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What is Hemoglobin? Definition, Structure and Function

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  • Last Updated : 02 Aug, 2022
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Hemoglobin is a two-way respiratory transporter, carrying oxygen from the lungs to the tissues and working with the return carry of carbon dioxide. In the blood vessel circulation, hemoglobin has a high affinity for oxygen and a low affinity for carbon dioxide, natural phosphates, and hydrogen and chloride ions. In the venous circulation, these overall affinities are reversed.

Hemoglobin is a protein inside red blood cells that transports oxygen from the lungs to tissues and organs in the body and transports carbon dioxide back to the lungs. Testing for the level of hemoglobin count in the blood is generally a part of the Complete Blood Cell (CBC) test.



Structure of Hemoglobin

Max Perutz portrayed the atomic construction of hemoglobin in 1959. Hemoglobin is a tetrameric protein. The primary type of hemoglobin in adults comprises two subunits each of Alpha-globin and Beta-globin polypeptide chains. Every polypeptide bind is connected to a heme prosthetic group.

  • α subunit – It is comprised of an alpha polypeptide chain having 141 amino acid deposits.
  • β subunit – It is comprised of a beta polypeptide chain having 146 amino acid deposits.
  • Heme group – It is an iron-containing prosthetic group, which is joined to every polypeptide chain. It contains iron in the focal point of the porphyrin ring.

In the quaternary structure, there are areas of strength among α and β subunits. On gentle treatment with urea, hemoglobin to some degree separates, however, Alpha-Beta dimer stay intact. The subunits are held together by mostly hydrophobic interactions, hydrogen bonding, and a couple of ions or salt links.

In newborn children, there are 2 alpha and 2 gamma chains, which get reinstated by beta chains.



Hemoglobin is available in two conformities, for example, R state and T state.

T-State (tense)

Oxygen has less affinity to the T state than the R state. The T-state is the deoxy type of hemoglobin (implying that it comes up short on oxygen species) and is moreover known as Deoxyhemoglobin.

R-State (relaxed)

Oxygen has a greater affinity to the R state. The R-state is the oxy type of hemoglobin that is fully oxygenated and is also known as Oxyhemoglobin.

Functions of Hemoglobin

The principal function of Hemoglobin (Hb) is to carry and supply oxygen to different tissues. The binding of oxygen to Hb is helpful or Cooperative binding. The binding and arrival of oxygen from Hb in the lungs and tissues separately is because of the change between low oxygen affinity T state (Tense) and high oxygen liking R state (Relaxed).

Transport of Oxygen

The affinity of oxygen to Hb is impacted by pH, 2,3 BPG (2,3-Bisphosphoglyceric corrosive). Low pH, high BPG, and CO2 present in tissues favor T-state, and oxygen is delivered, while R-state is leaned toward the alveoli because of high, pH, low CO2, and BPG concentration, which prompts the binding of oxygen to Hb. Restricting of oxygen is likewise controlled by the partial pressure of oxygen. In the lungs where pO2 is high, oxygen, ties with Hb, and in tissues, where pO2 is low, oxygen is delivered.

Every 100 ml of oxygenated blood carries 5 ml of O2 to the tissues

Binding of the main oxygen molecule to the heme unit of one subunit of the deoxyhemoglobin (T- state) causes conformational changes prompting an expansion in the partiality, accordingly the second molecule binds more quickly. The limiting of the fourth molecule happens, when it is in the R state. The limiting of oxygen to Hb shows a sigmoid bend. This sort of restricting is known as allosteric binding, where binding at one site influences the affinities of the leftover binding sites.

The amount of oxygen levels in the blood is measured by Pulse Oximeter. It helps in diagnosing hypoxia. It depends on the fact that oxyhemoglobin and deoxyhemoglobin have different absorption spectra. This is a significant device that specialists are utilizing to check the oxygen saturation level of COVID-19  patients and furthermore in the people who are in danger.

Transport of Carbon dioxide

Around 20-25% of CO2 is carried bound to hemoglobin as carbaminohemoglobin. In tissues where pCO2 is more and pO2 is less, restricting carbon dioxide is favored and in, alveoli separation of carbaminohemoglobin happens because of high pO2 and low pCO2. The rest of the CO2 is shipped as bicarbonate, which is worked with by an enzyme called carbonic anhydrase.

