Welcome to Haemoglobin and Gas Transport!

In the previous chapters, we looked at how the heart pumps blood around the body. But why do we bother moving all that blood? The main reason is to deliver oxygen (\(O_2\)) to every cell and remove carbon dioxide (\(CO_2\)). Because humans are large, we can't rely on simple diffusion alone; we need a specialized transport protein to do the heavy lifting. That's where haemoglobin comes in!

In these notes, we will explore how haemoglobin picks up and drops off gases, why its "stickiness" changes depending on where it is in the body, and how a baby in the womb manages to get oxygen from its mother.

1. What is Haemoglobin?

Haemoglobin is a complex globular protein found in red blood cells. (Quick cross-reference: You’ll learn more about globular proteins in the Proteins and Enzymes chapter!)

Its primary job is to transport oxygen from the lungs to the rest of the body. Here is how it is built:

  • It consists of four polypeptide chains (four parts joined together).
  • Each chain contains a special non-protein group called a haeme group.
  • Each haeme group contains an iron ion (\(Fe^{2+}\)).
  • It is this iron ion that actually binds to the oxygen.

Since there are four haeme groups, one single haemoglobin molecule can carry up to four oxygen molecules (\(4 \times O_2\)). When it is carrying oxygen, we call it oxyhaemoglobin.

The Reaction:
\(Hb + 4O_2 \rightleftharpoons Hb(O_2)_4\)
(Haemoglobin + Oxygen \(\rightleftharpoons\) Oxyhaemoglobin)

Key Takeaway

Haemoglobin is an oxygen-shuttle. It binds oxygen where there is plenty of it (lungs) and releases it where it is needed (tissues).


2. Affinity and Partial Pressure

To understand gas transport, we need to use two important terms:

1. Affinity: This is simply how "sticky" the haemoglobin is for oxygen. High affinity means it holds onto oxygen tightly; low affinity means it lets go easily.
2. Partial Pressure (\(pO_2\)): This is a measure of the concentration of oxygen. In the lungs, \(pO_2\) is high. In hard-working muscles, \(pO_2\) is low.

How it works:

  • In the Lungs (High \(pO_2\)): Haemoglobin has a high affinity for oxygen, so it associates (binds) with it to form oxyhaemoglobin.
  • In the Tissues (Low \(pO_2\)): Haemoglobin has a lower affinity for oxygen, so it dissociates (unloads/releases) the oxygen so cells can use it for respiration.

3. The Oxygen Dissociation Curve

If we plot a graph of the percentage saturation of haemoglobin against the partial pressure of oxygen, we get an S-shaped (sigmoid) curve.

Why is it S-shaped?
This is due to cooperative binding. Don't worry if this sounds complex; think of it like this:
The haemoglobin molecule is a bit "shy" at first. It is actually quite difficult for the very first oxygen molecule to bind. However, once that first oxygen joins, it changes the shape of the whole haemoglobin molecule, making it much easier for the second and third oxygen molecules to hop on. The fourth one is a bit harder to fit because most sites are full. This results in the steep climb in the middle of the graph.

Quick Review Box

Low \(pO_2\): Small change in \(pO_2\) = small change in saturation.
Medium \(pO_2\): Small change in \(pO_2\) = BIG change in saturation (the steep part of the S).
High \(pO_2\): Graph levels off as haemoglobin becomes 100% saturated.


4. The Bohr Effect

When you exercise, your tissues produce a lot of carbon dioxide (\(CO_2\)). High levels of \(CO_2\) actually change how haemoglobin behaves. This is called the Bohr Effect.

When \(CO_2\) concentration is high:

  1. The dissociation curve shifts to the RIGHT.
  2. Haemoglobin's affinity for oxygen decreases.
  3. Oxygen is released more easily to the tissues.

Analogy: Imagine oxygen is a delivery package. If a "customer" (muscle) is working really hard and shouting (producing \(CO_2\)), the delivery driver (haemoglobin) drops the package off even faster! This is helpful because working muscles need that extra oxygen for aerobic respiration.


5. Fetal Haemoglobin vs. Adult Haemoglobin

A fetus (unborn baby) gets its oxygen from its mother’s blood across the placenta. By the time the mother's blood reaches the placenta, it has already lost some oxygen.

For the baby to survive, its haemoglobin must be better at grabbing oxygen than the mother's haemoglobin is. Therefore, fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin.

  • The fetal dissociation curve is shifted to the LEFT of the adult curve.
  • At the same partial pressure, fetal haemoglobin will be more saturated with oxygen than the mother's.
  • This allows the fetus to "steal" oxygen from the mother's blood.

Memory Trick: Left = Loves oxygen more (higher affinity). Fetal haemoglobin is on the left!


6. Transport of Carbon Dioxide

Haemoglobin doesn't just carry oxygen; it also helps transport \(CO_2\) back to the lungs. Carbon dioxide is transported in three main ways:

  1. Dissolved in plasma: About \(5\%\) to \(10\%\) just dissolves directly in the blood.
  2. Bound to haemoglobin: It can bind directly to the amino acids in the polypeptide chains (not the haeme group!) to form carbaminohaemoglobin.
  3. As hydrogencarbonate ions (\(HCO_3^-\)): This is how most (\(approx. 85\%\)) of the \(CO_2\) is carried. Inside red blood cells, \(CO_2\) reacts with water to form carbonic acid, which then breaks down into \(H^+\) and \(HCO_3^-\) ions.

Summary Checklist & Common Pitfalls

Checklist:

  • Can you describe the structure of haemoglobin? (4 chains, 4 haeme groups, \(Fe^{2+}\) ions).
  • Do you know the difference between association and dissociation?
  • Can you explain why the dissociation curve is S-shaped? (Cooperative binding).
  • Can you explain the Bohr Effect? (High \(CO_2\) = shift Right = lower affinity).
  • Do you know why the fetal curve is on the Left? (Higher affinity to get oxygen from mum).

Common Mistakes to Avoid:

  • Mixing up the shifts: Remember, a shift to the Right means Lower affinity (Bohr effect). A shift to the Left means Higher affinity (Fetal Hb).
  • Confusing binding sites: Oxygen binds to the iron/haeme group, but carbon dioxide binds to the protein/globin part of the molecule.
  • Using the wrong terms: Always use partial pressure (\(pO_2\)) instead of "concentration" when talking about gases in the blood.
Did you know?

The reason your blood looks bright red is the oxyhaemoglobin. When haemoglobin loses its oxygen (deoxygenated blood), it turns a much deeper, darker purplish-red. It is not actually blue—that is just an optical illusion caused by the way light interacts with your skin and veins!