What is Hemoglobin?
Hemoglobin is the red pigment of blood. Commonly it is expressed by a short term Hb.
Chemical composition of Hemoglobin :
Hemoglobin is a chromoprotein or metalloprotein type of conjugated protein consisting of two parts—a prosthetic group which is an iron containing pigment called heme(or haem) and a histone type simple protein called globin. Heme is a ferrous iron ,Fe ++ containing porphyrin compound. Each hemoglobin molecule consists of 4 heme units and a globin. The globin is a protein made up of 4 polypeptides. Each heme remains attached to one polypeptide. The molecular weight of hemoglobin is 68000. Hemoglobin has the unique property of combining with oxygen reversibly.
Normal values of Hemoglobin content of blood :
The amount of Hemoglogin present in blood is expressed in grams of Hb per 100 ml blood (g%). Normal human blood contains about 14.5g% Hb. In normal adult males, it varies between 14-18g/100 ml ; while in females, the value is varying between 12-15g/100 ml. The instrument used to determine hemoglobin concentration of blood is called hemometer. Clinically, the amount of Hemoglobin in a blood sample is also expressed as percentage of normal (i.e., Hb%) and the normal value of 14.5g% is taken as 100% for comparison.
Variations of the Hemoglobin concentration :
The concentration of Hb in blood may vary under certain normal physiological conditions as follows :  Age : In the foetus, Hb concentration is highest. At birth, it is about 23 g%. Then if falls below normal and finally rises again to the normal level in adults.  Sex : In females, the amount of Hb is slightly lower than males.  Altitude : In high altitude (hill area) dwellers, the Hb level is higher than those dwelling at sea level.  Exercise, excitement etc., increase the blood level of Hb.  Diurnal variation : In the morning, the Hb level is lowest and in the evening it is highest. It should be noted that the above mentioned variations of Hb content in blood is mainly due to alteration of RBC count and not due to any change in the amount of Hb in each RBC. The Hb content is proportional to the number of RBC.
What is the 0rigin of Hemoglobin ?
Hemoglobin is synthesized in the developing RBCs in the red bone marrow.
What are the functions of Hemoglobin :
 The main function of hemoglobin is transportation of O2 from lung to tissues in the form of oxyhemogiobin.  It also helps in transport of CO2 by combining with CO2 to form carbaminohemoglobin.  Hemoglobin forms an important shock absorber system in blood for maintenance of acid-base balance in the body.  Various pigments of bile, urine and stool are found from Hb.
Normal varieties of Hb
In normal humans, two types of hemoglobins are found—adult hemoglobin (HbA) and toetal hemoglobin (HbF). In the foetal blood, the hemoglobin remain almost entirely in the form of HbF which is replaced by the HbA after birth. A small quantity of HbF persists in the blood for some weeks or months after birth. The HbF and HbA differ chemically due to some variation in their globin portion. HbF has a greater affinity for O2 and releases CO2 more eagerly than HbA. This property is of great importance because due to this, the foetal blood (HbF) can take up O2 from and give up CO2 to the maternal blood (HbA).
Compounds derived from Hemoglobin :
i) Oxyhemoglobin-It is an unstable compound formed by combination of O2 with the ferrous iron of hemoglobin ii) Methemoglobin-It is a stable compound formed by combination of O2 with the iron of Hb when it is in ferric form. iii) Carboxyhemoglobin -This compound is formed by combination of carbon monoxide (CO) with the ferrous iron of Hb. It is a very stable compound. Formation of this leads to poisoning of Hb when the Hb fails to bind O2. iv) Carbohymoglobin -It is formed by combination of CO2 with the globin portion of Hb. v) Nitric oxide hemoglobin-It is formed during nitric oxide (NO) poisoning by combination of NO with the iron of Hb. vi) Other compounds-In addition to the above mentioned iron containing compounds of hemoglobin, various other iron-containing and iron-free products may be derived by reactions involving denaturation and breakdown of Hb. The iron-containing products include— hematin hemin,  emochromogen,  heme,  cathemoglobinetc. The iron-free derivatives of hemoglobin include— hematoporphyrin,  hemopyrrole, hematoidin, [D] bilirubin etc.