Module 8

The maintenance of a relatively constant internal state, regardless of the changes in the external environment. This is vital for the optimal functioning of enzymes within an organism, allowing for increased metabolic efficiency.

Enzymes are proteins that catalyse the many reactions that occur within cells constantly.

1. Detecting a change: receptors (sensory cells)
2. Counteracting the change:

Refers to how receptors (sensory cells) detect a chance in one variable in the internal environment (e.g. the temperature or the pH). This change is called a stimulus.

Refers to a response that will reverse (counteract) the change. The change is brought about by an effector (muscles or glands) and will restore the body to the normal range.

The control centre is responsible receiving information from the receptors, detecting a change, either too far above or below the set point. It then determines the appropriate response and sends a message to the effectors which then carry out activities to return the levels to the set point.

Once the correct levels are achieved, the information is detected by the receptors and fed back to the control centre which then directs the effectors to stop the action.

A negative feedback loop is a normal biological response in which the effects of a reaction slow or cease when an optimal, set point is reached, to maintain homeostasis.

- Changes in temperature (the stimulus) are detected by thermoreceptors in the skin and in the hypothalamus (control centre)
- If body temperature is too high, messages are sent to the front of the hypothalamus to the heat-loss centre, then messages are sent via nerves to effectors that begin to cool the body down.
- If the body temperature is too low, messages are sent to the back of the hypothalamus to the heat-gain centre, then messages are sent via nerves to initiate processes to conserve heat and warm the body
- Once the set point of 37 degrees is reached, signals are sent back to the hypothalamus to stop its processes.

Vasodilation - the blood vessels dilate (expand), bringing blood closer to the surface and allowing heat to be lost by radiation
Sweating - sweat glands activated to release sweat, which removes heat from the body when it evaporates
Reduction in amount of thyroxin produced - the thyroid gland releases less thyroxin which lowers metabolism and so less heat is generated

Vasoconstriction - narrowing (constriction) of the blood vessels, moving blood away from the surface to reduce heat loss by radiation
Piloerection - contraction of the hair erector cells which causes hair to stand on end, trapping a layer of warm air and reducing heat loss
Release of TSH - the pituitary gland releases thyroid stimulating hormone to increase the amount of thyroxin released by the thyroid and increasing metabolism, generating more heat
Shivering - rapid contraction of muscles, generating more heat in the body

Blood glucose has a set value of 90mg/100ml
- If blood glucose levels are too high (hyperglycaemia):
- it will be detected by insulin-secreting (beta) cells of the pancreas (receptors)
- These cells release insulin into the bloodstream
- If blood glucose levels are too low (hypoglycaemia):
- It will be detected by glucagon-releasing (alpha) cells of the pancreas
- These cells release glucagon into the bloodstream

Insulin is released if blood glucose is too high and causes the body cells to take up more glucose. Excess glucose is taken up by the liver and stored as glycogen.

Blood glucose levels then decline to the set point and the message is sent to stop releasing insulin.

Glucago is released if blood glucose levels are too low and instructs the liver to break down the stored glycogen, releasing glucose into the bloodstream

Blood glucose levels increase to the set point and the message is sent to stop releasing glucagon

An endotherm is an organism that can maintain its body temperature within a narrow range via metabolic activity, despite variation in the ambient temperature

An adaptation is a characteristic an organism has that increases its chance of survival (and therefore chance of reproducing and passing the characteristic on to its offspring) in the environment.
Three types of adaptations include:
- Structural - physical characteristics of an organism
- Behavioural - actions that the organism carries out
- Physiological - related to internal functions eg; metabolism

- Migration to warmer climates for winter months e.g. the grey plover and humpback whales
- Basking in the sun
- Huddling together for warmth e.g. penguins

- Migration to cooler climates to avoid extreme heat
- Lying in the shade e.g. the red kangaroo
- Nocturnal activity - some organisms are mostly inactive during the day to avoid the heat, coming out at night to find food e.g. the bilby.

