This document discusses biology concepts related to communication and homeostasis. It covers 3 main topics: 1) the need for cellular communication to maintain internal conditions, 2) examples of neuronal and hormonal communication systems that allow organisms to respond to environmental changes, and 3) the process of homeostasis, including negative feedback loops that help regulate internal conditions. Key points are stimulus-response relationships, mechanisms of signal transmission between cells, and the role of communication systems in coordinating responses to keep internal environments within safe ranges despite external fluctuations.

1. Biology – Communication, Homeostasis, Energy
Module 1 – Communication and Homeostasis
1) THE NEED FOR COMMUNICATION
Keeping Cells Active – enzymes need a specific set of conditions in which to work
efficiently, and cellular activities rely on the actions of enzymes. Therefore, all living
things must maintain certain conditions inside their cells, including:
- A suitable temperature
- A suitable pH
- An aqueous environment, containing substrates and products
- Freedom from toxins and excess inhibitors
Stimulus/Response
External Environments – external environments will change, which will place a stress
on the organism, so the organism must monitor these changes, and change its
behaviour or physiology to remain active and survive.
Stimulus – environmental change
Response – how the organism changes its behaviour/physiology
The environment may change gradually/slowly or more quickly, but either way, the
organism must respond to the change.
Internal Environments – cells in most multi-cellular organisms are bathed in tissue
fluid, the internal environment.
The internal environment changes due to cellular activities (metabolic reactions)
which use up substrate and produce product. If some products build up, they can
disrupt the action of enzymes, i.e. by altering the pH. So excess product creates a
STIMULUS and a RESPONSE can be to reduce cellular activity.
The tissue fluid’s composition – the internal environment – is maintained by the
blood. Any waste/toxins/excess product will be carried away and excreted.
Co-ordination – a communication system requires:
- A mechanism to detect change
- A mechanism to process information about change
- A mechanism to react to the change
A good communication system will:
- Cover the whole body

2. - Enable cells to communicate with each other
- Enable specific communication
- Enable rapid communication
- Enable both short-/long- term responses
Cell Signalling – messages sent between cells in order to co-ordinate cellular
functions.
There are two major systems for this:
- Neuronal: interconnected neurones, signal to each other across
synapses
- Hormonal: hormone released by organ, stimulates target cells
Neuronal Hormonal
Signals are sent through electrical Signals through hormones and chemicals
impulses
Rapid response to stimuli Slower-acting response to stimuli,
excluding adrenaline
Short-time lift Longer-term responses
Not incremental – all or nothing Incremental – can control how much/how
little
External stimuli from sensory receptors Internal stimuli by binding of chemicals to
receptors on cells
1) HOMEOSTASIS AND NEGATIVE FEEDBACK
Homeostasis – the maintenance of a constant, internal environment at a set point
(norm) regardless of external changes. It involves a series of receptors, effectors and
negative feedback control mechanisms.
Stimulus -> Receptor -> Pathway (cell signalling) -> Effector -> Response
Positive Feedback – this is less common, as it initiates a response to increase the
initial change, therefore, it is usually harmful, i.e. temperature falling leads to further
fall in temperature.

3. 2) MAINTAINING BODY TEMPERATURE – ECTOTHERMS
The need to maintain body temperature – enzymes are globular proteins, with 3D
shapes specific to their function. Heat can denature proteins, or low temperatures
can lead to fewer collisions of enzyme and substrate, leading to less enzyme-
substrate complexes forming, etc. Therefore, the level of metabolic activity is highly
dependent on body temperature.
Ectotherms – an organism that relies on external sources of heat to regulate its body
temperature.
Advantages:
- Use less food in respiration, as they do not require respiration to
maintain body heat, so they need to find less food and can survive
for up to two weeks without meals.
- They can grow larger, as energy used for respiration in endotherms
can be redirected towards growth.
Disadvantages:
- They are less active at lower temperatures, so they need to warm
up before becoming active, leaving them at risk of predation
- They may require hibernation during winter, so they must be able to
store sufficient stores of energy
Temperature Regulation – when an ectotherm is cold, it must change its behaviour
or physiology to increase the absorption of heat. When an ectotherm is hot, it must
change its behaviour or physiology to decrease the absorption of heat.
1) MAINTAINING BODY TEMPERATURE – ENDOTHERMS
Endotherms – organisms that can use internal sources of heat to maintain their body
temperature
Advantages:
- Constant body temperature, whatever external temperature
- Activity possible when external temperature is cool
- Ability to inhabit colder parts of the planet
Disadvantages:
- Cold temperatures demand more energy from food, in order to
create heat through exergonic reactions, i.e. metabolism
- More food required in general
- Less energy from food can be used for growth

