Haemoglobin is a red pigment made of protein found in red blood cells. It plays an important role in transporting oxygen from the lungs to the rest of the body and carrying carbon dioxide from respiring cells to the lungs. This is because oxygen doesn't dissolve well in blood plasma, so haemoglobin picks it up and distributes it where it's needed. Haemoglobin also gives red blood cells their distinct red color. So, it’s safe to say that haemoglobin is pretty important for keeping our bodies healthy and functioning properly.

Haemoglobin structure

The word 'haemoglobin' is made up of two parts: 'haemo' and 'globin'. 'Haemo' stands for the haem group, while 'globin' represents the protein. Essentially, haemoglobin is made up of haem groups and protein chains (see Figure 1).

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Haemoglobin is a large, globular, conjugated protein, which means that the 'globin' part of its name refers to its structure. The quaternary structure of haemoglobin is made up of four polypeptide chains, with each chain carrying a haem group. The haem groups are essential for haemoglobin's function because they loosely bind to one terminal polypeptide chain and carry four oxygen molecules in total. This allows haemoglobin to transport oxygen throughout the body, forming oxyhaemoglobin. Keep in mind that a quaternary protein structure is made up of multiple tertiary structures combined. If you need to refresh your memory on this, take a look at the 'Tertiary and quaternary protein structure' document.

Haem group structure

Haemoglobin's function is mainly attributed to the haem group, which helps to carry oxygen molecules. There are four haem groups in each haemoglobin molecule, with one haem group at the terminal of each polypeptide chain, as shown in Figure 2.

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The porphyrin ring provides structural support for the iron ion, which is the oxygen-carrying component of haem. During transport, oxygen binds to the iron ion of haem, as seen in Figure 3. Figure 3 shows that haemoglobin binds reversibly to oxygen during transport. Once oxygen is bound, it can also unbind from haemoglobin. Each haem group binds to one oxygen molecule, which contains two oxygen atoms. Therefore, given that there are four haemoglobin groups, one haemoglobin can carry four oxygen molecules (i.e., eight oxygen atoms) in total. In conclusion, the haem group is a crucial component of haemoglobin, responsible for carrying oxygen molecules and ensuring the proper functioning of our bodies.

Haemoglobin concentration definition

Haemoglobin concentration refers to the amount of haemoglobin present in an individual's blood. For adult males, the normal range for haemoglobin concentration is 8.7-11.2 mmol/L, while for adult females, it is 7.4-9.9 mmol/L.

In regions of high altitudes, the body undergoes a process called acclimatization, where it adapts to low oxygen levels by producing more haemoglobin. This increase in haemoglobin concentration allows for more oxygen carriers, enabling more oxygen from the atmosphere to be carried in the blood.

However, haemoglobin concentration beyond the normal range can indicate certain health conditions. Anaemia is a condition where the haemoglobin concentration is too low, resulting in a reduced ability of the blood to carry oxygen. On the other hand, polycythaemia is a condition where the haemoglobin concentration is too high due to an excessive production of red blood cells by the body.

In conclusion, haemoglobin concentration is an essential parameter that can indicate an individual's health status. Maintaining a normal range of haemoglobin concentration is crucial for the efficient transportation of oxygen throughout the body.

Haemoglobin dissociation curve

The structure of haemoglobin and its function in transporting oxygen can be quantified using the haemoglobin dissociation curve. This curve is a graph that shows the relationship between haemoglobin saturation and partial pressure of oxygen.

Haemoglobin saturation refers to the amount of oxygen that is bound to the haemoglobin in comparison to the total number of binding sites available. On the other hand, partial pressure refers to the concentration of oxygen in the body.

The haemoglobin dissociation curve is plotted with haemoglobin saturation on the y-axis and partial pressure of oxygen on the x-axis. The curve shows how the amount of oxygen bound to haemoglobin changes as the partial pressure of oxygen changes.

At high partial pressures of oxygen, such as in the lungs, haemoglobin binds to oxygen strongly, resulting in a high level of saturation. As the partial pressure of oxygen decreases, such as in the tissues, haemoglobin releases oxygen more readily, resulting in a lower level of saturation.

The shape of the haemoglobin dissociation curve is sigmoidal, which means that at intermediate partial pressures, small changes in partial pressure can result in significant changes in haemoglobin saturation. This characteristic allows haemoglobin to efficiently transport oxygen from the lungs to the tissues and organs where it is needed.

