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HOMEOSTASIS – refers to the maintenance of a relatively stable internal
environment, achieved by the regulation of numerous factors including
temperature, cell pH, the concentration of ions, water balance and
reproductive cycles. In order to maintain the internal environment within the
certain acceptable limits, the organism must respond to external factors.
GENERAL MODEL FOR HOMEOSTATIC REGULATION
STIMULUS – refers to a detectable change in the internal environment (which
may be caused by a change in the external environment) i.e. when a factor
under regulation falls above or below a point at which regulation is triggered.
SENSOR/RECEPTOR – specialized cells or tissues, capable of detecting a
change in factors under regulation and sending a message via the nervous
system or endocrine system (responsible for the production of hormones) to
an effector site.
TYPES OF RECEPTORS – there are a number of different types of specialized
cells in the body, capable of detecting different types of stimuli.
CHEMORECEPTORS – capable of detecting chemical changes e.g.
concentration of ions, blood glucose level, water concentration, CO2 level.
MECHANORECEPTORS – capable of detecting pressure and movement e.g.
vibrations in the ear, muscle stretching, blood pressure, inflation/deflation of
lung.
PHOTORECEPTORS – capable of detecting light stimuli e.g. in the eye.
THERMORECEPTORS – capable of detecting changes in temperature e.g.
thermoreceptors in skin, capable of detecting internal changes,
hypothalamus capable of detecting changes in core body temperature.
EFFECTOR SITE – refers to the site where a response to the stimulus occurs,
activated by neural impulse of hormone.
RESPONSE – refers to the change instigated in response to the stimulus and
will, ideally, bring the factor under regulation back within stable limits. If the
response that occurs is in the opposite (negative) direction to the stimulus,
then NEGATIVE FEEDBACK is said to have occurred.
NEGATIVE FEEDBACK- maintenance of a stable internal environment relies
on negative feedback mechanisms.
In order to maintain a stable
environment, the body must possess mechanisms by which it may counteract
external stimuli by producing a response in the opposite direction of the
stimulus, to reduce the effects of the stimulus.
POSITIVE FEEDBACK – whilst positive feedback systems to occur in nature, it
is important to note that they are not mechanisms of homeostatic regulation.
Generally, positive feedback occurs in response to abnormally stressful
conditions e.g. during childbirth. During childbirth pressure on the cervix
stimulates the production of oxytocin, a hormone, which produces further
contractions and increased pressure on the cervix. The process of blood
clotting is another process in which positive feedback occurs. The injured
tissue releases a chemical which activates platelets in the blood. The
activated platelets then release more of the chemical, activating more
platelets, leading to the formation of a blood clot.
ENDOCRINE SYSTEM – refers to a system of glands which produce
hormones, a means of regulating some bodily processes.
EXAMPLES
HYPOTHALAMUS – located in the brain, the hypothalamus acts as both a
receptor and producer of hormones. Osmoreceptors, responsible for the
detection of water concentration in the blood and thermoreceptors,
responsible for detecting changes in core body temperature are located in the
hypothalamus. The hypothalamus also produces hormones which act directly
on effector sites, as well as inhibitory and releasing hormones which control
the production of other hormones in the body.
PANCREAS – secretes insulin and glucagons, hormones which regulate the
concentration of glucose molecules in the blood.
HORMONE – in animals, refers to chemical compounds produced (usually) in
the cells of endocrine organs that may act on the same cell, be released into
the surrounding extra cellular fluid to exert an effect on a nearby cell or be
released into to the bloodstream, via lymph, in order to exert an effect on a
specific (target or effector) site. Some hormones have relatively immediate
effects e.g. regulation of glucose, ions etc., whilst others are involved in the
long term development of an organism e.g. growth hormones, hormones
involved in reproductive cycles and sexual development. hormones may be
amino acid derivatives, polypeptide/protein hormones or steroid based
hormones.
AMINO ACID DERIVATIVE AND POLYPEPTIDE/PROTEIN HORMONES – are
made/modified and packaged by golgi into vesicles are secreted form the cell
via exocytosis. Amino acid and peptide hormones have a relatively short life
span (they are broken down by the organism).
These hormones are water soluble (polar) and therefore able to travel in the
bloodstream, dissolved in plasma. This, however means they are unable to
pass through hydrophobic cell membranes via simple diffusion. They must,
instead, bind with receptor molecules embedded in the cell membrane, which
may be carbohydrates or proteins, and exert their effects on the contents of
the cell via a process referred to as signal transduction.
