BLOOD
Function of Blood
Single cell organisms, such as an amoeba, live a fairly independent life. However, cells that are part of
a larger organism require blood to perform various specialised functions as they cannot, for example,
move away from toxic areas, find their own food etc. Blood therefore:
transports oxygen from lungs to tissues
transports carbon dioxide from tissues to lungs
transports nutrients from digestive organs to cells
transports waste products from cells to kidneys, lungs and sweat glands
transports hormones from endocrine glands to cells
regulates body pH
regulates body temperature
regulates water content of cells
prevents body fluid loss
protects against toxins and microbes
Composition of Blood
Blood, which makes up ca 8% of our body weight (4-6 litres), is not a homogenous substance. Its
various components can be separated in a haematocrit formed by centrifugation. It is composed of;
1.
Plasma (55%)
2.
Formed Elements (45%)
a)
b)
c)
Erythrocytes (red blood cells)
Leukocytes (white blood cells)
i)
Granulocytes
Neutrophils (62%)
Eosinophils (2.3%)
Basophils (0.4%)
ii)
Agranulocytes
Lymphocytes (30%)
Monocytes (5.3%)
Thrombocytes (blood platelets)
Blood Plasma
This is 91.5% water. The rest is solutes, most of which are various plasma proteins (e.g. albumins to
create capillary osmotic pressure, globulins to act as carrier molecules and antibodies, fibrinogen
involved in clotting). Other things dissolved in the plasma include; hormones, nutrients, respiratory
gases, waste products, and electrolytes.
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Red Blood Cells (Erythrocytes)
These cells, which compose 99% of the blood’s formed elements,
are anucleate biconcave discs, that can change their shape to
‘squeeze through’ capillaries. The average male has 5,200,000
erythrocytes mm-3 of blood, while females posses on average
4,700,000 mm-3. These very high densities vary depending on a
number of factors such as; age, height you live at, and of course
your state of health (too few = anaemia)
The major function of red blood cells is to transport oxygen.
Each erythrocyte
contains around
280 million
molecules of
haemoglobin for
this purpose. Every
molecule of haemoglobin is composed of a globin
portion bound to 4 haem groups. Oxygen binds to the
iron (Fe) of each haem group (thus each rbc transports
ca. 1 million molecules of oxygen from the lungs to the
tissues). We will talk more about this is the respiration lectures
Haemopoiesis is the formation of blood cells.
In the adult this occurs from stem cells in the red bone marrow which is found in the flat bones of the
body (pelvis, skull, clavicles etc.).
Since there are many types of blood cell, the question is, how many stem cells give rise to these
different types? There are 2 extreme possibilities;
1.
2.
One stem cells gives rise to all blood cell types (monophyletic), or
Each type of blood cells has its own stem cell (polyphyletic)
The answer is in between. There are two classes of stem cell; one giving rise of lymphocytes and the
other to all other types of blood cell . Haemopoiesis is thus a limited polyphyletic system.
There are 5 phases to the development of every blood cell type.
1.
2.
3.
4.
5.
Commitment of the stem cell
Proliferation
Differentiation
Maturation
Release
The formation of red blood cells (erythropoiesis) shows all these phases. Erythrocytes survive ca. 100120 days. Thus, around 5x1011 rbcs are destroyed every day by the spleen .
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The stem cell, known as a haemocytoblast, commits to becoming a rbc and develops into a
proerythroblast (diameter 14-17 µm), which has a large nucleus occupying about 80% of the cell’s
volume. The cell then develops its proliferative phase, forming an early (basophilic) erythroblast.
Eventually this results in a late (polychromatic) erythroblast in which the cell differentiates through
haemoglobin synthesis. Then the cell matures into a normoblast, in which the nucleus only occupies
25% of its volume, before finally ceasing
haemoglobin synthesis and extruding its
nucleus. The resulting reticulocyte is
released from the bone marrow.
Unusually, however, it is still immature and
must circulate in the vessels for ca. 3 days
before becoming fully mature.
Any condition which results in less oxygen
being delivered to the tissues (i.e climbing
a mountain) causes the release of
erythrogenin from the kidney. This
converts one of the plasma proteins to
erythropoietin (EPO), which acts on the
bone marrow to enhance erythropoiesis.
Blood can be classified as belonging to various blood groups depending on the presence of antigens on
the surface of red blood cells. Although
there are many such antigens
(agglutinogens), only 2 groups (ABO
and rhesus) are commonly used
clinically (although others have
relevance in the legal profession)
ABO blood grouping – This involves 2
antigens; A and B. Individuals who
posses the A antigen are said to have
type A blood, those with B antigen are
type B, those with both antigens are AB,
and those with neither antigen are type
O.
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The body of a person carries the antibody (agglutinin) to the antigen they do not posses. Thus if type A
blood is given to a type B individual an immune reaction will occur and the blood will coagulate. Since
individuals with type AB blood have no antibodies they are ‘universal recipients’ as they can receive
any blood group, while type O blood is the ‘universal donor’ as it posses neither antigen.
