Now, red cells are packed with haemoglobin.
Haemoglobin consists of four protein chains
and each of those carries a pocket for
a molecule called HAEM.
Now, these four protein chains vary slightly during
our development in embryonic, fetal times
You have slightly different forms of haemoglobin.
but I'm not going to go into that today
but in the adult, the 4 proteins consists of two alpha
chains and two beta chains as represented on the right.
and haem contains iron. Iron is critical
for the carriage of oxygen within haemoglobin.
In fact much of the iron within your body is
found within your haemoglobin.
We'll be discussing that later in the
and one thing we just need to consider is how
haemoglobin can take up and release oxygen
and it releases oxygen to the tissues.
On the right you'll see a graph, it is called
an oxygen dissociation curve and on the x-axis,
we will see the partial pressure of oxygen,
basically how much oxygen there is around.
And on the y-axis the percentage saturation
of haemoglobin at that given partial pressure
You'll see the dark green line
of course as the oxygen concentration goes up
the saturation goes up.
You can see it's not exactly a linear line, it is not
a straight line, it is what we call a sigmoid curve,
it starts and then it accelerates.
That's because when one globin molecule binds oxygen
it helps the others to bind, it's a cooperative
interaction and the haemoglobin can almost readily
grab the oxygen.
Another important point I want you to look at
on that slide is the amount of saturation of
haemoglobin within arterial blood and venous blood.
So in the right you'll see the dotted line
showing arterial blood.
So it's a p02 of around 100 millimetres of mercury
which is what we see in arterial blood,
haemoglobin is almost completely saturated whereas
if we look at blood coming back to the heart,
venous blood, the partial pressure of oxygen is
around 40 millimetres of mercury and at that point,
the haemoglobin is around 75% percent saturated.
It's around 25% of its capacity of oxygen carriage
has been lost as it's gone around the body.
There's still quite a lot of oxygen still remaining
in venous blood even though when we take it
of course it looks very blue compared to
red blood from arteries but there is excess
capacity to release more oxygen, perhaps if
there is intense exercise. So we have some capacity
to increase oxygen delivery.
There are molecules within red cells such as
2-3-Diphosp hoglycerate which can shift this curve
slightly to the left or the right so the body can
regulate how much oxygen it releases into the tissues
and we often find patients who have chronic anaemias
who have low levels of haemoglobin and red cells
that their 2-3-DPG levels are such that they can
release more oxygen into the tissues to help their
bodies cope with metabolism.
Finally on that slide there is a dotted green-line
above the haemoglobin curve and that is myoglobin
You can see there the myoglobin curve there's
not sigmoid and it has much more avidity for
oxygen than haemoglobin.
Haemoglobin needs to release oxygen into
Well it's not just about getting oxygen to tissues,
of course you know from your biochemistry lectures
on respiration that carbon dioxide is formed and so
haemoglobin helps carbon dioxide to be transferred
to the lungs and out of the body. So carbon dioxide
diffuses from tissues into red cells and there is an
enzyme carbonic anhydrase which generates the
carbonic acid, that dissociates into bicarbonate ion
and a proton. The protons are buffered by haemoglobin.
So haemoglobin has an important role in buffering
the production of that acidic proton.
So as well as helping with oxygen delivery,
haemoglobin helps with removal of carbon dioxide