Excretion
The lungs, kidneys and skin are all organs of
excretion.
Excretion: the removal of metabolic waste
created in cells such as urea, carbon dioxide and water. The lungs, kidneys and
skin are all organs of excretion.
Renal/Urinary system:
Function:
The urinary system is used to create, store and
remove urine which in turn removes wastes such as urea. Blood flows to the
kidneys which separates it into the components of urine: water, urea, some
salts. The urine them moves down the ureters and is stored in the bladder until
the two sphincter muscles relax where it then exits the body.
The components and various concentrations of
urine may vary depending on the amount of water and salt intake and whether you
have kidney failure or diabetes.
Kidneys: the kidney is involved in both
excretion and osmoregulation.
Nephron: there are millions of these in each
kidney and they are the main areas of the kidney where filtration occurs.
Ultrafiltration is the separation of smaller molecules such as glucose, water,
urea, oxygen, salts, and amino acids from the blood at the glomerulus in high
pressure.
Selective
reabsorption is
where molecules in the glomerular substrate are retaken by the capillaries at
the proximal convoluted tubule. It is selective as only some molecules such as
glucose, amino acids and some salts are retaken.
Water is reabsorbed into the blood both through
selective reabsorption and through osmosis at the collecting ducts. The latter
is controlled by ADH (anti-diuretic hormone). When the hypothalamus detects the
blood is too concentrated, it tells the pituitary gland to release ADH which
travels via the blood stream to the kidneys. ADH makes the collecting ducts more
permeable to water meaning more water is absorbed. This is how the kidneys act
in osmoregulation.
Finally, after ultrafiltration and selective
reabsorbing has occurred, the remaining substances arrive at the collecting
ducts where it travels down the ureter to the bladder excreted. The excreted
urine contains water, urea and salts.
Coordination and
Response:
Organisms are able to respond to a change in
their environment (stimuli).
A coordinated response requires a stimulus, a
receptor and an effector.
Coordination in plants:
Plants respond to stimuli. Stimuli which acts in
a particular orientation are known as directional stimuli. A tropism is a plant
growth response caused by a directional stimulus.
Roots are positively geotropic meaning they grow
in the same direction as gravity. Auxin, a plant growth substance is produced
behind the root tip and diffuses to the lower side of the growth region. This
inhibits growth or elongation of the underside. The top side, however,
elongates faster, causing the roots to grow downwards.
Shoots are negatively geotropic. Auxin produced
behind the shoot tip diffuses to the lower side of the growth region. This
causes more rapid elongation and division of cells on the lower side. This
causes the shoots to grow upwards.
Note that at the roots auxin inhibits growth
while at the shoots auxin encourages it.
Shoots are also positively phototropic. In
unidirectional light auxin diffuses from behind the shoot tip to the growth
region and accumulates on the shaded side. Cells then divide and elongate on
the side causing the shoot to bend towards the light.
In uniform light, auxin diffuses from behind the
shoot too to the growth region and distributes evenly. This causes the shoot to
grow upwards towards the light.
Coordination in Humans:
Comparison of
nervous and endocrine system:
•
Both move instructions around
the body and participate in homeostasis.
•
Neurons use electrical impulses
while hormones use chemicals.
•
Neurons target specific
collection of cells while hormones target specific cells.
•
Hormones are relatively slow
while neurons are very fast and rapid.
•
Hormonal effects are relatively
long lasting while neurons only create a short effect.
Neurons:
 |
| Sensory neuron |
 |
| Motor neuron |
 |
| Relay neurone |
Central Nervous System:
This contains the brain and spinal cord and is what
gives out orders to other parts of the body.
How neurons work:
An electrical impulse is sent from a nerve
receptor and travels along the axon. At the nerve ending, there is a gap
(synapse). A synapse works as the electrical impulse triggers the release of
neurotransmitters. These chemicals diffuse across the synaptic cleft and binds
with receptors on the second neurone. This forms a neurotransmitter-receptor
complex which stimulates the second neurone to transmit the electrical impulse.
The unused neurotransmitters are then either reabsorbed or broken down by
enzymes.
Spinal cord: the
spinal cord marks the site of coordination where information sent from sensory
neurones are passed via relay neurones to motor neurones for the impulses to be
sent to effectors.
Reflex Arc:
1. Receptor cells detect stimulus
2. Receptor stimulated and an electrical impulse sent along sensory neurons
to the central nervous system (CNS).
3. At the CNS it travels along synapses to the relay neurone.
4. The relay neurone passes it to the motor neurone.
5. The impulse travels via the motor neurone to the target muscle or
gland effector.
6. This creates an effect.
Examples of this are the knee jerk reflex and
the withdrawal reflex. Both of these are spinal reflexes and so are
involuntary. This is because you only receive information (to your brain) after
the reflex has occurred due to the speed. In fact, the knee jerk reflex can
take just 50ms.
The Eye:
Function:
The eye is a photoreceptor (detects changes in
light) and transduces light energy into electrical impulses. These are then
interpreted as images by the brain. The eye is also able to respond to changes
in light and is able to focus and bend light reflected of objects to allow the
brain to produce a discernible image.
Focusing near and distant objects:
If an object is near, ciliary muscle will
contract which relaxes the suspensory ligaments so that the lens becomes fat
and slightly curved (convex). If the object is far, ciliary muscle will relax,
making the suspensory ligaments tighten so that the lens stretches out and
becomes thin and flat.
Responding to changes in light intensity:
In dim light, the iris dilates the pupil to
allow more light to reach the retina. It does this by contracting the radial
muscles and relaxing the longitudinal muscles. In bright light, the iris will
shrink the pupil.
Skin:
Function:
1. Protection
2. Waterproof barrier
3. Thermoregulation
Thermoregulation:
1. Sweat glands release sweat when the internal temperature is too
high. The evaporation of sweat uses heat energy of the skin. This takes away
heat energy.
2. Vasodilation occurs when the body is too warm. The shunt vessel
constricts and the capillaries widen meaning more blood flows through the
surface of the skin and so more heat is lost through the radiation of heat.
Vasoconstriction occurs when you are too cold. The opposite occurs meaning less
heat is lost through radiation.
3. Hair erector muscles relax to flatten the hair when it is warm. This
means less trapped air is present and therefore there is less insulation. The
opposite occurs when it is cold.
4. Adrenaline is used to raise metabolism meaning more energy is
released and so more heat is also released.
5. Shivering - more respiration occurs in muscles meaning more heat is
released.
Hormones:
Pituitary:
• Anti-Diuretic Hormones (ADH) - involved in osmoregulation by
controlling the permeability of collecting ducts.
• Gonadotrophin - controls the production of sperm and ovulation and
triggers sexual maturation.
Adrenal:
• Adrenaline - prepared the body for the "fight or flight"
response in times of stress. It increases heart rate, raises blood pressure,
enlarges pupil size, and raises the body's metabolism.
Thyroid gland:
• Thyroxine - controls the body's metabolic rate.
Pancreas:
• Insulin - helps control the body's blood sugar level by signaling
the liver, muscle and fat cells to take in glucose and convert it to glycogen.
• Glucagon - helps control the body's blood sugar level by telling the
liver to convert stored glycogen into glucose which is then released into the
blood stream.
Hormones are vitally important for growth and
development. The main hormones involved are the pituitary growth hormone,
thyroid hormone and sex hormones.
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