Haemoglobin (Hb)

Haemoglobin (Hb) is a protein found in the red blood cells of all vertebrates (and in the tissues of some invertebrates) that is responsible for the transport of oxygen from the lungs to the rest of the body. It also plays a crucial role in transporting carbon dioxide from the body's tissues back to the lungs, where it can be exhaled. Haemoglobin's ability to bind and release oxygen is due to the presence of iron atoms in its structure, which can reversibly bind oxygen molecules.

Structure

Haemoglobin is a tetramer, consisting of four subunits, each of which contains an iron-containing heme group. In humans, the most common form of haemoglobin is called haemoglobin A (HbA), which is a tetramer made up of two alpha (α) and two beta (β) chains. The structure of haemoglobin allows it to change shape as it binds and releases oxygen, a property that is critical for its function in oxygen transport.

Function

The primary function of haemoglobin is to transport oxygen from the lungs to the body's tissues. Oxygen binds to the iron in the heme groups of haemoglobin, and the oxygenated haemoglobin then travels through the bloodstream to deliver oxygen to cells. In the tissues, where oxygen levels are lower, oxygen is released from haemoglobin. Haemoglobin then binds to carbon dioxide, a waste product of cellular metabolism, and transports it back to the lungs to be exhaled. This process is vital for cellular respiration and energy production in the body.

Types of Haemoglobin

Besides HbA, there are several other forms of haemoglobin, including:

• Haemoglobin A2 (HbA2) - a minor component of adult haemoglobin, consisting of two alpha and two delta chains.
• Haemoglobin F (HbF) - fetal haemoglobin, which has a higher affinity for oxygen than adult haemoglobin. It consists of two alpha and two gamma chains and is the predominant form of haemoglobin in the fetus.
• Sickle-cell haemoglobin (HbS) - a variant of haemoglobin found in individuals with sickle-cell disease. It is less effective at oxygen transport and can cause red blood cells to assume a sickle shape, leading to various health complications.

Clinical Significance

Haemoglobin levels in the blood are an important indicator of overall health. Abnormal levels can be a sign of diseases such as anemia, polycythemia, and sickle-cell disease. Measurement of haemoglobin concentration is a common part of routine blood tests. Additionally, the study of haemoglobin and its variants has been crucial in understanding various genetic disorders and in developing treatments for conditions like sickle-cell disease.

See Also

• Red blood cell
• Oxygen transport
• Carbon dioxide transport
• Anemia
• Sickle-cell disease