Now, red cells are packed with hemoglobin.
Hemoglobin consists of four protein chains,
and each of those carries a pocket
for a molecule called heme.
Now these four protein chains
vary slightly during our development
an embryonic fetal times.
You have slightly different
forms of hemoglobin,
but I'm not going to go into that today.
But in the adults, the four proteins
consist of two alpha chains
and two beta chains
as represented on the right.
And heme contains iron.
Iron is critical
for the carriage of oxygen
within hemoglobin, in fact,
much of the iron within your blood body
is found within your hemoglobin,
and we'll be discussing
that later in the lecture series.
And one thing we just need to consider
is how hemoglobin
can take up and release oxygen.
And it releases oxygen to the tissues.
On the right, you will see a graph, it's
called an oxygen dissociation curve.
And on the x-axis, we will see
the partial pressure of oxygen.
how much oxygen there is around.
And on the y-axis, the percentage saturation of hemoglobin at that given partial pressure of oxygen.
So, you'll see the blue line that's representing hemoglobin.
Of course, as the oxygen concentration goes up, the saturation goes up.
You can say it's not exactly linear line. It's not a straight line.
It's 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 hemoglobin can almost greedily grab the oxygen.
Another important point I want you to look at on that slide is the amount of saturation
of hemoglobin within arterial blood and venous blood.
So, on the right, you will see the dotted line showing arterial blood.
So, it appear to be around 100 mmHg, which is what we see in arterial blood.
Hemoglobin is almost completely saturated.
If we look at blood coming back to the heart, venous blood,
the partial pressure of oxygen is around 40 mmHg.
And at that point, the hemoglobin is around 75% saturated.
So, 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 the risk of 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-diphosphoglycerate,
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 anemia
who have low levels of hemoglobin and red cells,
but that 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's a dotted blue line above the hemoglobin curve
and that's myoglobin. And of course, you can see there, the myoglobin curve,
that's not sigmoid and it has much more avidity for oxygen than hemoglobin.
Hemoglobin needs to release oxygen into the tissue.
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, hemoglobin 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 then dissociates into bicarbonate ion and a proton.
The protons are buffered by hemoglobin.
So, hemoglobin has an important role in buffering the production of that acidic proton.
So, as well as helping with oxygen delivery,
hemoglobin helps with removal of oxygen -- carbon dioxide from tissues.