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The Structure and Functions of Blood
Note: Knowledge of the
structure and function of blood and aspects of the heart
and vascular system are part of training in various
therapies, (incl. e.g. Massage, Aromatherapy, Acupuncture,
Shiatsu, etc.). This page is intended to include detail
suitable for introductory courses, and some ITEC Diplomas.
This page is divided into the following sections:
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The Functions of
Blood
(generally - as opposed to the functions of particular
components of blood).
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The Composition of
Blood
(incl. the different types of blood cells and their
properties and functions).
-
Process of Oxygenation
of Tissues due to Circulation of Blood
-
Types of Leucocytes
(White Blood Cells)
1. Functions of
Blood
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 1.
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| Transports: |
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Dissolved
gases (e.g. oxygen,
carbon dioxide); |
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Waste
products of metabolism (e.g. water, urea); |
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Hormones; |
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Enzymes; |
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Nutrients
(such as glucose,
amino acids, micro-nutrients
(vitamins
& minerals),
fatty acids, glycerol); |
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Plasma
proteins (associated
with defence, such
as blood-clotting
and anti-bodies); |
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Blood
cells (incl. white
blood cells 'leucocytes',
and red blood cells
'erythrocytes'). |
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2. |
Maintains
Body Temperature |
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3. |
| Controls
pH |
| The pH of blood must
remain in the range 6.8
to 7.4, otherwise it begins
to damage cells. |
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4. |
| Removes
toxins from the body |
| The kidneys filter
all of the blood in the
body (approx. 8 pints),
36 times every 24 hours.
Toxins removed from the
blood by the kidneys leave
the body in the urine.
(Toxins also leave the
body in the form of sweat.) |
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5. |
| Regulation
of Body Fluid Electrolytes |
| Excess salt is removed
from the body in urine,
which may contain around
10g salt per day
(such as in the cases
of people on western diets
containing more salt than
the body requires). |
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2. Composition of Blood
Blood consists of many components (constituents).
These include: |
55% |
Plasma |
45% |
Components (sometimes called "formed elements"), i.e. 'Blood
Cells'. Of these,
 99% are erythrocytes (red
blood cells)
and 1% are leucocytes
(white blood cells) and thrombocytes
(blood platelets) |
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This is summarised in the following
diagram, and described in further detail
below.
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The summary chart above includes: erythrocytes (red blood cells), thrombocytes (blood platelets)
and leucocytes (white blood cells). It also
includes categories of leucocytes: agranulocytes and granulocytes (also known as polymorphonucleocytes),
which may also be sub-divided into lymphocytes,
monocytes, basophils, neutrophils and eosinophils.
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The following table includes further general
information about the constituents of blood.
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Structure
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Functions
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Plasma |
Normal
blood plasma is 90-92 % water.
This is the straw-coloured
fluid in which the blood cells
are suspended, and consists
of: |
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The
medium in which the
blood cells are transported
around the body (by
the blood
vessels) and are
able to operate effectively. |
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Helps
to maintain optimum
body temperature throughout
the organism. |
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Helps
to control the pH of
the blood and the body
tissues, maintaining
this within a range
at which the cells can
thrive. |
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Helps
to maintain an ideal
balance of electrolytes
in the blood and tissues
of the body.
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Dissolved
substances including electrolytes
such as sodium, chlorine,
potassiun, manganese, and
calcium ions; |
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Blood plasma
proteins (albumin, globulin,
fibrinogen); |
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Hormones. |
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Erythrocytes
(Red
blood
cells) |
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Immature erythrocytes
have a nucleus but mature
erythrocytes have no nucleus. |
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Carry oxygen
(process described in more
detail - below). |
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Haem |
Erythrocytes
have a "prosthetic
group" (meaning
"in addition to"
- in this case, in addition
to the cell). The active
component of this prosthetic
group is Haem. |
Haem
relies on the presence
of iron (Fe). |
Haem
combines with oxygen
to form oxyhaemoglobin: |
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...
continued in section
below. |
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Erythrocytes
are eventually broken down
by the spleen into the blood
pigments bilinubin and bilviridin,
and iron. These components
are then transported by the
blood to the liver where the
iron is re-cycled for use
by new erythrocytes, and the
blood pigments form bile salts.