4. Temperature Regulation:
Sweat: secrete more from glands if hot, but less if it is cold. Heat evaporates water
from the skin, using latent heat.
Lungs, mouth, nose: panting increases evaporation of water from moist surfaces,
using latent heat.
Hairs on skin: raise when cold, to trap a layer of insulating air, but they lie flat to the
skin if cold.
Arterioles: dilate if hot, and constrict if cold, in order to control flow of blood to the
skin, where heat is lost by convection and radiation.
Liver cells: low temperatures stimulate increased rate of metabolism, however, high
temperatures stimulate a decrease in the rate of metabolism.
Skeletal muscles: if the temperature is low, there will be spontaneous contractions of
muscles (shivering) which will generate heat as the muscle cells will have to respire
more. The opposite applies at high temperatures, i.e. there is no shivering, so no
more respiration in muscle cells.
1) SENSORY NERVES
Sensory Receptors – sensory receptors are specialised cells that can detect
changes to the external environment. They are energy TRANSDUCERS, meaning
they convert energy from one form to another. The changes in the external
environment can be: light levels, pressure on the skin, or many others. Changes in
energy levels in the external environment create a STIMULUS. The specialised
receptors transduce the energy from the stimulus into electrical energy (an
IMPULSE).
Generating Nerve Impulses
Changing Membrane Permeability – neurones have specialised channel proteins
across their membranes, which are specific to either potassium or sodium ions. Also,
they possess a “gate,” which can open or close the channel, thus changing the
permeability of the membrane to a particular ion. The channels are usually kept
closed.
Polarisation – in addition, nerve cells contain carrier proteins (sodium/potassium ion
pumps) that actively transport sodium ions OUT of the cell, and potassium ions INTO
the cell (remember this by Next of Kin). More sodium ions are actively transported
out of the cell than potassium ions are transported into the cell. This creates a
negatively charged intracellular environment, thus POLARISING the plasma
membrane.
Depolarisation – nerve impulses are created by altering the permeability of the
membrane to sodium ions. The sodium ion channels open and move across the
membrane down a concentration gradient (diffusion); this creates a change in the
charge across the membrane, and the inside of the cell becomes less negatively
charged. The cell surface membrane becomes DEPOLARISED.

5. Generator Potentials – receptors cells respond to changes in the environment,
opening the gated sodium ion channels, allowing sodium ions to diffuse across the
membrane. A small change in potential, for example if one or two sodium channels
open, creates a GENERATOR POTENTIAL. The larger the change to the external
environment (the stimulus), the more gated channels will open. If enough sodium
ions enter the cell, the charge will change significantly, initiating an impulse or
ACTION POTENTIAL.
Sensory and Motor Neurones
- Sensory neurones carry the action potential from a sensory
receptor in the peripheral nervous system (PNS) to the central
nervous system (CNS).
- Relay neurones connect sensory and motor neurones together in
the CNS.
- Motor neurones carry an action potential from the CNS, back to the
PNS, to an effector such as a muscle or gland.
A motor neurone A sensory neurone
The function of a neurone is to transmit an impulse from one area of the body to the
other, so neurones all have a very similar structure and are specialised to their
function with certain features:
- Very long so that they can transmit over a long distance.
- The plasma membrane has gated ion channels that control the
entry/exit of sodium, potassium and calcium ions.
- They have sodium/potassium ion pumps, allowing them to pump
ions in and out of the cell respectively, through using ATP. This
maintains a polarised cell membrane.
- They are surrounded by fatty, insulating Schwann cells to insulate
the neurone from the electrical activity of nearby cells.
- Their cell body contains a nucleus, many ribosomes and many
mitochondria.
- Motor neurones’ cell body is in the CNS, with a long axon that
carries an action potential out to the effector.