Every 100 ml of deoxygenated blood carries 4 ml of CO2 to the alveoli

Hemoglobin likewise transports nitric oxide bound to the globin protein. It ties to the thiol groups present in the globin chains. Carbon monoxide can likewise bind to hemoglobin and structure the carboxyhemoglobin complex. Hemoglobin has a 250 times higher liking for carbon monoxide than oxygen. So even the smallest concentration of CO can influence the limiting of oxygen. Thus, rousing air wealth in CO can cause cerebral pain, unconsciousness, or even nausea. It can block 20% of active restricting sites of oxygen in chain smokers.

Hemoglobin Diseases

There can be different explanations behind hemoglobin inadequacy. Hemoglobin inadequacy prompts the lower oxygen-conveying limit of the blood. It tends to be because of nutritional inadequacy, kidney disorder, cancer, or any hereditary imperfections.

Greater than normal hemoglobin level is related to different heart and pulmonary illnesses.

Sickle cell Anemia: It is because of a deformity in the hemoglobin gene. There is a solitary nucleotide or point transformation in the beta-globin chain. ‘GAG’ gets changed over into ‘GTG’ prompting the substitution of glutamic acid by valine at the sixth position.

Thalassemia: It is caused because of less creation of hemoglobin. Thalassemia is of two types, Alpha-Thalassemia and Beta-Thalassemia. It is likewise caused because of damaged genes, and its seriousness relies heavily on the number of genes that are absent or defective. Hemoglobin level is ordinarily utilized as an indicative device. The HbA1c level, for example, glycosylated Hb or Hb connected with sugar is a marker for normal glucose level in the blood of a diabetic patient.

To summarize, hemoglobin is a fundamental color, which is expected for oxygen transport and to complete ordinary body capabilities.

Frequently Asked Questions

Question 1: Mention any 3 types of Hemoglobin. 


     The 3 main types of Hemoglobin are:

  • Hemoglobin A (Alpha2Beta2) – It represents 95-98% of hemoglobin in adults. It is made out of two alpha and two beta polypeptide chains.
  • Hemoglobin A2 (Alpha2Sigma2) – It represents 2-3% of hemoglobin present in adults. It is made out of two alpha and two delta polypeptide chains.
  • Hemoglobin F (Alpha2Gamma2) – It is known as fetal hemoglobin. 2-3% of hemoglobin F is available in adults. It is made out of two alpha and two gamma chains. Fetal hemoglobin has greater partiality for oxygen compared to  HbA.

Question 2: What is the normal level of hemoglobin?


The normal hemoglobin level shifts with the age and sex of the individual. The normal hemoglobin levels per dl of blood are:

  • Infant – 17-22 g/dl
  • Children  – 11-13 g/dl
  • Grown-up male – 14-18 g/dl
  • Grown-up female – 12-16 g/dl
  • Old age males – 12.4-14.9 g/dl
  • Old age females – 11.7-13.8 g/dl

Question 3: Discuss the abnormal hemoglobin.


Abnormal hemoglobin is a hemoglobin type or the mutant structure, which causes a blood problem or hemoglobinopathy. These can get acquired for posterity from parents. A few of the instances of abnormal hemoglobin are:

  • Hemoglobin S – The essential cause of sickle cell anemia. There is a point change in the globin chain. ‘GAG’ gets changed over into ‘GTG’ prompting the substitution of glutamic acid by valine at the sixth position. It causes deformation in the red blood cells prompting sickle shape of RBCs.
  • Hemoglobin C – Heterozygotes for the gene are asymptomatic. Hemoglobin C illness is caused by homozygotes. It prompts gentle hemolytic paleness and growth of the spleen.
  • Hemoglobin E – It is a beta chain variant of the normal hemoglobin. It is normally present in individuals of Southeast Asia. Causes gentle hemolytic anemia in homozygotes.
    Thalassemia is additionally caused because of different hereditary defects in the genes coding for alpha and beta chains of hemoglobin.

Question 4: Mention the various symptoms of low hemoglobin.


A low hemoglobin level means iron deficiency. The fundamental symptoms of low hemoglobin are:

  • Weakness
  • Unsteadiness
  • Breathing shortness
  • Pale or yellow skin
  • Pain in the chest

Question 5: How does hemoglobin function?


Hemoglobin transports oxygen from the lungs to the various parts of our body through the blood.  Hemoglobin ties to four molecules of oxygen through cooperative restricting in the lungs where the fractional pressure of oxygen (pO2) is high and deliveries oxygen in the tissues where the partial strain of oxygen (pO2)  is low. Around 20-25% of CO2 is moved bound to hemoglobin as carbaminohemoglobin. In tissues where pCO2 is high, restricting carbon dioxide is leaned toward and in the lungs, separation of carbaminohemoglobin happens because of low pCO2.

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