- Blubber which is a thick layer of fat, providing a layer of insulation to prevent heat loss e.g. seals and whales
- Larger animals with a smaller surface area to volume ratio, reducing the surface for heat loss e.g. polar bears. Smaller animals in these regions have smaller ears to reduce heat loss e.g. the mountain pygmy possum

Animals tend to be smaller with a large surface area to volume ratio to increase the surface for heat loss.

Eg: Larger animals have large, thin ears to promote heat loss, such as elephants

- Shivering which, along with increased metabolic rate, produces heat.
- Hibernation is an extended period of inactivity where body temperature stays above 30°C but the heart rate and oxygen consumption drop significantly, reducing the amount of energy required to keep warm (e.g. mountain pygmy possum).
- Torpor is a short-term, involuntary hibernation and lasts for just a few hours during the daytime.

- Evaporative cooling e.g. sweating, panting or licking saliva onto the body surface. As the water evaporates from the body, heat is carried away with it.
- Vasodilation (expansion) of capillaries brings the blood (which carries heat) closer to the surface. As body temperature is generally higher than the ambient temperature, heat will be lost to the environment over the surface of the skin.

Homeostasis is maintained by negative feedback loops which are controlled by communications between various systems via pathways created by the nervous and endocrine systems working together or separately. Messages in the nervous system are transported by nerves as electrical impulses while messages in the endocrine system are transported as hormones in the bloodstream

Receptors are responsible for detecting stimuli - that is, changes from the set points of any variables. There are different types of receptors depending on the stimuli that they detect. While some receptors can detect changes in the external environment, interoceptors are responsible for detecting internal stimuli associated with homeostasis.

Thermoreceptors - Detect changes in temperature

Chemoreceptors - Detect the concentration of chemicals in the body e.g. oxygen and carbon dioxide, as well as the pH of blood. They are located in the blood vessels.

Provides the pathways that allow messages to travel in the body, and acts as a coordination system to maintain homeostasis. There are two main parts to the nervous system: the central nervous system (CNS) which is comprised of the brain and the spinal cord, and the peripheral nervous system (PNS), which is composed of all of the nerves throughout the body.

The nervous system is made up of millions of units called neurons (nerve cells) where no two neurons are the same

Sensory neurons - Carry impulses from sensory cells in the PNS, towards the CNS.

Motor Neurons - Carry messages from the CNS to effectors (muscles and glands)

Interneurons - located within the CNS, connecting sensory and motor neurons

- grey matter is cell bodies and dendrites
- white matter is myelinated axons (myelin = fatty layer)

In the brain, the grey matter tends to be on the outside with the white matter on the inside.

In the spinal cord the grey matter is on the inside with the white matter on the outside.

The endocrine glands are ductless and release hormones directly into the bloodstream. From here they will travel all over the body, but will only effect their target tissues and organs.

Endocrine glands may be stimulated to secrete hormones by signals from the nervous systems or by other hormones.

Receptors must be present in an endocrine gland to respond to these other hormones. These receptors include osmoreceptors and chemoreceptors.

INSULIN AND GLUCAGON RELEASE BY THE PANCREAS

Insulin = hormone to lower blood sugar

Glucagon - hormone to heighten blood sugar

Plants lose water by transpiration whereby water is evaporated from the stomata in leaves

- It lifts water and dissolved ions from the roots up the stem to the top of the plant (transpiration stream)

- It is a form of evaporative cooling, helping to regulate the temperature in plants.

Plants that live in arid areas or excessively wet areas have adaptations to survive in these hostile environments are called xerophytes and hydrophytes respectively.

- Reducing internal temperature to reduce the amount of water needed for cooling, while keeping their temperature within the optimal range.
- Reducing the exposure to sunlight
- Reducing water loss
- Regulating the opening and closing of stomata

- Waxy or thick, leathery cuticles on leaves, waterproofing the leaves and preventing evaporation.
- White hairs on the leaves to reflect sunlight, reducing the temperature on the leaf's surface and thus evaporation.