The rhesus blood grouping was first described in rhesus monkeys and relies on the presence of 6
antigens; C, D, E, c, d, e. Only C, D, and E cause immune reactions. If an individual posses any of
these antigens they are said to be Rh+, otherwise they are Rh-. Most people are Rh+. The body does
not usually contain the antibodies to these antigens, and they take several months to form. The wrong
rhesus group can therefore be administered once (thereafter the antibodies are present).
Historically people have long looked for a substitute for blood because, before the recognition of
different blood groups, transfusions so often failed. Ale, wine, opium and even milk were all used as
blood substitutes at one time or another. Recently (1998) solutions of free haemoglobin have been
successfully employed. The haemoglobin in such solutions has the ability to carry oxygen but has no
antigens. Polymer coatings for rbcs have also been used to neutralise the antigens
White Blood cells (Leukocytes)
There are 5 types of white blood cell divided into granulocytes (basophils, eosinophils and neutrophils)
and agranulocytes (lymphocytes and monocytes). The former have lobed nuclei and granules in their
cytoplasm, while the latter have more regular nuclei and no granules. In contrast to red blood cells,
leukocytes are generally larger, and much less numerous (7000 mm-3 of blood).
White blood cells are the major form of defence against ‘attack’ by potentially harmful foreign
organisms such as bacteria, viruses, parasites, fungi etc.. The granulocytes and monocytes, protect the
body by phagocytosis , while lymphocytes are involved in the immune response.
Neutrophils have a granular cytoplasm and account for 60-70% of all
leukocytes. They have a diameter of 10-15 µm and a nucleus
composed of 2-5 sausage-shaped lobes. They are powerful
phagocytes.
Like all leucocytes,
neutrophils often leave the
vascular system by the process
of diapedesis. Before this cells
adhere to capillary walls. This
is known as margination
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Prior to phagocytosis objects have to be recognised
as foreign. This is done by; roughness, differences
in charge, and the presence of antibodies acting as
‘flags’
When a white blood cell encounters a ‘foreign’
object it grows pseudopodia, enclosing the object in
a phagocytic vesicle. Proteolytic enzymes released
by lysozymes then destroy the object. (Hydrogen
peroxide also results in the production of chlorine.)
Eventually the neutrophil itself is destroyed and in
turn phagocytosed by monocytes.
Monocytes are
agranular and mature into large macrophages (80 mm) only within the
extravascular tissue where they form what is sometimes referred to as
the Tissue Macrophage System. They are very powerful phagocytes.
Basophils have a diameter of ca 10-15
µm and account for 0.5-2% of all
leukocytes. Their nucleus is often Sshaped and their cytoplasm contains
large granules. They are weak
phagocytes that contain histamine, heparin, serotonin, and are
therefore important in the inflammatory response & allergic reactions.
They may mature into mast cells
Eosinophils are granulocytes and make up
1-4% of all white blood cells. They are 9
µm in diameter and have a bilobed nucleus. Their function is to detoxify
foreign proteins and they have a possible role in blood clotting and
phagocytosis of the antibody-antigen complex.
Lymphocytes develop from their own stem cell. T-lymphocytes mature in
the thymus, B-lymphocytes mature in the bone marrow (Bursa of Fabricius
in birds). T-lymphocytes mediate cellular immunity in which the whole cell ‘attacks’ the invader. Blymphocytes mediate humoural immunity via plasma cells producing antibodies. You will hear much
more about these cells in your lectures on the immune system
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Blood Platelets (thrombocytes)
These are very
small (2 µm diam),
anucleate cell
fragments with a
density of 250,00 400,000 mm-3
blood. The have an
average life span of
5-14 days.
As other blood cell
types, platelets are produced in the bone marrow. A megakaryocyte can be up to 160 mm in diameter
and disintegrates to produce around 4000 platelets.
Platelets are involved in the prevention of blood loss (Haemostasis), which has 3 phases.
1. Vascular phase (endothelial cells produce chemicals which cause;)
- vascular spasm lasting ca. 30 mins to close down vessels
- division of endothelial cells, smooth muscles etc. to repair wound
- endothelial cells to become ‘sticky’
2. Platelet phase - platelets adhere to the damaged endothelium and aggregate to form a platelet plug.
(If the damage is not too severe this is enough to repair the ‘wound’)
3. Coagulation - this occurs via both an extrinsic
route (chemicals released by endothelial cells) and
an intrinsic pathway (signals initiated by the blood
itself)
Both intrinsic and extrinsic pathways involve
enzyme cascades that ultimately result in the
production of substance X and prothrombin
activator. This converts a plasma protein
(prothrombin) into thrombin, which converts
fibrinogen into fibrin threads. Fibrin threads
stabilise and form a net that makes up the eventual
clot. Depending on the pathway, clot formation
occurs in 15 secs - 18 mins. On average it takes 36 mins
The final phase of haemostasis is clot retraction,
and eventual wound healing
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