SIGNAL TRANSDUCTION – refers to the process by which hydrophilic
hormones are able to exert an effect on the internal environment of cells,
despite being unable to pass through their membranes. Having attached to a
receptor protein of carbohydrates on the outer side of the cell membrane, a
g-protein on the inner side of the membrane is activated. The activation of
the g-protein triggers a response, being the creation of a signaling or relay
molecule which triggers a second response and so on, until the final response
occurs. The final response may be the activation of a gene resulting in the
creation of mRNA in order to synthesize a protein, which may then act as an
enzyme for a chemical reaction in the cell.
STERIOD HORMONES – are lipid based hormones, formed from precursors
within cells, and being hydrophobic, are able to pass through the
phospholipid bi-layer membrane of cells and attach to a cytosol receptor, a
receptor molecule floating in the cytosol, forming a hormone-receptor
complex. The hormone-receptor complex then enters the nucleus to activate
a gene, inducing production of mRNA coding for the synthesis of a protein,
which may act as an enzyme for cell reactions. Generally, steroid hormones
regulate long term development such as growth and sexual development.
PLANT REGULATION – hormones are also involved in the regulation of plants
and are generally transported via xylem or phloem and sometimes both,
however particularly via phloem. Plant hormones may produce different
effects in different types of plant tissues, differeing form animal hormones
which are generally very specific in their functions.
AUXINS – are a type of plant hormone, of which indoleacetic acid (IAA) is
one. A major function of auxins is to control enlargement and elongation of
cells. In stems and leaves of plants, auxins stimulate cell elongations,
however may inhibit cell elongation in the roots of plants.
TROPISMS – the term tropism refers plant growth in response to a stimulus
such as water of light. Tropisms are terms to be positive if the plant grows
towards the stimulus and negative if the plant grows away from the stimulus.
PHOTOTROPISM – a positive tropism referring to the tendency for plants to
grow towards a light stimulus. This tendency occurs due to the presence of
indoleacetic acid (IAA), an auxin, which occurs in higher concentrations in
cells that have less exposure to light. If a plant is exposed to an overall even
intensity of light, the plant will grow straight, due to even concentrations of
IAA. If a light source is of greater intensity on one side of the plant, auxin
will become concentrated in cells with less exposure to light. In plant stems,
IAA stimulates elongation of cells, thus, cell with higher concentrations of IAA
will become more elongated than those exposed to light, causing the plant to
bend towards the light stimulus.
GEOTROPISM – refers to a tendency for plants to grow in the vertical
direction i.e. if a seed is planted so that both the roots and the shoot emerge
horizontally, after a period of time, the roots will turn to grow downwards
and the shoot upwards. This tropism is due to the effects of auxin, which
accumulates on the lower sides of horizontally growing shoots and roots. In
the shoots of plants, auxin stimulates cell elongation, causing cells on the
lower side of the shoot to become more elongated. This causes the shoot to
turn upwards. Auxins, however, have an inverse effect when present in the
roots of plants. Accumulated auxins on the lower side of horizontally growing
roots inhibit the elongation of cells, causing the root to turn downwards.
CYTOKININS – a type of plant hormone which stimulates cell growth and cell
reproduction.
GIBBERELLINS – a class of plant hormones which stimulate cell growth and
reproduction. Gibberellins also promote seed germination and flowering.
Gibberellins promote cell growth by stimulating the synthesis of the enzyme
amylase, a catalyst in the breakdown of starch. In breaking down starch
molecules, glucose, a reactant in cellular respiration, is made available to the
cell.
ABSCISIC ACID – may inhibit germination of seeds, leading to seed
dormancy and seasonal dormancy. Abscisic acid may also influence the
opening and closure of stomata.
ETHYLENE – also known as ethene, is secreted as a gas and promotes
ripening of fruits and the abscission (shedding) of leaves through stimulating
the formation of a protective layer referred to as an abscission layer, at the
site of abscission.
FLORIGEN – a hormone believed to stimulate the flowering of plants, its
precise identity is as of yet unknown.
PHEROMONES – are chemicals produced in the exocrine glands of animals,
especially insects, which are then secreted to the external environment to
influence the behaviour of other members of that species.