6. - Sensory neurones’ long dendrites carry the action potential from a
sensory receptor to the cell body, positioned just outside the CNS.
1) RESTING POTENTIALS AND ACTION POTENTIALS
Resting Neurones – when sodium/potassium ion pumps are pumping ions across the
membrane at a ratio of 3:2, using ATP, the interior of the cell is maintained at a
negative potential compared to the outside. The resting potential is a potential
difference across the membrane of -60mV.
Action Potential - if some of the sodium ions channels are opened then sodium ions
will quickly diffuse down the concentration gradient into the cell, causing
depolarisation of the membrane. In the GENERATOR REGION of the receptor cells,
energy changes from the external environment opens the gated channels, allowing
sodium ions to diffuse across the membrane. Gates further along the neurone are
opened by the changes in the charge across the membrane. These channels are
called VOLTAGE-GATED CHANNELS, as they respond to the depolarisations of
the membrane.
Resting
Potential
Depolarisation
Threshold Potential – if the depolarisation of the membrane is great enough, it will
reach THRESHOLD POTENTIAL, and it will begin to have an effect on the nearby
voltage-gated channels, opening them. This causes a large influx of sodium ions
diffusing down their concentration gradient. If the depolarisation reaches +40mV, an
action potential is produced, and is then transmit to the very end of the neurone.

7. Ionic Movements – an action potential is
basically a set of ionic movements. The
ions move across the cell
membrane when the correct
channels are opened.
The graph above shows the
stages:
1) The membrane begins in its resting state – i.e. it is polarised at a
potential difference of -60mV
2) Sodium ions channels open and sodium ions diffuse into the cell down
a concentration gradient.
3) The membrane, therefore, depolarises, becoming less negative
compared to the extracellular environment, eventually reaching a
threshold potential of -50mV.
4) Voltage-gated sodium ions channels open and many more sodium ions
flood in. As more sodium ions enter, the cell becomes positively
charged inside the cell.
5) When the charge across the membrane reaches +40mV, the inside of
the cell is positive compared with the outside.
6) At this point, sodium ion channels close and potassium ion channels
open.
7) Through potassium ions diffusing out of the cell, the potential difference
becomes more negative compared with the outside –
REPOLARISATION.
8) The potential difference becomes too negative momentarily, known as
HYPERPOLARISATION.
9) The original potential difference is restored through the actions of
sodium/potassium ion pumps, returning to its resting potential state.
For a short time after an action potential, it is impossible to stimulate the cell
membrane to reach another action potential: a REFRACTORY PERIOD, which also
allows the cell to recover after an action potential, ensuring that action potentials are
only transmitted in one direction along the axon.
1) TRANSMITTING ACTION POTENTIALS

