The human circulatory system Part 4.
What substances are in the blood?
What do they do? Explaining the functions of red blood cells, white blood
cells, plasma & platelets
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of notes on human circulatory system: heart, lungs & blood vessels
4. What substances are in the blood? What do they do?
Blood is considered a liquid tissue (unique in your body!)
Blood is a mixture of red blood cells, white blood cells,
platelets, and plasma, which is a straw coloured fluid (when
separated from red blood cells, mainly of water, but containing solutes (dissolved
Know the structure and function of the following
parts of the blood.
Red blood cells
Red blood cells carry oxygen from the lungs to
every living cell of the body via the blood stream.
It's the red blood cells that give blood
its red colour and unlike white blood cells, they have no nucleus.
Red blood cells
are a squashed from both sides 'doughnut' shape (biconcave disc) to give them a
large surface area for the iron
containing haemoglobin molecules to capture the oxygen.
The large surface area gives efficient
absorption of oxygen to combine with the haemoglobin molecules
by the reversible reaction:
oxyhaemoglobin (bright red
The oxygen molecule is weakly bonded to
the haemoglobin molecule by a reversible reaction, so it
is easily released to be consumed in respiration.
A high oxygen concentration favours the
formation of oxyhaemoglobin (reaction moves to right), and low
oxygen concentration favours its dissociation to free oxygen
(reaction moves to left) - this follows from Le Chatelier's
Principle you learning in chemistry about reversible
reactions and chemical equilibrium. See GCSE chemistry notes:
Reversible reactions and
blood cells do not have a nucleus, allowing more space for
Note the four red blood cell adaptations:
(i) the haemoglobin molecule is adapted to
carry the oxygen molecule O2 for cellular respiration
in all the body's tissues, the complex formed is called
(ii) the biconcave disc shape gives a large surface area
/ volume ratio for absorbing oxygen molecules - increases
efficiency of diffusion of oxygen in and out of the red blood cells and
reduces distance to centre of cell,
(iii) no nucleus - extra space
for oxygen carrying haemoglobin molecules - nucleus not required,
(iv) red blood
cells are very small and flexible and can easily pass through tiny
In the lungs the oxygen molecule attaches
itself to the iron atom at the centre of the large complex
haemoglobin molecule to form oxyhaemoglobin.
The process is reversed in
the body tissues to release oxygen for cell respiration (equation above).
If we don't have
enough iron in our diet we can suffer from anaemia, a potentially
serious condition where the blood can't carry enough oxygen needed for
all the respiration demands of the cells.
White blood cells
White blood cells have a nucleus and are part of the body's
immune system to fight diseases.
Some white cells can change shape to engulf
and destroy potentially harmful microorganisms.
These cells are called phagocytes
and their action is called phagocytosis.
Other types of white blood cells called lymphocytes
and have variety of functions.
Type B lymphocytes produce antibodies
(type of protein) to inhibit and fight the action of invasive
microorganisms e.g. harmful bacteria.
Some white blood cells produce antitoxins to counteract
the effect of
toxins produced by microorganisms.
When your body is subjected to an infection,
your white cells multiply to fight it - giving you a high white
blood cell count.
Measurement of your white blood cell count is
an important diagnostic indicator of the state of your body's
Having a low white blood cell
count increases your risk of infection, but having a very high white
blood cell count may mean you have an infection or even a more
serious condition like a blood cancer e.g. leukaemia.
For more on fighting pathogens-infections see
Keeping healthy - communicable diseases -
fighting pathogen infections
Platelets do not have a nucleus.
Blood platelets are ever present small
fragments of cells that help to clot blood and cover over an open
wound in tissues e.g. cut in the skin.
This reduces blood loss from bleeding AND
prevents potentially harmful bacteria (microorganisms) getting into your body via the
If this didn't happen, blood would just keep
pouring out of your body!
If your platelet concentration is low you may suffer
from excessive bleeding (blood loss) and bruising and require
The liver is one of the most important sites for the
assimilation of amino acids to synthesise many different proteins
including plasma proteins, e.g. fibrinogen protein is essential
for blood clot formation, wound healing, and blood vessel growth in
response to injury, infection and inflammation..
Blood plasma is the straw coloured (pale
yellow liquid fluid that carries everything in the blood, so its
the major transportation fluid in the body.
Plasma looks straw coloured when separated
from the red blood cells.
The constituents of blood can be separated
in a centrifuge - high speed rotation of a container (of liquid
mixture), that separates substances out by sedimentation, with the
most dense material moving the most outwards in the radial
About 55% of blood is plasma, which itself
is 90% water.
Blood plasma carries everything else needed for
every cell in the body:
red cells (oxygen), white cells (and the antibodies and antitoxins
they produce) and
(ii) soluble nutrients e.g. amino acid and sugar
products from the digestive
system - the soluble products of digestion from the gut,
(iii) waste products (carbon dioxide from
the organs to the lungs, urea from the
liver to the kidneys where it is removed in urine),
(iv) proteins for enzymes and tissue
(v) control system hormones ('chemical
messengers') from the glands to
their target and activate the functions of organs.
Summary of learning objectives and key words or phrases
Know what substances are in blood and their function
including red blood cells, white blood cells, plasma and formation and
function of platelets.
Know and understand that blood is a tissue and
consists of a fluid called plasma in which red blood cells, white blood
cells, and platelets are suspended.
Blood is considered an irregular
tissue because it doesn't physically support or connect things but it does
involve groups of cells performing particular functions.
Plasma is pale straw-coloured
liquid which transports everything around the body
What is the function of blood
plasma? .... What does it do? ...
Know and understand
that blood plasma
carbon dioxide from the organs to the lungs,
of digestion from the small intestine to other organs eg glucose, amino
acids, mineral salt ions etc.
urea from the liver
to the kidney prior to excretion in urine,
hormones which control the
function of various organs and their associated chemical processes in the
and obviously, as already
mentioned, red blood cells, white blood cells and platelets are constantly
being carried around the body as well associated oxygen as oxyhaemoglobin
and antibodies and antitoxins produced by the white blood cells.
Know and understand that red blood cells transport
oxygen from the lungs to the organs.
What do red blood cells do? What
is their function? Know that red blood cells have no nucleus but a large
surface area to chemically absorb oxygen.
Know that red blood cells are packed with a red pigment called haemoglobin
which readily combines with oxygen
Know that in the lungs haemoglobin
combines with oxygen to form oxyhaemoglobin.
Know that in other organs and
all body tissue oxyhaemoglobin splits up into
haemoglobin and oxygen for cell respiration.
Know that white blood cells have a nucleus.
What do white blood cells do? What is their function?
Know that white blood cells form part of
the body’s defence system against microorganisms eg harmful bacteria.
White blood cells can attack and
destroy harmful 'foreign' microorganisms.
White blood cells can produce
antibodies to fight microorganisms.
White blood cells produce
antitoxins to combat the effect of waste toxins produced by microorganisms.
Know that platelets are small fragments of cells.
Know that platelets have no
Know and understand that platelets help blood to clot at the site of
a wound and a lack of platelets is potentially dangerous from excessive bleeding
The clotting action of platelets
allows a skin to form over a wound which hardens into a scab and this
prevents infection of the wound by harmful bacteria or any other
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