- Orientation of the leaves so stomata are not exposed to direct sunlight
- Reduced surface area of organs that have the most stomata
- Loss of transpiring plant organs such as leaves

- Reducing leaf size (smaller leaves)
- Reduced flowers or flowers without petals reducing the amount of water needed and the amount of evaporation from flower surfaces

Some plants only open their stomata during the cooler parts of the day e.g. early morning or late afternoon. When the temperature is lower, evaporation is reduced and water loss is minimised.

Non-infectious diseases are those that are not caused by pathogens and are not transmitted from one host to the next. Non-infectious diseases may be genetic, caused by environmental exposure, nutritional or cancers.

Usually due to a mutation in either:
Gene - each gene codes for the production of a specific protein so an abnormality in the gene may cause the protein to be non-functional or not produced at all.
Chromosome - abnormal cell division may lead to an incorrect number of chromosomes or abnormalities in the chromosomes.

Many single-gene abnormalities are a result of mutated genes inherited from an individual's parents. Some are recessive alleles so will only appear in the individual if the mutated gene is inherited from both parents. Others are dominant so will be expressed if only one allele has the mutation.

Cystic Fibrosis is caused by inheriting the mutated CFTR gene on chromosome 7. It is a recessive condition so an individual must receive the faulty gene from both parents.
The faulty gene causes a change to the protein that regulates the movement of sodium chloride into and out of cells, resulting in the production of abnormally thick mucus. The thick mucus blocks the passageways in the lungs and digestive tract, creating issues with breathing and absorption of nutrients.

Mutations that produce changes in whole chromosomes, these include;
Non-disjunction - incorrect seperation during meoisis
Trisomy - an extra chromosome in a normal pair
Monosomy - when a chromosome of a pair is missing
Structural changes - deletion, duplication, translocation & inversion

Down syndrome is the most common trisomy syndrome. It is caused by an extra copy of chromosome 21 in the zygote, resulting in 47 chromosomes in each cell. Down Syndrome results in:
Physical defects: small, flattened skull, enlarged, protruding tongue, small ears, short stature and short,stubby hands
- impaired intellectual ability (varying greatly from individual to individual)
- heart defects
- sexual underdevelopment and infertility (though some may reproduce)

Diseases may be caused by exposure to substances in the environment. These can include:
Lifestyle diseases - caused by abuse of substance e.g. smoking can lead to lung cancer
Physical substances in the environment e.g. UV light and other forms of radiation can cause mutations in DNA and lead to disease.
Chemical substances in the environment e.g. through air, water, soil and in food sources. Chemicals known to cause disease include lead, mercury, DDT and asbestos.

Asbestos is a chemical that was previously used in a number of common items, such as building materials. Inhalation of even a small amount of asbestos can lead to asbestosis, mesothelioma (type of cancer) and lung cancer.
Asbestosis occurs when inhaled fibres of asbestos cause an inflammatory response in the lungs, leading to scarring and stiffening of lung tissue. This leads to difficulty in breathing and receving sufficient oxygen, which cause shortness of breath, a persistent cough, chest pain and clubbing of fingertips due to insufficient oxygen

Cancer occurs due to uncontrolled cell division of abnormal cells which then invade nearby cells, spread to other tissues and disrupt functioning of the body. Uncontrolled cell replication results in cells that are undifferentiated (so unable to specialise) and may result in formation of tumours. Tumours may be benign (don't spread to other cells, but can sometimes become cancerous if not treated) or malignant (cancerous, spreading to other tissues).

Melanoma occurs due to uncontrolled division of cells in skin cells called melanocytes (containing the pigment melanin). Melanoma can occur anywhere on the body, but most commonly on areas exposed to sunlight e.g. face, neck and trunk in men, and legs and arms in women. It can lead to:
- Unexplained pain and weightloss,
- Hard or swollen lymph nodes
- Sickness and fatigue

Malnutrition refers to any imbalance in the diet, and may fall into one of two categories:
1. Undernutrition - insufficient intake of food, or a particular nutrient in food.
2. Overnutrition - excessive intake of food.