NERVOUS SYSTEM – refers to the system of the brain and nerves, which is
responsible for detecting, transmitting, processing and responding to
information which an organism receives. The nervous system can divided
into two main parts: the central nervous system (CNS) consisting of the
brain and spinal cord and the peripheral nervous system (consisting of all
other nerve tissue within the organism.)
PERIPHERAL NERVOUS SYSTEM – can be furthered divided into parts, based
on the various functions it carries out.
SOMATIC NERVOUS SYSTEM – consists of sensory (afferent) neurones and
motor (efferent) neurones. Sensory neurones carry sensory information
from the sensory organs (e.g. nose, touch receptors in the skin etc.) to the
spinal cord, which will then carry the neural messages to the brain where
they may be processed. Motor neurones carry neural messages to skeletal
muscles, in order to control voluntary movement.
AUTONOMIC NERVOUS SYSTEM – regulates involuntary processes within the
body e.g. digestion, heart rate, breathing etc. and can divided into two
further categories: the sympathetic nervous system and the parasympathetic
nervous system. The function of the parasympathetic nervous system is to
heighten the body’s state of arousal (e.g. by increasing heart rate, release of
adrenalin etc.) in order to prepare the organism for action. The function of a
parasympathetic is to calm the organism after having been in a state of
arousal, and increase or decrease the rate of process under its regulation as
part of maintaining homeostasis. Note: whilst the general function the
parasympathetic nervous system is an inhibitory one, the parasympathetic
nervous system may also increase factors such as heart rate, as part of
maintain a relatively constant internal environment of the organism.
NEURONS – refer to the type of cell which makes up the nerve tissue of an
organism. Their function is to relay neural impulses, and thus, they have
specialised features which enable them to do so.
DENDRITES – are the receptors of the neuron; they receive neural impulses
from other neurons and sensory information from receptor cells.
AXON – transmits the neural impulse down the nerve cell, to the terminal
buds, and may be protected by a layer of Schwann cells, referred to as the
myelin sheath.
AXON TERMINALS/TERMINAL BUDS – are found at the end of the axon, and
contain neurotransmitters, which are released in order to transmit impulses
to surrounding nerve cells.
SYNAPSE – refers to the junction of two nerve cells. The space between the
two, containing tissue fluid, is referred to as the synaptic gap.
NEUROTRANSMITTERS – are chemicals secreted from axon terminal buds
which pass across the synaptic gap (containing tissue fluid) to stimulate the
cell membrane of an adjacent neuron.
NEUROHORMONES – are chemicals secreted from nerve cells, however they
differ from neurotransmitters in that they can be released into the
bloodstream to exert an effect on a target organ. Essentially,
neurohormones function in the same way as other animal hormones.
NEURONE POLARITY – when not in a state of conducting a neural impulse,
the neurone is resting, however it must also maintain a charge difference
between the outside of the cell membrane which is positively charged, and
the inside of the membrane which is negatively charged. This charge
difference is maintained via the active transport of sodium (Na+) ions.
NEURAL IMPULSE – when a neural impulse is transmitted down the axon of a
nerve cell, a rapid change in the polarity of the cell occurs. As the impulse
travels down the axon, a change in the permeability of the membrane allows
positively charged ions to pass across the membrane. This changes the
overall charge of the outside of the nerve cell from positive to negative. This
difference in charge is then reversed once the neural impulse has been
transmitted. The process occurs in a period of milliseconds.
SOME DETAILED EXAMPLES OF HOMEOSTATIC REGULATION
FRIGHT STIMULUS – the body’s response to a frightening stimulus is a good
example
of
nervous and endocrine
systems
working
together.
Photoreceptors in the eyes or mechanoreceptors in the ears (capable of
detecting vibrations) will detect a frightening stimulus and send a message
via sensory (afferent) nerves to the brain. The sympathetic nervous system
will then send neural messages to the heart to increase heart rate, to the
lungs to increase breathing rate etc. Hormones (adrenalin) are released
from the adrenal medulla, increasing the blood flow to muscles. A hormone
is released from the hypothalamus to stimulate the production of another
hormone, which in turn stimulates the production of another hormone,
cortisol, which acts on the liver to release glucose into the blood. The
combined effects of the hormonal and nervous systems prepare the organism
to respond to a frightening stimulus.
