worn over the penis during intercourse to prevent sperm from entering the vagina. Femidoms work the same way but are just inserted into the vagina. The only form of contraception that protects from STI's
Diaphragm - a shallow plastic cup that fits over the cervix. Has to be used with spermicide.
Spermicide - can be used alone or with a diaphragm but is not as effective
More effective ways:
Sterilisation - involves cutting or tying the fallopian tubes or the sperm duct. A permanent procedure, however, there is a very small chance that the tubes can rejoin.
'Natural' methods - Pregnancy may be avoided by tracking the menstrual cycle and finding out when the woman will be most fertile and avoiding sexual intercourse on those days. Not very effective.
Abstinence - only way to be completely sure that the sperm and egg don't meet.

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Controlling fertility - Increasing fertility

Some women have FSH levels that are too low so they can't get pregnant. Can be given in a fertility drug
Pro: Helps women get pregnant
Con: Doesn't always work

'in vitro fertilisation' Involves collecting eggs from the woman's ovaries and fertilising them in a lab with the man's sperm. Fertilised eggs are grown into embryos in the lab. FSH and LH are given before egg collection to stimulate several eggs growing.
Pro: Can give an infertile couple a child,
Cons: Multiple births can happen if all the eggs implant.
The success rate is low.
Some women have a strong reaction to the hormones.

Some people are against IVF. Unused embryos are often destroyed. Genetic testing before could lead to the selection of preferred characteristics.

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