Insufficient intake in the correct type of food, quantity of food, or a combination of both. This includes both macro and micro-nutrients
- Iron deficiency causes anaemia. Symptoms include pale skin, weakness, tiredness, sensitivity to cold and shortness of breath. Iron forms part of the haemoglobin molecule and is responsible for transporting oxygen in the body.

Vitamin C deficiency can lead to a disease called scurvy. This causes slow wound healing, joint pain, bleeding gums, loose teeth, bruising, etc. and may result in death if not treated. Fortunately, treatment is as simple as increasing the intake of vitamin C by diet or supplmentation.

A state in which nutritional intake greatly exceeds the body's needs. Obesity may result from continued excessive consumption and is associated with numerous health conditions including heart disease, diabetes, stroke, joint issues and increased risk of some cancers.

- Incidence is the number of new cases of a named disease during a specific period of time. This is usually expressed as a proportion or fraction of the population (e.g. number per 100,000).
- Prevalence is the number of people living with a named disease at a given time. This is also usually expressed as a proportion or fraction of the population (e.g. number per 100,000).
- Mortality is the number of deaths occurring as a result of a disease during a specified time period. This is most commonly expressed as a percentage of the total population during the specified time period.
- Morbidity is the condition of suffering from a disease or medical condition in a given population

Mortality rates have decreased due to progress in prevention and early intervention
Prevalence = Lung cancer remains the most common occuring cause of cancer where cancer is the leading cause of all time death in Australia and 3rd leading cause in males currently
Incidence = Incidence rates are decreasing as less and less people are engaging in risk factors such as tabacco smoking.

Kwashiorkor (severe deficiency in protein)
Incidence = Low incidence rate and very low mortality
Mortality = In areas where Kwashiorkor is high, mortality rate is relatively high
Prevalence = Most prevalent in overpopulated regions eg; Africa & Asia, & young children (1-3 years old)

Treatment of a disease is the administration of remedies (pharmaceutical, behavioural, surgical etc.) to an individual suffering from a disease. Some diseases such as genetic diseases have very few possible treatments, and thus management of the disease is a more appropriate response to improve quality of life. Management is the care and attention given to an individual suffering from a disease.

Melanoma is a potentially deadly form of skin cancer originating in the melanocytes. There are different treatment options available depending on the stage at which melanoma is diagnosed.
Early detection - the most common treatment is surgery. This surgery involves removing the tumour and the skin around it to ensure none of the cancerous cells are left behind, reducing the risk to recurrence.
Later detection - variety of treatments including radiation, chemotherapy, targeted therapies and immunotherapy:

The use of electromagnetic radiation to treat a disease. Upon exposure to X-rays, normal cells repair their DNA more easily than cancerous cells. As a result these X-rays can kill the cancerous cells.

is the use of chemical substances of pharmaceuticals to treat a disease. They work by slowing the growth of cancer cells, aiming to inhibit their growth and kill the cancer cells. However, this method is much less effective against melanomas than other cancers so is used less often

Future directions of study currently involve researching further into targeted therapies. The use of targeted therapies is based on the different types of mutations causing uncontrolled cell division in melanoma, and targeting these mutations with specific drugs. However, only a few of these mutations have been identified and there are many more that have not. Current research is aiming to identify these other mutations in order to produce more drugs that will target these specific mutations.

To be considered an effective study, epidemiological studies must include the following:
· Be conducted over a long period of time (5-10 years ideally). However, some studies that require results more urgently (for example in a pandemic) may be conducted over shorter periods of time
· Study very large sample sizes (approximately 5000-10,000 people). This is done to ensure the data accurately represents an entire population. Using small sample sizes means data may not be representative of the occurrence of a particular disease in a population
· Data must be collected from affected and unaffected people. The unaffected people act as a control group in order to ensure validity
· Data must be collected from a range of groups, including people of different ages, sex, diet, occupation, lifestyle and exercise habits. This ensures data does not come from one specific group of people which do not accurately represent the population, preventing possible bias and ensuring validity
· Collect data on the incidence, prevalence, mortality and morbidity rates of the disease being studied