BLOOD GLUCOSE – RISE IN BLOOD GLUCOSE
Concentrations of blood glucose greater than 6.8 mmol/L(stimulus) are
detected by the pancreas(receptor). Alpha cells in the pancreas decrease
production of glucagon, a hormone (message) which stimulates the
breakdown of glycogen energy stores, stored in the liver and skeletal
muscles (effectors). As a result, the level of glucose in the blood lowers, due
to a decrease in the amount of glycogen being broken down into glucose and
released into the blood (response).
Beta cells, also located in the pancreas, increase production of insulin.
Insulin, a hormone (message) stimulates uptake of glucose by cells
(effectors). As glucose is absorbed by the cells and converted into glycogen
stores, the level of glucose in the blood decreases (response).
BLOOD GLUCOSE – FALL IN BLOOD GLUCOSE
Concentrations of blood glucose less than 3.6 mmol/L(stimulus) are detected
by the pancreas(receptor). Beta cells in the pancreas decrease production of
insulin (message) which stimulates the uptake of glucose by cells (effectors).
As a result, the level of glucose in the blood increases, due to a decrease in
the amount of glucose being absorbed by cells (response).
Alpha cells increase production of glucagons which stimulates the breakdown
of glycogen stored in the liver and skeletal muscles (effectors) into glucose,
which is then released into the blood. As glucose is released into the blood,
the level of glucose in the blood increases (response).
WATER BALANCE – the water concentration within an organism may be
detected via two different stimuli. Firstly, the hypothalamus is capable of
detecting a change in osmotic pressure, due to changes in the concentrations
of salts in the body. High osmotic pressure due to an increase in salt
concentration (decreased water concentration) is detected by the
hypothalamus which stimulates the release of vasopressin, a hormone, from
the pituitary gland. Vasopressin causes the distal tubules of the nephrons in
the kidney to become more permeable to water, thus allowing more water to
be reabsorbed from the filtrate into the bloodstream. Secondly, blood
volume, which affects blood pressure, may be detected by pressure receptors
in the kidney. The kidneys then increase production of renin, an enzyme,
which stimulates the production of a hormone aldosterone, in the adrenal
cortex. Aldosterone stimulates active transport of sodium ions back into the
blood from the kidney filtrate, which leads to increased reabsorption of water
from the nephron tubules.
These factors combined will increase the
concentration of water within the body, by decreasing the concentration of
water excreted in the urine of the organism.
The increase in water
concentration will decrease the concentration of salts and therefore osmotic
pressure, and increase blood volume and therefore pressure.
CORE BODY TEMPERATURE –a stable core body temperature will be
maintained at the expense of factors under regulation. In extreme heat, an
organism may sweat to the point of dehydration, in extreme cold, circulation
to extremities such as fingers and toes ceases in order to reduce heat loss
and maintain a stable core temperature.
FALL IN CORE BODY TEMPERATURE – is detected by the hypothalamus,
which sends messages via neurohormones to the thyroid gland, which
stimulate the production of thyroxin. Thyroxin is then released into the
bloodstream where it increases the organism’s metabolic rate. The process
of cellular respiration generates heat, thus negative feedback occurs.
Messages are also sent via neural impulses to sweat glands stimulating a
decrease in sweat production to minimize heat loss due to evaporation of
sweat, and to blood vessels, which constrict peripheral circulation, to
minimize heat transfer from blood to the cooler air.
RISE IN CORE BODY TEMPERATURE – is detected by the hypothalamus,
which decreases production of neurohormones responsible for stimulating the
production of thyroxin. Production of thyroxin is therefore decreased which
decreases the organism’s metabolic rate. The process of cellular respiration
generates heat, thus a decrease in cellular respiration would decrease the
amount of heat produced. Messages are also sent via neural impulses to
sweat glands stimulating an increase in sweat production to maximise heat
loss due to evaporation of sweat, and to blood vessels, which dilate aterioles
to increase peripheral circulation, to maximise heat transfer from blood to
the cooler air.
BLOOD PRESSURE – a decrease in blood pressure is detected by receptors in
blood vessel walls. The drop in pressure is then conveyed to the central
nervous system which sends neural impulses to the heart to increase the
heart rate. Neurohormones are also released by the CNS which stimulates
the constriction of arterioles which increases resistance to the blood flow,
thus increasing the pressure.