In summary, the haemoglobin dissociation curve is a useful tool for quantifying the role of haemoglobin in the transport of oxygen, showing how the amount of oxygen bound to haemoglobin changes as the partial pressure of oxygen changes.

The shape of the curve

It would be expected that when Hb saturation in the blood increases, the partial pressure will increase, creating a linear regression such as in Figure 4. This is NOT the case.

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Instead, the graph takes a ‘weird’ shape known as a sigmoid (S-shaped) curve (Figure 5). This shape is crucial in the rationale behind oxygen transport.

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The haemoglobin dissociation curve provides valuable information about the unloading and loading of oxygen by haemoglobin.

The leftward region of the curve, which lies to the left of the dotted line at 50% haemoglobin saturation, represents the unloading of oxygen. In this region, the partial pressure of oxygen is low, and haemoglobin releases oxygen to the tissues where it is needed for cellular respiration.

The rightward region of the curve, which lies to the right of the dotted line at almost 100% haemoglobin saturation, represents the loading of oxygen. In this region, the partial pressure of oxygen is high, such as in the lungs, and haemoglobin binds to oxygen strongly, enabling efficient oxygen uptake.

The central region of the curve, which lies between the two dotted lines, represents the transport of oxygen in blood vessels. In this region, the partial pressure of oxygen is intermediate, and small changes in partial pressure can result in significant changes in haemoglobin saturation. This characteristic allows haemoglobin to efficiently transport oxygen from areas of high partial pressure, such as the lungs, to areas of low partial pressure, such as the tissues.

The reversible nature of oxygen binding to haemoglobin is also reflected in the haemoglobin dissociation curve. As oxygen binds to haemoglobin, the haemoglobin molecule undergoes a conformational change, which enables it to bind more oxygen molecules. On the other hand, as oxygen is released from haemoglobin, the molecule returns to its original conformation, making it easier for oxygen to dissociate from haemoglobin.

In summary, the haemoglobin dissociation curve provides valuable information about the unloading, loading, and transport of oxygen by haemoglobin, and reflects the reversible nature of oxygen binding to haemoglobin.

Unloading and loading of oxygen

Unloading is the process where oxygen gets released into respiring cells. One can identify the region of the haemoglobin dissociation curve where unloading occurs (Figure 6).

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To summarize, the haemoglobin dissociation curve shows the relationship between haemoglobin saturation and partial pressure of oxygen. The curve is split into three regions - unloading, transport, and loading - by two dotted lines.

The unloading of oxygen occurs at low partial pressures of oxygen until 50% saturation, in metabolically active and aerobically respiring cells. At low partial pressure, the affinity of haemoglobin for oxygen is low, making it difficult for oxygen to bind to haemoglobin. The graph moves from less steep to increasing steepness until the 50% mark due to positive cooperativity, where the binding of one oxygen molecule makes it easier for the next oxygen molecule to bind.

Loading occurs at high partial pressures of oxygen (95% saturation) until the maximum haemoglobin saturation, in the lungs where partial pressures of oxygen are high. At high partial pressure, the affinity of haemoglobin for oxygen increases, making it easy for oxygen to bind to haemoglobin in the pulmonary capillaries. The plateau of the sigmoid curve is beneficial in loading, as oxygen can be loaded even though the partial pressure of oxygen may drop slightly.

It is essential to keep in mind that unloading is the release of oxygen into cells, whereas loading is when oxygen from the lungs binds to haemoglobin for its transport.

Bohr shift

What would happen to the curve if there was an increased CO2 concentration? High CO2 levels would shift the sigmoid curve shift rightwards. This phenomenon is the Bohr shift (Figure 7).

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The Bohr shift would mean a higher partial pressure to achieve 50% saturation. In other words, the affinity of haemoglobin for oxygen drops, causing haemoglobin to be loaded with oxygen less readily. Instead, the body unloads oxygen for respiring cells more efficiently, allowing these cells to continue aerobic respiration and produce ATP.

Variants of haemoglobin

The activity of haemoglobin variants can be identified by comparing the dissociation curve of the variant with the curve of adult human haemoglobin.

Leftward shift

Certain variants of haemoglobin experience a leftward shift when compared to adult human haemoglobin (Figure 8).

