Hello! Welcome to this lecture,
which will serve as an introduction to blood.
Blood is a complex mixture of cells
which circulate in plasma.
We'll learn in this lecture that plasma contains
a range of proteins that help the blood to clot,
and serum is the fluid that remains after blood has clotted.
Blood cells are made from haemopoietic stem cells
and the three main cell types are red cells,
white cells and platelets.
Let's start with some general facts about
blood volume and blood composition.
First of all, the amount of blood within your body.
The blood volume is around 70 ml/kg.
A typical person might be 70 kg meaning 4900 or
5 litres of blood in the average person.
Now blood is composed of a range of cells which
circulate in fluid called plasma.
If you take blood from somebody's vein,
put it in a tube and then in a centrifuge,
You'll see the picture on the right.
At the bottom, the red blood cells, by far
the most common cell in the blood, representing
around 45% of the blood volume.
Above them, white blood cells and platelets, a small
fraction. They're very important as we shall see.
And above them, the straw colored fluid - the plasma,
which makes up the great volume, 55% of the blood.
Here's another representation of the blood flowing
down a vessel and you'll see the three main cell types:
red cells, white cells and platelets within the plasma.
And this is an example of the closed circulation of blood
from arteries and capillaries and
back into veins.
As we'll see later, there is one organ - the spleen
which has a very unusual open blood circulation
which is of great interest.
Now I do want to just emphasize the difference
between these terms serum and plasma.
Let me show you the difference.
On the left diagram that we've already seen,
plasma at the top, making up 55% of blood volume.
But that's what we see if we centrifuge blood
which is being anticoagulated, but cannot clot.
We take blood into a tube, it starts to clot very rapidly
as you can see on the right - clotted blood.
All the cells have been drawn down into that clot
and coagulation factors, the range of proteins
involved in blood coagulation have been taken
out of plasma and incorporated into that blood clot.
and what is left is the serum above there.
So serum is deficient in blood coagulation factors.
So if we need to find a liquid to give to
patients who have blood coagulation factors,
you'll see immediately that we need to give plasma,
fresh frozen plasma
Let's spend some time discussing haemopoiesis -
the formation of blood.
This is a classic chart of haemopoiesis that you
would probably find in every haematology laboratory
around the world and it starts at the top as always
with a haemopoietic stem cell.
It's termed PLURIPOTENT - that means it has the ability
to develop into many different types of cells
and of course it has to self-replicate.
Everytime it divides, it has to create a daughter cell
to maintain its numbers.
Now a key point for us to remember at this stage
is these two major lineages during haemopoiesis.
You have the lymphoid lineage on the left
leading to T-cells, B-cells and NK or natural killer cells
and on the right the great mass of myeloid cells
that are derived from that stem cell.
Working from the left, you see monocytes, neutrophils,
basophils, eosinophils, megakaryocytes leading to
platelets and of course, the red cells - the erythrocytes.
Let's just spend a minute talking about the
haemopoietic stem cell in a little more detail
because this is such a crucial cell.
It's a rare subtype of cell within the bone marrow
despite the fact that all the blood cells are derived from it.
It expresses a range of proteins on its surface,
but the key one that we use in science and also
clinically is this molecule CD34.
Now that's interesting because we can do stem cell
transplants where we take blood from one person
and put it into another person and often we will
select the stem cells on the basis of CD34 selection
and that can lead to reconstitution of
haemopoiesis after transfer.
Now as we will see later on, haemopoietic stem cells
exist within a specialized microenvironment.
It is called a stem cell niche, and look at the
range of cells, which are involved in supporting
the survival and division of the
haemopoietic stem cell.
We will also see that stem cells get disturbed
in a range of conditions and that can lead to
dysfunction of haemopoiesis and a reduction in
the blood count in the blood.