In this lecture, I am going to talk about blood.
Blood is a very specialized connective tissue.
It is a fluid and it flows within
blood vessels throughout the entire body.
At the end of this lecture, I want you to
be able to understand that blood consists of
plasma, a fluid which either has lots and
lots of proteins and other substances that
have very important roles throughout the body.
Blood also has cells, red blood cells and
the white blood cells. And I want you also to
understand their functions and how to identify them.
Blood contains platelets and also it
would be a good idea for you to be able to
understand the relative numbers of each of
these blood cells in blood and also some idea
of their relative sizes. Well, blood has many
different functions. As I mentioned
earlier, it is a fluid flowing within blood
vessels right throughout the body and therefore
it can deliver certain substances to all the
tissues of the body. It can take away waste
products. Importantly it carries cells with
it that helps us fight, a defence against
invading pathogens, the immune cells. It also
carries around lots of chemical messengers
like hormones, that are secreted from endocrine
glands and then deliver to their target organs
some distance away from those glands. We will
learn that it is very important in maintaining
various roles in hemostasis in the body. For
instance, maintaining a relatively even tissue
volume of fluid proportions. Also it is involved
with thermoregulation like sweat glands in
skin. And we will also appreciate that the
erythrocyte or the red blood cell is the important
cell that transport oxygen and carbon dioxide
to the tissues and from the tissues.
Well let us first of all look at the general characteristics
of blood. If we take a blood sample, if we
take some blood out of our vein for instance
and we put it in a test tube and then centrifuge
that blood, it separates blood into three
major components. The greatest volume of blood
is the plasma. It occupies somewhere around
55 percent of the total blood. The other major
proportion of blood are the erythrocytes,
those cells that transport oxygen and also
carbon dioxide and they occupy roughly about
45 percent of total blood. And then there
is this very very tiny layer called the buffy
coat. That has less than 1 percent of total
blood volume. That little buffy coat contains
the white blood cells and platelets, very
small proportion of blood. But those white
cells as we will see later on are very very
important because they are involved with protecting
our body against invading pathogens and they
also have other roles that I will talk about
in this lecture and also when we talk about
the immune system. Sometimes hematologists
will measure the percentage of total blood
volume that is occupied by erythrocytes. Here
it is about 45 percent, but it varies.
It varies depending on whether you are a male
or a female. Also, it varies depending on
whether or not you may have some diseases
of blood such as anemia where the proportion
of red blood cells is decreased, it gets very
low and that is called the hematocrit.
The hematocrit is the percent of total blood that
is occupied by erythrocytes. And that hematocrit
if it is found to be low, can be an indication
of blood diseases such as anemia. Well let us have
a look at plasma. As I said earlier in the
lecture, it is the property of blood, the
fluid property that allows blood to circulate
through blood vessel to all parts of the body.
It is basically water, 95 percent water, but
it contains very very important components.
One of the most important components are the
proteins in blood. Blood proteins, by far the
most abundant protein in the blood is albumin.
Albumin is the smallest protein in blood,
but it has a very important role. It is involved
in what we call the colloid osmotic pressure
gradient in the body. Albumin is responsible
for making sure that there is an even balance
between fluid in the blood, the plasma, water
in the plasma and water in the interstitium,
that is the interstitial compartment that
surrounds cells. It is important that the
amount of water in that interstitial component
is controlled and regulated very very finely.
If albumin leaks out of blood or is lost,
then fluids starts to accumulate within the
tissues and it can give rise to edema. Sometimes
that colloid osmotic pressure can actually
be inhibited or at least overridden for instance
when we stand for a long time without moving.
We tend to get fluid then accumulating in
our ankles, we get swollen ankles and it is
not if we moved around. That fluid can then
be returned into the vascular system.
By the way, I will talk about when we look at the
immune system. Now albumin also has another
role. It can transport substances through
the blood to the different parts of the body.
For instance, they can transport a hormone
called thyroxine. They can transport metabolites
such as bilirubin and they can also transport
drugs such as the barbiturates.
The globulins, those plasma proteins termed globulins are
antibodies and we will talk about those again
when we look at the immune system. Fibrinogen
is the largest blood protein. It is a very
important blood protein. It is soluble, but
when a blood vessel becomes damaged, that fibrinogen
can then be transformed into fibrin and that
has a role in the clotting mechanism and I
will mention that later on. Hemoglobin is
a protein that transports the gases, oxygen
and carbon dioxide. Oxyhemoglobin, the combination
of oxygen binding to hemoglobin makes those
red blood cells appear bright red. Well listed
are there other substances that blood
contains, the plasma contains and I just want
to focus on the bottom one, chemical messengers
because that is really an important component
of blood that I will refer to when we talk
about the endocrine glands and the secretion
of these products from the endocrine glands
called hormones that as I mentioned earlier,
have their effect on target tissue elsewhere
in the body.