The nervous system
Sense organs detect stimuli.
A stimulus is a change in your environment which you may need to react to.
You have five different sense organs- eyes, ears, nose, tongue and skin.
All sense organs contain receptors:
receptors are groups of cells which are sensitive to stimulus.
They change stimulus energy into electrical impulses.
A stimulus can be light, sound, touch, pressure or a change in position or temperature.
The five sense organs and the receptors they contain:
-Eyes: light receptors.
-Ears: sound and “balance” receptors.
-Nose: smell receptors- sensitive to chemical stimuli.
-Tongue: taste receptors- sensitive to bitter, salt, sweet and sour.
-Skin: sensitive to touch, pressure and temperature change.
The nervous system
The central Nervous System is where all the information from the sense organs is sent, and where the reflexes and actions are coordinated.
The Central Nervous System consists of the brain and spinal cord only.
Neurones transmit the information as electrical impulses very quickly from the CNS.
Instructions: from the CNS are sent to the effectors which respond accordingly.
The three types of neurone and what they do:
-Sensory Neurones: the nerve cells that carry signals as electrical impulses from the receptors in the sense organs to the CNS.
-Motor Neurones: the nerve cells that carry signals to the effector muscles or glands.
-Effectors: Muscles and glands are known as effectors, they respond in different ways. Muscles contract in response to a nervous impulse whereas glands secrete hormones.
Reflexes are automatic responses to a certain stimuli- they can reduce the chances of being injured.
The passage of information in a reflex, from receptor to effector, is called a reflex arc.
The neurones in reflex arcs go through the spinal cord or an unconscious part of the brain When a stimulus is detected by receptors, an impulse is sent along a sensory neurone to the spinal cord.
In the spinal cord the sensory neurone passes on the message to another type of neurone (a relay neurone).
The relay neurone relays the impulse to a motor neurone. The impulse then travels along the motor neurone to the effector. The effector then reacts accordingly.
Because you don’t have to think about the response it is much quicker The connection between two neurones is called a synapse.
The nerve signal is transferred by chemicals that diffuse across the gap.
These chemicals then set off a new electrical signal in the next neurone.
Hormones are chemicals released directly into the blood.
They are carried in the blood plasma to other parts of the body, but only affect particular cells called target cells in particular places.
Hormones control things in organs and cells that need constant adjustment.
Hormones are produced in various glands.
Hormones have relatively long lasting effects.
Glands which release hormones and the hormones they release:
-The pituitary gland: This produces many important hormones including LH, FSH ADH which controls water content.
-Pancreas: Produces insulin for the control of blood sugar.
-Ovaries (females only): Produce oestrogen, which controls the menstrual cycle and promotes all female secondary sexual characteristics during puberty.
-Testes (males only): Produce testosterone, which promotes all male secondary characteristics during puberty.
The menstrual cycle
The menstrual cycle has four stages:
-Stage 1: Starts at day 1 when the bleeding starts. The uterus lining breaks down for about for days.
-Stage 2: The lining of the womb builds up again. From day 4 to day 14, into a thick spongy layer full of blood vessels, ready to receive a fertilised egg.
-Stage 3: An egg is developed and then released from the ovary at day 14.
-Stage 4: The wall is then maintained for about 14 days, until day 28. If no fertilised egg has landed on the uterus wall by day 28, the spongy lining starts to break down again and the whole cycle starts again.
here are three main hormones involved:
-FSH (Follicle-Stimulating Hormone): Produced by the pituitary gland. Causes an egg to mature in one of the ovaries. Stimulates the ovaries to produce oestrogen.
-Oestrogen: Produced in the ovaries. Causes pituitary gland to produce LH. Inhibits the further release of FSH.
-LH (Luteinising Hormone): Produced by the pituitary gland. Stimulates the release of an egg at around the middle of the menstrual cycle.
The hormone oestrogen can be used to prevent the release of an egg.
So oestrogen can be used as a method of contraception.
The pill is an oral contraceptive that contains oestrogen.
Advantages of the pill:
-The pill is over 99% effective at preventing pregnancy.
-It reduces the risk of getting some types of cancer.
Disadvantages of the pill:
-It isn’t 100% effective.
-It can cause side affects like headaches, nausea, irregular menstrual bleeding and fluid retention.
-It doesn't protect against STIs.
IVF can also help couples have children. Hormones are given before the egg collection to stimulate egg production so that more than one egg can be collected.
The hormones in IVF can have adverse effects on some women. They can suffer abdominal pain, vomiting and dehydration.
Homeostasis means all the function of your body that try to maintain a constant internal environment.
All your body cells are bathed in tissue fluid which is blood plasma that has leaked out of the capillaries (on purpose).
To keep all of your cells working properly this fluid must be ‘just right’.
Bodily levels that need to be controlled include:
-Ion content which is regulated by the kidneys. Ions are taken into the body through food then absorbed into the blood. Excess ions need to be removed, some are lost in sweat and some are removed in your urine.
-Water content. Water is taken into the body through food and drink. The body needs to constantly balance the water coming in against the water going out, it does this through sweat, breathing and urine.
-Sugar content, eating foods containing carbohydrate puts glucose into the blood. Normal metabolism of cells removes glucose from the blood but vigorous exercise will remove a lot more glucose. The hormone inulin controls sugar levels in the body. Diabetes (type 1) is where the body doesn't produce enough insulin.
-Body temperature which is controlled by the brain. All enzymes work best at a certain temperature. The ones in the human body work best at about 37 degrees centigrade and so this is the temperature your body tries to maintain.
Diet and exercise
To be in good health your diet must provide all the energy you need and no more.
All the different food groups have different uses in the body so you need to have the right amount of food from each group as well.
-Enough carbohydrates and fats to keep warm and provide energy.
-Enough protein for growth, cell repair and cell replacement.
-Enough fibre to keep everything moving smoothly through your digestive system.
-Tiny amounts of various vitamins and minerals to keep your skin, bones, blood and everything else happy.
The amount of energy needed varies from person to person. You need energy to fuel the chemical reactions in the body that keep you alive. These reactions are called your metabolism and the speed at which they occur is called your metabolic rate.
Health problems due to diet are different in different parts of the world.
In developed countries obesity is a serious problem because of things like fast food which are full of bad fat, sugar and salt.
Hormonal problems can lead to obesity although the usual cause is overeating and lack of exercise.
Health problems that obesity can cause include arthritis, high blood pressure and heart disease.
In developing countries the problem is more underweight people.
This can be cause by a lack of one or more specific types of food called malnutrition or not enough food called starvation.
The worst sufferers are usually women, children and the elderly.
The effects of malnutrition vary depending on what foods are missing but include slow growth, fatigue, poor resistance to infections and irregular periods in women.
Cholesterol and salt
<span>Cholesterol is a fatty substance that is essential for good health.</span>
It is found in every cell in the body but <span>too much of it is very bad for you.</span>
<span>High cholesterol levels in the blood causes an increased risks of problems like coronary heart disease.</span>
This is due to blood vessels getting <span>clogged with fatty cholesterol deposits. This reduces blood flow which can lead to chest pain (angina) or is the vessels are blocked completely a heart attack.</span>
The <span>liver is vital in controlling the amount of cholesterol in the body.</span>
It makes new cholesterol and removes some from the blood so that it can be removed from the body.
Cholesterol is transported around the body by <span>lipoproteins. These can be high density lipoproteins or low density lipoproteins.</span>
Another risk factor of heart disease is <span>high blood pressure.</span> Eating too much salt can cause <span>hypertension.</span>
This is is a problem for about 30% of the UK population who need to carefully monitor how much salt they eat.