Hello in the second lecture in our series
on haematology we're going to look at the
physiology and function of some of the major
blood cells, and the learning outcomes that
we'll be getting from this lecture are as follows:
We will look at red cell production and how
that's regulated by erythropoietin, and we
will explore how red cells contain haemoglobin
and how that helps them to carry oxygen to
We will look at the subsets of white cells which
play specific roles in protection against infection
and we will explore how lymphocytes mediate the
function of the immune system
and find out how neutrophils are critical in the
first line of defence against bacteria and fungi.
In this lecture we are not going to focus
on platelets because they're the subject of
a later lecture on blood clotting.
Let's look at red cells in a little bit more detail,
by far the most common cell within the blood.
Red blood cells derive of course from a nucleated
cell within the bone marrow
and the first cell that is committed to forming
red cells is the erythroblast
and there are rare forms of leukaemia in fact
derived from these cells.
Red cells are packed with haemoglobin
and they carry oxygen to tissue.
The reason that we've evolved red cells is that it
allows haemoglobin to be packaged effectively
within these blood cells rather than being
free within the blood,
the major production of red cells is through this
hormone called erythropoietin and we'll explore
the regulation of erythropoietin in some detail.
On the right, you'll see a lovely scanning
electron micrograph of red cells and you'll see
that classic biconcave disc structure of the red
cell which allows it to be flexible and flow
through the capillaries as well as diffusing
oxygen into tissue.
Now, haematologists can't stop making blood films
and there you’ll see a blood film on the right
and you'll see how red cells look down the
microscope and you'll see the characteristic pale
centre because of their shape.
They are around seven microns in diameter
and highly flexible structures with the biconcave
Now, some red cells in the blood are very young,
because they've just come out of the bone marrow
and we call those reticulocytes.
They carry a lot of RNA and that RNA gradually gets
degraded over 2-3 days and the final red cell
then becomes apparent.
So you'll see on the left reticulocytes can live
for two or three days normally within the blood.
Around 2% of the red cells are reticulocytes
and we can detect them by staining what we call
the supravital staining with this wonderful dye
called brilliant cresyl blue and you'll see
on the right those cells with the intense blue
stain, those are reticulocytes.
There's an awful lot in that picture and many
more of them we'd find in your own blood I'm sure.
Now as well as being physiologically important,
reticulocytes are important clinically because
they can give us a guide as to the type of anaemia
that we may be dealing with in a patient.
Because if the reticulocyte count has increased,
it shows that the patient's bone marrow is
very active and pouring out a lot of red cells into
the blood, whereas if we can't find many reticulocytes,
it means that there's a problem with the production
of red cells from the bone marrow.
So when we discuss anaemia in later lectures,
you can see why the reticulocyte count is a
very useful test for trying to understand
the etiology of the anaemia.
Now, the production of red cells is described
by the term erythropeoisis.
And again, it's represented by those green cells.
Erythroblasts are the first precursors and
those differentiate into normoblast.
Normoblasts have a nucleus but then as they
themselves differentiate, the nucleus is ejected
and at that stage, the reticulocytes can be
released into the blood.
Now we don't know why the nucleus is ejected in
red cell production but it may be that that allows
more room for haemoglobin production and that
has been selected during evolution as a very
effective way to increase oxygen transport
within the blood.
Now the regulation of this process of haemopoiesis
is really controlled through this protein
called erythropoietin or EPO.
This hormone EPO is produced largely from the kidney.
Again very surprising finding, the kidney should
be regulating our blood production.
But if cells in the kidney detect that the blood
is hypoxic in any way, they will then release
erythropoietin into the blood.
Erythropoietin circulates to the bone marrow
and it stimulates the production of red cells.
As you could probably imagine, EPO or erythropoietin
levels are increased in people who are short of oxygen.
For instance, those who live at high altitude
or patients who have lung disease or perhaps, smoke.
You'll see on the right there, a representation of
stimuli to hypoxia, factors such as anaemia,
or low atmospheric oxygen tension.
That hypoxia is detected by the kidney and
erythropoietin is produced.
Erythropoietin stimulates haemopoiesis through a
number of mechanisms.
It increases the differentiation pathway through
erythropoiesis to make more erythropoietic cells.
It accelerates cell division and it accelerates
release of cells into the blood.
That can usually address the hypoxia.
Of course, the negative feedback loop is established.
Erythropoietin is actually a very useful molecule
for clinical therapy and we can use it to stimulate
haemopoiesis in patients with some forms of anaemia.