8. A stimulus must be intense enough to reverse the polarity of the membrane – the all
or none effect, where a stimulus threshold has to be reached before an impulse
arises.
1) Depolarisation of an axon membrane following the stimulus opens
sodium voltage-gated channels. Na+ enters the axon, and positive
feedback occurs, as more Na+ channels opening, allowing more
sodium ions to cross the membrane into the neurone.
2) As the sodium ions diffuse across the membrane, the membrane
becomes reverse polarised, with the intracellular environment
becoming more positively charged than the extracellular. The potential
difference is +40mV.
3) The production of an action potential causes the flow of a small,
localised electric current, which depolarises the adjacent membrane.
This is caused by the Na+ ions flowing away from the positively
charged area in the axon. This opens the voltage-gated sodium
channels in the adjacent membrane, so more sodium ions diffuse in,
creating an action potential in this membrane thus stimulating adjacent
membranes, etc. As the process repeats, this is how the action
potential is transmitted down the axon.
The Myelin Sheath
The myelin sheath is an insulating layer of fatty material. Sodium and potassium ions
cannot diffuse through it, meaning the ionic movements that transmit an action
potential along the neurone cannot occur over much of the length of the neurone.
This is why there are gaps between the Schwann cells, known as Nodes of Ranvier.
Therefore, the ionic movements can only take place at these Nodes of Ranvier. In
myelinated neurones, the local currents must be elongated and sodium ions diffuse
along the axon from one node of Ranvier to the next. This is known as SALTATORY
CONDUCTION, as it appears that the impulse “jumps” from one node of Ranvier to
the next.
This is advantageous as a non-myelinated neurone cannot transmit an action
potential as quickly as a myelinated neurone’s rate of 120m/s. Therefore, myelinated
neurones are important in co-ordinating reflex reactions and quickly transmitting
impulses from sensory receptors.
1) NERVE JUNCTIONS
A synapse is a junction, around 20nm wide, between two neurones, which allows
substances known as neurotransmitters to diffuse across the SYNAPTIC CLEFT to
create a new action potential in the POST-synaptic neurone.
The presynaptic neurone ends in a “swelling,” known as the synaptic knob. It
contains many mitochondria (required as synaptic transmission is an active process,
requiring ATP), a large amount of endoplasmic reticulum (as vesicles are required),
neurotransmitter (which diffuses across the synaptic cleft) and voltage-gated calcium
ion channels in the presynaptic membrane.

9. The process of synaptic transmission begins when an action potential arrives at the
synaptic knob of the PRE-synaptic neurone. This action potential opens voltage
gated CALCIUM ion (Ca2+) channels in the membrane to open, allowing the
facilitated diffusion of calcium ions into the synaptic knob across the presynaptic
membrane. These CALCIUM ions cause vesicles containing neurotransmitter
(usually acetylcholine), to move to and fuse with the presynaptic membrane, where
they are released by exocytosis.
The neurotransmitter diffuses across the SYNAPTIC CLEFT towards receptor sites
on the sodium ion channels in the POST-synaptic membrane, causing these sodium
ion channels to open.
Sodium ions diffuse down a concentration gradient, from an area of higher
concentration to an area of lower concentration into the POST-synaptic neurone.
This creates a generator potential, also known as excitatory postsynaptic potential
(EPSP) in the POST-synaptic neurone. If the generator potential reaches a threshold
potential, then an action potential is created in the POST-synaptic neurone.
ACETYLCHOLINESTERASE is an enzyme present in the synaptic cleft. It
hydrolyses the neurotransmitter acetylcholine into ethanoic acid and choline. This
stops the constant transmission of impulses in the postsynaptic neurone, as it
prevents acetylcholine from binding to receptor sites on the postsynaptic neurone,
and so on. The ethanoic acid and choline diffuse back into the synaptic knob, where
they are recycled back into acetylcholine using ATP from respiration. The recycled
acetylcholine is stored in synaptic vesicles for future use.
2) SIGNALS AND MESSAGES
An action potential is an all or nothing response. Once an action potential begins, it
will be transmitted all the way to the end of the neurone; it does not vary in size or
intensity.
The Role of Synapses
Several presynaptic neurones may converge into one postsynaptic neurone after
neurotransmitters diffuse across the synaptic cleft, creating the same response. This
could be useful where several different stimuli are warning us of danger, for
example.
One presynaptic neurone might diverge into several postsynaptic neurones, allowing
one signal to be transmitted to several parts of the body. This is useful in the reflex
arc: one postsynaptic neurone elicits the response while another informs the brain.
Synapses ensure that signals are transmitted in the correct direction.
Synapses can filter out any unwanted, low-level signals. Even if a low-level stimulus
creates an action potential in the presynaptic neurone, it is unlikely to pass across a
synapse to the next neurone because several vesicles containing neurotransmitter
must be released to create an action potential in the postsynaptic neurone.