(Bile breaks down fats.) |
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Have a longevity
of approx. 120 days. |
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There are
approx. 4.5 - 5.8 million
erythrocytes per micro-litre
of healthy blood (though there
are variations between racial
groups and men/women). |
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Leucocytes
(White
blood
cells) |
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There
are different types
of leucocytes (described
in more detail - below),
classified as: |
Granular:
e.g. Neutrophils, Eosinophils,
Basophils. |
Agranular (do not contain granules):
e.g. Monocytes, Lymphocytes. |
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Major part
of the immune system. |
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Have a longevity
of a few hours to a few days
(but some can remain for many
years). |
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There are
approx. 5,000 - 10,000 leucocytes
per micro-litre of blood. |
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Trombocytes
(Platelets) |
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Blood platelets
are cell fragments; |
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To facilitate
blood clotting - the purpose
of which is to prevent loss
of body fluids. |
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Approx disk-shaped fragments; |
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Diameter 2-4 um
(1 micro-metre = 1 um = 0.000001m); |
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Have many
granules but no nucleus; |
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Have a longevity
of approx. 5-9 days. |
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There are
approx. 150,000 - 400,000
platelets per micro-litre
of blood. |
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3. The
Oxygenation of Blood
| The oxygenation of blood is the
function of the erythrocytes (red blood cells)
and takes place in the lungs.
The sequence of events of the blood becoming oxygenated
(in the lungs) then oxygenating the tissues (in
the body) is as follows:
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The Right Ventricle (of
the heart) sends de-oxygenated blood to
the lungs. |
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While in the lungs:
1. Carbon Dioxide diffuses out of the blood
into the lungs, and
2. Oxygen (breathed into the lungs) combines
with haemoglobin in the blood as it passes
through the lung capillaries. |
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Oxyhaemoglobin returns to
the heart via the pulmonary vein and then
enters the systemic circulation via the
aorta. |
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There is a low concentration
of oxygen in the body tissues. They also
contain waste products of the metabolism
(such as carbon dioxide). |
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Due to the high concentration
of oxygen in the blood and the low concentration
of oxygen in the tissues, |
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... the high concentration
of carbon dioxide in the tissues diffuses
into the blood. (95% of this carbon dioxide
dissolves in the blood plasma.) |
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Blood returns from the tissues
back to the heart via the superior vena
cava (from the upper-body) and the inferior
vena cava (from the lower-body) |
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4.
Types of Leucocytes (White Blood Cells) |
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| Lymphocytes: |
Monocytes: |
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*Basophils: |
*Neutrophils: |
*Eosinophils: |
| Approx. 24%
of leucocytes are lymphocytes. These produce
anti-bodies and include:
*T-Cells
*B-Cells
*Natural
Killer Cells |
Approx.
4% of leucocytes are monoocytes.
These are
also known as phagocytes.
They combat microbes by the process of phagocytosis. |
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0.5-1%
of leucocytes are basophils.
Diameter 8-10 micro-metres.
Liberate heparin, histamine, and seratonin
in allergic reactions, intensifying inflammatory
response. |
60-70%
of leucocytes are neutrophils.
Diameter 10-12 micro-metres.
Phagocytosis. Destruction of bacteria with
lysozyme and strong oxidants. |
2-4%
of leucocytes are eosinophils.
Diameter 10-12 micro-metres.
Combat the effects of histamine in allergic
reactions;
Phagocytize antigen-antibody complexes; Destroy some parasitic worms. |
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* It is only possible to
observe the differences between these by
staining them.
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Further notes about the types of leucocytes
identified above:
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| Lymphocytes: |
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The term
"antigen" refers to something
that is not naturally present and
'should not be in the body'. |
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T Cells
(lymphocytes) are activated by the
thymus gland. |
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B Cells
(lymphocytes) are activated by other
lymphoid tissue. The 'B' indicates
'bone marrow' cells. |
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Both
T-cells and B-cells:
(1) destroy antigens, and
(2) produce 'memory cells' and anti-bodies. |
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| Basophils: |
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An increased
(higher than usual) percentage of
basophils in the blood may indicate
an inflammatory condition somewhere
in the body. |
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| Neutrophils
& Monocytes: |
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Neutrophils
are the first leucocytes to respond
to bacterial invasion of the body.
They act by carrying out the process
of phagocytosis (see opposite), and
also be releasing enzymes - such as
lysozyme, that destroy certain bacteria.
Monocytes take longer to reach
the site of infection than neutrophils
- but they eventually arrive in much
larger numbers.Monocytes that migrate
into infected tissues develop into
cells called wandering macrophages
that can phagocytize many more microbes
than neutrophils are able to.
Monocytes also clear up cellular debris
after an infection. |
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| Eosinophils: |
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An increased
(higher than usual) percentage of
eosinophils in the blood may indicate
parasitic infection somewhere in the
body. |
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| Phagocytosis:
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A phagocyte is a cell able
to engulf and digest bacteria, protozoa,
cells, cell debris, and other small
particles. Phagocytes include many
leucocytes (white blood cells) and
macrophages - which play a major role
in the body's defence system. |
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Phagocytosis is the engulfment
and digestion of bacteria and other
anigens by phagocytes.
This is illustrated below. |
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This is the end of this article but further information
about blood
vessels, the structure
and functions
of the heart,
systemic circulation, and the vascular system generally
are included on other pages of this website.
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