How they eye works
Accommodation is the process of changing the shape of the lens to focus on near/distant
objects.
To focus on a near object - the lens becomes thicker - this allows the light rays to refract
(bend) more strongly.
To focus on a distant object - the lens is pulled thin - this allows the light rays to refract less
strongly.
Position
Near
Ciliary
muscles
Contract
Suspensory
ligaments
Slacken/loosen
Muscle
tension
Low
Distant
Relax
Stretched/tighten High
Lens shape
Refraction
Fat/Thicker
Light is
refracted
strongly
Light is
refracted
slightly
Thin
Common defects of the eye and how they can be overcome
Correcting vision defects
Two common defects of the eyes are:
1) Myopia (Short-sightedness)
2) Hyperopia (Long-sightedness)
In both cases, rays of light do not focus on the retina so a clear imagine is not formed.
Treatment = Spectacle lenses, which refract (bend) the light rays so that they do focus on the
retina.
Short sight
Someone with short-sight can see near objects clearly, but cannot focus properly on distant
objects.
Caused by:
1) Lens is too thick and curved, the light is focused in front of the retina.
2) Eyeball is elongated, so that the distance between the lens and the retina is too great.
Myopia
Concave lens cures short sightedness
Long-sight
Someone who is long sighted can see distant objects clearly, but cannot focus properly on
near objects.
Causes:
1) A loss of elasticity in the lens - the lens is too thin to focus (which is often agerelated, hence why older people use reading glasses).
2) Eyeball being too short - the distance between the lens and the retina is too small
As a result, the lens focuses light behind the retina instead of onto it.
Long sightedness is corrected by putting a convex lens in front of the eye.
A convex lens corrects long-sightedness.
Modern solutions:
1) Contact lenses - on the surface of your cornea, focus and refract light
2) Laser surgery - reshape cornea surgically
3) Replacement lenses - artificial lens placed in front of the original lens, to correct an
eye defect
The human endocrine system
The endocrine system secretes hormones into the bloodstream from glands throughout the
body. Hormones produce an effect on specific target organs in the body.
Hormones and nerves
A hormone is a chemical substance produced by a gland and carried in the bloodstream,
which alters the activity of a specific target organ.
Eg: Adrenaline - released by the adrenal gland. One of its target organs is the heart, where it
increases the heart rate.
Once a hormone has been used, it is destroyed by the liver.
Hormones can control the body, and the effects are much slower than the nervous system, but
they last for longer.
Important differences between nervous and hormonal control:
Type of signal
Transmission of signal
Effectors
Type of response
Speed of response
Duration of response
Nervous
Electrical and Chemical
(neurotransmitter in the
synapse)
Nerve cells (aka neurons)
Muscles or glands
Muscle contraction/secretion
Very quick
Short (until nerve impulses
stop)
Hormonal
Chemical
Bloodstream
Cells in particular tissues
Organ reaction
Slower
Long (until the hormone is
broken down by the liver)
Master gland
The pituitary gland in the brain is known as the master gland.
Secretes several hormones in response to the body’s conditions, such as blood water levels.
These hormones can also act on other glands to stimulate the release of other hormones.
Different hormones
Range of different chemical hormones that travel in the blood stream and affect a number of
different organs or cells in the body.