10. Low-level signals can be amplified by a process called SUMMATION. If a low-level
stimulus is persistent it will generate several successive action potentials in the
presynaptic neurone. The release of many vesicles of acetylcholine over a short
period of time will enable the postsynaptic generator potentials to combine together
to produce an action potential. The same can happen when a little acetylcholine is
released from many presynaptic neurones converging on one postsynaptic neurone.
Acclimatisation – after repeated stimulation, a synapse may run out of vesicles
containing neurotransmitter. A synapse in this state is said to be fatigued. The
means the nervous system no longer responds to the stimulus, explaining why we
become acclimatised to a background noise or persistent smell. It may also prevent
overstimulation of an effector organ, preventing damage from overstimulation.
The Frequency of Transmission – when a stimulus is at higher intensity, the
sensory receptor will produce more generator potentials. This causes more action
potentials in the sensory neurone. When this arrives at a synapse, it causes the
exocytosis of more vesicles. Our brain can determine the intensity of the stimulus
from the frequency of signals arriving. A higher frequency of signals means a more
intense stimulus.
Myelinated and Non-Myelinated Neurones
Around one third of peripheral neurones are myelinated. The remainder and the
neurones in the CNS are not myelinated. Action potentials in non-myelinated
neurones travel as a wave, rather than jumping from node to node as seen in
myelinated neurones. Myelinated neurones can transmit an action potential much
faster than a non-myelinated neurone, 120m/s compared to 2 – 20 m/s. Myelinated
neurones tend to carry impulses over a long distance, up to 1m, whereas non-
myelinated neurones tend to carry impulses over a short distance. This is because
stimuli must be responded to quickly in comparison to carrying out responses such
as breathing and digestion.
3) THE ENDOCRINE SYSTEM
Signalling Using Hormones – the endocrine system uses the bloodstream to
transport its signals. Endocrine glands secrete hormones into the blood stream,
allowing them to travel to any cell in the body to initiate a response, so long as it has
a complementary receptor. The glands are a ductless group of cells that produce the
hormones from organic compounds, usually proteins, in small quantities.
Endo-/exo- crine – endocrine glands are ductless, meaning their hormones are
secreted directly into the bloodstream, travelling to a distant organ/tissue effector.
Exocrine glands have ducts into which the hormone is secreted. The duct carries the
secretion to another please, i.e. saliva flows along a duct from the saliva glands to
the mouth. Or the tear duct carries tears from the gland to the eye.
Targeting the Signal – hormones can only bind to cells with complementary
receptors to the hormone’s 3D shape. Therefore, certain hormones can only affect
TARGET CELLS that are usually grouped together to form TARGET TISSUES. If

11. every cell has a complementary receptor to a hormone, then the hormone can cause
a response in every cell in the body. However, if only a certain target tissue has
complementary receptors, then a very specific response can be initiated.
The Action of Adrenaline – adrenaline is unable to enter target cells, as it is an
amino acid derivative and therefore not a steroid hormone. The adrenaline molecule
binds to a complementary receptor on the plasma membrane’s extracellular side,
which has enzyme (ADENYL CYCLASE) associated with it on the intracellular side.
As adrenaline in the blood – the first messenger – binds to the plasma membrane
receptor, the enzyme adenyl cyclase is activated. This enzyme is responsible for
converting ATP to cyclic AMP or cAMP. The cAMP is the second messenger in the
cell. The effect of cAMP is to activate specific enzyme action inside the cell.
The Functions of the Adrenal Glands – there are two adrenal glands, lying above
the kidneys on either side of the body. Each gland has a MEDULLA REGION and a
CORTEX REGION.
The Adrenal Medulla – the cells in the medulla specialise in manufacturing the
hormone adrenaline, which is released in response to pain, shock or environmental
stresses. Most cells in the human body contain an adrenaline receptor, allowing its
effect to be widespread. Its effects include:-
- Relaxing of the smooth muscle in the bronchioles.
- Increasing stroke volume of the heart
- Increasing heart rate
- Raising blood pressure by general vasodilation
- Stimulating the conversion of glycogen to glucose
- Dilating the pupils in the eyes
- Increasing mental awareness an improving perception
- Inhibiting the action of the gut, i.e. digestion
- Causing body hair to stand erect.
The Adrenal Cortex – the adrenal cortex manufactures steroid hormones using
cholesterol, which have various roles in the body.
Mineralcorticoids help to control the concentrations of sodium and potassium in the
blood.
Glucocorticoids, also known as cortisol, help to control the metabolism of
carbohydrates and proteins in the liver.
Advantages of the Endocrine System:
1) Process can occur over a long period of time
2) Hormones can reach a wide range of organs to give a full body
response
3) A small amount of chemical can cover a large area
4) A slow response allows gradual increase/decrease in the response
1) THE REGULATION AND HOMEOSTATIC CONTROL OF BLOOD
GLUCOSE LEVELS