Epidemiological studies are generally highly beneficial as (if done properly) they provide a valid and reliable indication of the link between a specific factor (cause) and the occurrence of the disease. This is beneficial as it can:
- Help develop informed strategies to control disease and promote the heal of the public. For example, the National tabbacco strategy was developed and implemented to respond to lung cancer incidence and mortality rates.
- Evaluate current health strategies that are in place and alleviate fears about certain factors in causing disease. For example, a study investigating the link between the MMR vaccine and autism conclusively showed that there was no relationship between the two. This alleviated a lot of fear in parents that had led to a significant rise in cases of measles, mumps and rubella as many children had not received the vaccine due to a fear of developing autism

Studies can be experimental or observational. Observational studies is divided into descriptive and analytical. Analytical studies are further broken down into cohort and case-control studies

Case-control - comparison of people who have the disease (cases) with people who do not have the disease (controls). + quick and cheap, good for uncommon diseases

Cohort studies - study two or more similar groups who do not have the disease. One group is then exposed to the possible cause, while the other group is not exposed. The groups are then followed up over a long period of time to determine the incidence of the disease.

Educational programs and campaigns use strategies to educate the public about the effects of a disease, causes, risk factors that may increase the chance of getting the disease and behaviours (protective factors) that may reduce the chance.

The national tabacco strategy (2012-2018) informs and educates individuals of smokings risk factors and equips them with greater health literacy and the protective factors to enhance their health and prevent LUNG CANCER. This included:
- Taxation on tabacco products
- Limiting visibillity of tabacco products and its glorofied messgaes (ban of advertisments)
- Plain packaging to reduce appeal or with graphic pictures and warning labels
- regulating the place of use (no smoking areas)
- education programs such as compulsory k-10 PDHPE or online advertisments/articles
- The provision of support such as QUIT Helplines

Golden rice is a transgenic strain of rice that has had a gene from a daffodil and soil bacteria inserted into its genome so that it will produce beta-carotene in the edible parts of the rice giving it a distinctive golden colour. Human bodies synthesise beta-carotene in vitamin A. Given the widespread use of rice in developing countries - where vitamin A deficiency is most prevalent - golden rice had the potential to be a sustainable and long term solution to vitamin A deficiency
However, there have been some problems with the adoption of this crop due to mistrust of genetically engineered foods, a smaller crop yield than normal rice and some questions as to whether it actually works in the way that it was thought it would.

- The cochlea is the fluid filled inner part of the ear that contains receptors that convert sound waves into electrical impulses.
- Situated at the back of the ear, the auditory nerve consists of nerve fibres connecting the cochlea to the brain. It transmits electrical signals from the cochlea to the brain for interpretation as sound.

Two main parts, the ossicles and the eustachian tube. The ossicles is the set of small bones in the middle ear that magnify vibrations from the ear drum and transfer these to the oval window. The ossicles consist of three small bones, the hammer, stirrup and anvil

The eustachian tube connects the ear to the nose and throat, responsible for equalising pressure in the air and draining fluid.

Hammer (malleus) = Transmits sound vibrations from the eardrum to the anvil
Anvil (incus) = Transmits sound vibrations from the hammer to the stirrup
Stirrup (stapes) = the last of the three ossicles which passes vibrations onto the oval window

Pinna = Collects sound waves and directs them into the ear canal
Ear canal = Channels sound waves to the eardrum
Eardrum/tympanic membrane = Vibrates in response to sound waves, transmitting these vibrations to the ossicles (hammer, anvil, stirrup)

Conductive hearing loss is caused by a problem with the conduction of vibrations through the outer and middle ear. This usually affects the loudness of the sound.

Sensorineural hearing loss is caused by damage or malformation of the inner ear that prevents kinetic energy from the vibrations from being converted to electrical impulses that can be sent to the brain. The damage is usually permanent resulting in permanent hearing loss, affecting loudness or clarity of the sound.