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To summarize, a leftward shift of the haemoglobin dissociation curve means that haemoglobin has a higher affinity for oxygen. This shift occurs when the partial pressure of oxygen required to achieve 50% saturation is lower, indicating that oxygen binds to haemoglobin more readily.

Organisms that live in environments with low oxygen supply, such as foetuses and alpacas, have haemoglobin that experiences a leftward shift to ensure their cells still receive sufficient oxygen. Foetuses have their subtype of haemoglobin, which has a higher affinity for oxygen as the partial pressure of oxygen in the placenta is low.

Myoglobin, another proteinous pigment that acts as an oxygen carrier in humans, is found in muscle filaments. The curve of myoglobin also has a leftward shift compared to haemoglobin, indicating that myoglobin has a higher affinity for oxygen than haemoglobin. This allows myoglobin to store oxygen molecules and only release oxygen at a very low partial pressure of oxygen, allowing muscle cells to continue respiring anaerobically.

Understanding the leftward shift of the haemoglobin dissociation curve is essential in understanding oxygen transport and delivery in different organisms and tissues.

Rightward shift

Other haemoglobin variants experience a rightward shift compared with adult human haemoglobin (Figure 9).

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To summarize, a rightward shift of the haemoglobin dissociation curve means that haemoglobin has a lower affinity for oxygen. This shift occurs when a higher partial pressure of oxygen is required to achieve 50% saturation, indicating that oxygen binds to haemoglobin less readily.

Organisms with high oxygen demand or a high metabolic rate, such as small and active animals like birds and mice, have haemoglobin variants that experience a rightward shift in the haemoglobin dissociation curve. This allows for more efficient unloading of oxygen to respiring tissues.

Understanding the rightward shift of the haemoglobin dissociation curve is essential in understanding oxygen transport and delivery in different organisms and tissues, particularly those with high metabolic rates or oxygen demands.

Does haemoglobin take part in the transport of carbon dioxide in the blood?

To summarize, haemoglobin does transport carbon dioxide, but the amount of carbon dioxide transported by haemoglobin is relatively low, at around 15% of the total PCO2. Carbon dioxide binds to another site in the haemoglobin molecule, forming carbaminohaemoglobin.

The majority of carbon dioxide is transported as the soluble bicarbonate ion, HCO3-. Carbon dioxide reacts with water to form carbonic acid, which then dissociates into bicarbonate and an H+ ion with the help of the enzyme carbonic anhydrase. These reactions are reversible, allowing carbon dioxide to be regenerated from the bicarbonate ion and exhaled through the lungs.

Understanding the mechanisms of carbon dioxide transport is essential in maintaining acid-base balance and ensuring efficient gas exchange in the body.

Problems with the oxygen transport

In summary, the haem group in haemoglobin is responsible for carrying oxygen, but it can also bind other substances. The most significant problem occurs when carbon monoxide binds to haemoglobin, as it has a higher affinity for haemoglobin than oxygen. Carbon monoxide poisoning can result in insufficient oxygen being carried in red blood cells, even if the partial pressure of oxygen is high.

Understanding haemoglobin and its function is crucial in maintaining proper oxygen transport and delivery in the body. The haemoglobin dissociation curve represents the activity of haemoglobin and can help us understand how haemoglobin responds to changes in oxygen and carbon dioxide partial pressures. Different haemoglobin variants have different activity levels, which can be determined by the direction of the shift compared to normal haemoglobin.

Haemoglobin

What is the average haemoglobin level?

The average haemoglobin level in adults is 8.7-11.2 mmol/L for males and 12-16 g/dL for females.

What does it mean when your haemoglobin is low? 

When the haemoglobin levels are below the average range, this is a medical condition called anaemia.

How do you treat low haemoglobin? 

The main treatment for low haemoglobin would be iron supplements. 

What does it mean when your haemoglobin is high? 

There are many explanations for high haemoglobin. These include acclimatisation at higher altitudes or polycythaemia where the body produces too many red blood cells.

Which part of the blood transports the most oxygen? 

The part of the blood that transports the most oxygen is the haemoglobin.

Does plasma transport oxygen? 

Only a very small percentage of oxygen is transported by the plasma as oxygen is poorly soluble. 

What transports oxygenated blood from the lungs to the heart? 

The pulmonary vein transports oxygenated blood from the lungs to the heart.

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