12. The Pancreas is the organ of interest in this topic. It lies just below the stomach, and
is an organ that can utilise both endocrine and exocrine functions, i.e. is has glands
with ducts and glands without.
Secretion of Enzymes – the majority of the cells in the pancreas manufacture and
release digestive enzymes. The cells responsible for this function are found in
groups surrounding tubules, which then combine to form the pancreatic duct. This
carries the pancreatic fluid, which contain the enzymes, to the small intestine. The
fluid contains amylase, trypsinogen (and inactive protease) and lipase, all
enzymes necessary for digestion. The fluid is alkaline, as it contains sodium
hydrogencarbonate, in order to neutralise the acidic contents of the small intestine
which have just left the acidic environment of the stomach. This process is exocrine.
Secretion of Hormones – the ISLETS OF LANGERHANS are responsible for the
secretion of hormones from the pancreas. They contain different types of cells,
namely α-cells and β-cells. α-cells manufacture and secrete glucagon, when blood
sugar becomes too low, and β-cells manufacture and secrete insulin, when blood
sugar is too high. As the hormones are secreted directly into the numerous
capillaries surrounding the Islets of Langerhans, this process is endocrine.
Control of Blood Glucose – this is important, as it is the principle respiratory
substrate, and also because brain cells cannot metabolise any other metabolites. As
well as this, glucose exerts an osmotic pressure on the blood and tissue fluids,
ensuring the cells do not haemolyse or become crenated.
Levels of Glucose Rise Too High:-
- The high levels of blood glucose is detected by the beta-cells in the
Islets of Langerhans.
- The beta-cells then secrete insulin into the surrounding capillaries,
which travels to the liver cells – hepatocytes – muscle cells and
some other body cells, such as some brain cells.
- The cells that the insulin travels to contain receptors
complementary to the 3D shape of insulin, where insulin binds as
the first messenger.
- The binding of insulin results in the activation of the adenyl cyclase
enzyme, converting ATP to cAMP which subsequently activates
other enzymes in the cell, leading to the following responses:
1) More glucose channels are placed into the cell surface
membrane
2) More glucose enters the cell
3) Glucose in the cell is converted to glycogen for storage
4) More glucose is converted to fat
5) More glucose is used in respiration
This, therefore, causes the reduction of blood glucose concentration.

13. Levels of Glucose Too Low:-
- The low concentration of blood sugar is detected by the alpha-cells,
which secrete the hormone glucagon in response.
- The hormone then travels to the hepatocytes in the blood, after
being secreted directly into the bloodstream.
- The glucagon then binds to a complementary receptor in the
plasma membrane of the hepatocytes, initiating a second
messenger which initiates the
following responses by
activating enzymes:
1) Conversion of glycogen to
glucose
2) Use of more fatty acids in
respiration
3) The production of glucose by
conversion from amino acids
and fats
1) REGULATION OF INSULIN
LEVELS
Insulin brings about the effect of reducing blood sugar concentrations, therefore, it is
important to be able to control how much insulin in the blood when the blood sugar
concentration returns to its set point, or norm.
The control of insulin secretion:
- The cell membrane of beta-cells contain both potassium and
calcium ion channels.
- The potassium ion channels, under normal conditions, are open
and the calcium ion channels closed. There is a potential difference
across the cell membrane of around -70mV, as potassium ions
diffuse out of the cell, making the intracellular space negatively
charged.
- When glucose concentrations in the surrounding tissue fluid are
high, it quickly diffuses across the membrane.
- It is metabolised (respired) to produce ATP.
- This excess ATP is causes the potassium ion channels to close,
causing the potential difference across the membrane to become
less negative.
- This change in voltage opens the voltage-gated calcium ion
channels, allowing calcium ions to diffuse into the cell down a
concentration gradient.
- The calcium ions cause vesicles containing insulin to move to and
fuse with the cell surface membrane, releasing insulin into the
tissue fluid and therefore blood stream by exocytosis.