Conductive hearing loss can be caused by:
- Malformed structures in the ear
- A perforated ear drum or trauma to the ossicles
- Infection

Sensorineural hearing loss can be caused by:
- Excessive noise exposure
- Birth defects / genetic causes
- Infections & tumours
- Ageing

1) The process begins when sound waves are collected by the pinna (the outer part of the ear). The pinna's shape helps funnel these sound waves into the ear canal. The ear canal channels the sound towards the ear drum

2) When the sound waves reach the eardrum, they cause it to vibrate. The eardrum transforms the sound wave energy (acoustic energy) into mechanical energy through its vibrations. The vibrating eardrum is connected to a series of three tiny bones in the middle ear: the hammer (malleus), anvil (incus), and stirrup (stapes) which work to amplify the vibrations and continue to transmit the mechanical energy

3) The stirrup is connected to the oval window, a membrane-covered opening that leads into the cochlea. Inside the cochlea, the fluid waves move through the spiral structure, causing the hair cells (specialized sensory cells) to move. The bending of these hair cells converts the mechanical energy into electrical energy by triggering electrical impulses.

4) The electrical signals generated by the hair cells are then transmitted to the brain through the auditory nerve. The brain receives and interprets these electrical signals as sound, allowing us to perceive and recognize what we hear

The assistive technologies for hearing loss include:
- Hearing Aids
- Bone conduction
- Cochlea Implants

The hearing aid receives sound through a microphone, which converts the sound waves to electrical signals and sends them to an amplifier. The amplifier increases the power of the signals and then sends them to the ear through a speaker

A bone conduction implant has a microphone that detects sound waves and transforms them to vibrations which pass through the implant to the bone above the ear, and then to the cochlea where they are then processed as normal. This technology is useful when the cochlea is functional, but the outer or middle ear is damaged or malformed.

A cochlear Implants is designed for those with moderate to severe sensorineural hearing loss (loss due to damage in the inner ear) who have difficulty interpreting speech even with a hearing aid. A cochlear implant may be right for those who: Struggle to understand speech even with hearing aids that are properly fitted.

Cochlear implants essentially bypass your inner ear structure, creating a new pathway for sounds to make their way to your brain. It works like this:
- You wear a sound processer that sorts the sounds coming into your outer ear.
- The sound processor sends signals to a transmitter attached to your scalp with a magnet.
- The transmitter converts the signals into electric impulses that it sends to electrodes placed in your cochlea.
- The electrodes in your cochlea collect the impulses, sending them on to hearing nerves.
- The hearing nerves let your brain know sounds are coming in.

The human eye is one of the most complex and sophisticated organs in the body. Although it is small and delicate, the eye allows us to see the world without any conscious effort, adjusting to light automatically.

1) Sclera = a tough white protective coat that surrounds the eye, providing protection and making the eye difficult to damage
2) Choroid = middle layer of the eye that is responsible for absorbing light. This is done to reduce the reflection of light in the eye and thus reducing over-exposure
3) Retina = sensory membrane covering the inner portion of the eye which contains photoreceptors (rodes and cones) that allow light to be converted into electrical signals for the optic nerve and thus brain.

For a clear, focused image to form, light rays must be refracted so that a sharp focus is formed on the retina. Light from the environment passes into the eye through the cornea - the transparent structure of the sclera that covers the eye and is responsible for most of the refraction of light due to its curvature. The light then passes through the aqueous humour and the pupil - a hole in the centre of the iris where light enters the eye. The iris is the muscle that contracts to expand or dilate the pupil to control the amount of light entering the eye. The light moves through the lens - a highly elastic transparent biconvex disc that focusses the light onto the retina

The ability of the lens to change its curvature to focus on objects at different distances. This is controlled by the ciliary body and the suspensory ligaments:

Light entering from a distance needs to be refracted less, as the light rays are already close to parallel. Thus, the lens must be thinner. This is achieved by relaxing the ciliary muscles drawing the sclera back, holding the suspensory ligaments taught. This lengthens the lens, decreasing its curvature and thus its refraction

The light rays must be refracted more. The ciliary muscles contract, drawing the sclera forward and causing the suspensory ligaments to slacken. This makes the lens rounder, increasing the curvature (concavity) and thus the refractive power of the lens.

Iris = the muscle and coloured part of the eye that contracts or relaxes to control the amount of light entering the eye
Cornea = The transparent area of the slcera covering the iris and pupil which bends (refraction) most of the light onto the retina
Lens = The transparent, bi-convex disc controlled by cilliary muscles that fine focusses light onto the retina
Retina = the sensory membrane that lines the back of the eye containing photoreceptors (rodes and cones) allowing light to be converted to electrical impulses for the optic nerve.

Myopia (short-sigthedness)
Hyperopia (long-sightedness)
Cataracts (any loss of transparency of the lens, eg; clouding of the lens causing blurred vision for both near and far objects)
Astigmatism (when there is an irregular curvature of the cornea, lens or both resulting in multiple focal points)

Occurs when the eye can focus correctly on near objects, but when a distant object is viewed the focal length is too short resulting in a blurred image as the image falls before the retina.
Caused by the lens being too rounded, the eyeball being too elongated or the lens not flattening enough when ciliary muscles contract.

Occurs when the eye can focus on distant objects, but the focal length when viewing near objects is too long causing the image to fall behind the retina. The causes Include:
the lens being too flat or the eyeball being too rounded.

Myopia = Concave lenses bend the light rays outwards (diverge) before they reach the eye. This extends the focal length, causing the image to fall on the retina instead of in front of it (myopia)

Hyperopia = Convex lenses converge the light rays before they reach the eye, shortening the focal length and allowing the image to fall on the retina instead of behind it (hyperopia)

Laser eye surgery is a medical procedure that involves the use of a laser to reshape the surface of the eye to improve or correct visual disorders such as myopia, hyperopia and astigmatism

to remove nitrogenous waste from the bloodstream and maintain optimal levels of water and salt. There are 3 main regions in the kidney:
the cortex = the outer layer of the kidney with a strong blood supply
medulla = the middle layer of the kidney
pelvis = the inner layer of the kidney that collects urine

The functional unit of the kidney which filters the blood to form urine. Each kidney has millions of microscopic nephrons which extend from the cortex into the medulla, where ach nephron has a spherical, cup-like structure called the Bowman's capsule which contain a network of capillaries called the glomerulus.

1. Filtration (Bowmans capsule)
2. Reabsorbption (Tubules and Loop of henle)
3. Secretion (Distal tubule and collecting duct)
4. Excretion (bladder and urethra)

Filtration is a non-selective process where any substance small enough will pass through the glomerulus walls into the Bowman's capsule regardless of whether it is required in the body. Thus substances that are too large will not enter the Bowman's capsule e.g. proteins, blood cells, etc whilst glucose, water, salts and urea will form glomerular filtrate which is pased onto the tubules for reabsorption.

The selective process by which substances required by the body move from the tubules of the kidney back into the capillaries by diffusion, active transport and osmosis whilst those that are waste or excess will remain in the nephron and be excreted in the urine. The blood requires substances such as glucose, water and salt ions which are filtered out in the glomerulus, whilst urea and other watse is not needed.

Secretion is the removal of toxic substances and excess ions from the capillaries and surrounding tissues into the tubules so they can be eliminated from the bloodstream. The last part of the nephron, aka the collecting duct is heavily influenced by the hormone antidiuretic hormone (ADH) which regulates water reabsorption for secretion.

Once the filtrate has passed through the nephron and reabsorption and secretion have taken place, it is now called urine. The ureter is a tube that leads to the bladder, where urine is stored. Urine is eventually excreted from the body through the urethra during urination.

Water leaves the tubules through osmosis, and other molecules can either be reabsorbed through diffusion or active transport. 1. Diffusion is done when there is a positive concentration gradient (higher concentration in the tubules than the capillaries)
2. Active transport is done when there is a negative concentration gradient (lower concentration in the tubules than the capillaries) thus requiring energy to go against the concentration gradient, possibly involving membrane proteins to transport molecules across cells.