14. Type I Diabetes Mellitus is also known as insulin-dependent diabetes, as the body
is no longer able to produce sufficient insulin in the beta-cells of the islets of
Langerhans in the pancreas. It also cannot store excess glucose as glycogen, so the
excess glucose must be excreted in the urine. It is believed to be the result of an
auto-immune attack on the body’s own beta-cells, however, it could also be the
result of viral infection.
Type II Diabetes Mellitus is also known as non-insulin-dependent diabetes, as the
body can still produce insulin, however, the hepatocytes are no longer as responsive
to it. This is thought to be caused by the specific receptors on the surface of the liver
and muscle cells declining, thus losing their ability to respond to insulin in the blood.
Certain factors increase your risk of contracting type II diabetes:
- Obesity
- A diet high in sugars, particularly refined
- Being of Asian or Afro-Caribbean descent
- Family history of the illness
Type II diabetes is generally treatable with close monitoring and control of diet,
where carbohydrate intake and use are matched. However, this may eventually
require to be supplemented by insulin injections or drugs that absorb glucose from
the digestive system.
Type I diabetes, generally the more serious form, is treated using insulin injections.
The blood glucose levels must be monitored, generally done by using urine tests,
and the correct dose of insulin must be administered to ensure blood glucose
concentrations remain stable and the patient does not become hypo- or hyper-
glycemic.
Insulin used in insulin injections used to be extracted from the pancreas of pigs,
however, bacteria have been genetically engineered (reverse transcriptase) to
produce human insulin. There are certain advantages to this type of insulin:
- It matches human insulin exactly, so it is faster acting and thus
more effective.
- There is less chance of developing a tolerance to human insulin.
- There is less chance of rejection due to an immune response.
- There is a lower risk of infection.
- It is cheaper to manufacture and the process of manufacture is
more adaptable to demand.
- People will not have moral objections to using humalin compared to
insulin from animals.
1) CONTROL OF HEART RATE IN HUMANS
The human heart is involved in pumping blood around the circulatory system in
blood vessels, supplying cells with the necessary substrates and substances
required for their activities. It also removes the waste products of these cells’

15. activities, such as urea, CO2, etc. Therefore, the requirements of blood by these
cells vary according to their level of activity. For example, if you are running, you
need more glucose and oxygen due to a higher level of respiration of muscle cells. It
is important, then, for the heart to meet the requirements of the body cells.
The heart can adapt to the varying needs of cells by:
- An increase in the number of beats per minute, known as an
increase in heart rate.
- An increase in the strength of its contraction.
- An increase in the volume of blood pumped per beat, known as the
stroke volume.
How does the body control the heart rate? Heart muscle is myogenic, meaning it
initiates its own contraction, through excitation of the atria walls from an action
potential in the sinoatrial node (SAN). This travels to the atrioventricular node
(AVN) and down the Purkyne fibres to initiate contraction in the ventricles. However,
the SAN is connected to nerves from the medulla oblongata in the brain. While
these nerves do not initiate contraction, they can affect the frequency of contraction.
Action potentials sent down the accelerator nerve of the sympathetic nervous system
increase the heart rate, but action potentials sent down the vagus nerve of the
parasympathetic nervous system reduce the heart rate. The cardiac muscle can also
respond to the presence of adrenaline in the blood stream.
Many factors influence heart rate, due to impulses from around the body from the
ANS arriving at the cardiovascular centre in the medulla oblongata:
- Movement of limbs is detected by stretch receptors in the muscles.
This sends an impulse to the centre informing it that extra oxygen
will be required, tending to increase heart rate.
- CO2 is acidic when dissolved in the blood, so this leads to a lower
blood pH, which is detected by chemoreceptors in the carotid
arteries. These chemoreceptors send impulses to the
cardiovascular centre, eventually increasing the heart rate.