Hemoglobin 

Hemoglobin is the iron-containing oxygen transport metalloprotein found in red blood cells (RBC) of all vertebrates and tissues of some invertebrates. With some very rare exceptions of leptocephalus (larvae of eel), all classes of vertebrates possess hemoglobin in their RBCs, and their muscles contain myoglobin. 

1.0Hemoglobin Structure

• Hemoglobin is a conjugated protein made up of four subunits. 
• Each subunit is made up of a protein part called globin and an iron-containing protoporphyrin (heme) ring. 
• Heme attaches to the polypeptide chain by a nitrogen atom to form one hemoglobin subunit. Each of the four heme portions of the hemoglobin molecule contains an atom of Iron (Fe), which binds oxygen. 
• The iron in the heme is in a ferrous state (Fe²⁺). 
• The protein moiety or globin varies considerably in size, amino acid composition, solubility and other physical properties from animal to animal. 
• The iron content of mammalian hemoglobin is 0.336 %, and the heme content is 4 %. 
• In each heme unit, the iron atom is joined by four coordination bonds to the four nitrogen atoms of protoporphyrin. 
• One of the remaining six coordination bonds is joined to the globin molecule. 

• Since each hemoglobin molecule has four heme rings, it carries a total of four oxygen molecules. 
• Hemoglobin is beautifully adapted to the task of an oxygen carrier as it can readily pick up or release oxygen in response to the change in partial pressure of oxygen in lungs, blood, or tissues. 
• Hemoglobin can also combine with CO₂ or other gasses. 
• It can carry a small amount of the CO₂ out of tissues. 
• Hemoglobin can combine with 4 carbon monoxide (CO) molecules and form carboxyhemoglobin. 
• The affinity of hemoglobin is about 200 times higher for CO than oxygen. 
• It can, therefore, cause CO poisoning if inhaled in even small quantities and result in death by anorexia (lack of oxygen). 

2.0Gaseous Transport

• Oxygen has very low solubility in plasma (~0.3 ml/100ml). Most of the oxygen is carried by the hemoglobin in the form of oxyhemoglobin. 
• Most of the CO₂, on the other hand, is transported by plasma in the form of soluble bicarbonates. 
• Transport of Oxygen: Oxygen is carried by hemoglobin, which is found in red blood cells. 
• The maximum amount of O₂ that normal human blood can absorb is 20 ml per 100 ml of blood. 
• During the passage of oxygen from lung alveoli to lung capillaries, oxygen diffuses into the blood, and it is captured by hemoglobin to form oxyhemoglobin. 

• Each hemoglobin molecule has 4 iron atoms, which can carry 4 oxygen molecules. 
• Normally, the arterial blood exposed to the lungs' alveoli is not completely oxidized. 
• At the O₂ pressure of 100 mm Hg, blood is about 98 % saturated and, therefore, contains 19.6 ml of O₂ per 100 ml. About 0.2 -0.3 ml of O2 is dissolved in plasma. 
• The arterial blood and the alveoli have the same O2 pressure (100 mm Hg). 
• However, in the cells and tissues of the body, the O₂ tension is quite low (1 to 40 mm of Hg). 
• Since the oxygen coordination with the hemoglobin is reversible, it dissociates from the oxyhemoglobin and diffuses into the cells. 
• This phase is important to supply oxygen to cells and regenerate hemoglobin for further transport cycles.
• The reduced hemoglobin is transported back via blood to the lungs. 

3.0Oxygen Hemoglobin Dissociation Curve

• Oxygen binding to hemoglobin is influenced by four factors: partial pressure of oxygen and carbon dioxide, temperature, H⁺ concentration and 2,3-phosphoglyceraldehyde (2,3-DPG). 
• The quantity of oxygen that can be held by hemoglobin depends on the partial pressure of oxygen (PO₂). 
• The oxygen-hemoglobin dissociation curve can graphically represent the relationship between the two. 
• The curve is sigmoid (S-shaped) in nature. 
• The important feature of the oxygen-hemoglobin dissociation curve is that hemoglobin takes up oxygen when the partial pressure of the latter is high; oxyhemoglobin dissociates when the PO₂ is low. 
• The hemoglobin is almost completely saturated at an O₂ tension of 100 mm Hg. However, as the oxygen pressure drops below 60 mm of Hg, it dissociates rapidly, thus forming the steep slope of the curve. When PO₂ is zero, all of the oxyhemoglobin dissociates into hemoglobin. 
• The actual relationship between the partial pressure of O₂ and the degree of saturation of the hemoglobin with O₂ is shown by the Oxygen-hemoglobin dissociation curve.

• Haemoglobin gets saturated at about 100 mm Hg pressure, and no more oxygen can be taken up even if the pressure is increased. 
• Inside tissues where the partial pressure of oxygen is less, the oxygen is rapidly dissociated from the oxyhemoglobin, thus yielding larger quantities of the O₂ to the surrounding tissues and cells where it is needed most. 
• During exercise or hard physical work, the partial pressure of oxygen falls in tissues accompanied by increased pH, local temperature and increase in 2,3-DPG concentration. 
• All these factors combined promote the dissociation of oxyhemoglobin to release more oxygen. 

4.0Transport of Carbon Dioxide

• CO₂ is formed in the body due to various metabolic activities of the cell and diffuses into the blood. 
• CO₂ concentration in venous blood is about 60 ml/100 ml of blood, while in arterial blood, it is about 50 ml/100 ml. 
• CO₂ is transported in three ways, which are as follows: 

a) Transport as carbonic acid: As CO₂ enters the blood from the body tissues, it reacts with water present in the plasma to form carbonic acid (H₂CO₃). About 5% of the total CO₂ dissolved in blood is carried as carbonic acid. 

b) Transport as carbamino compounds: In Red Blood Cells (RBCs), CO₂ combines directly with the amino groups (-NH₂) of the haemoglobin to form the carbaminohemoglobin. About 10 % of the total CO2 is transported in this complex.

c) Transport as bicarbonates: About 85 % of the CO₂ is carried in the form of bicarbonates in plasma and RBCs. As CO₂ diffuses into the blood, it forms carbonic acid, as discussed earlier, which then dissociates to give bicarbonate ions (HCO₃ ^- ) and hydrogen ions (H⁺ ). The bicarbonate ion then combines with sodium or potassium to form respective bicarbonates.

• In normal conditions, most carbon dioxide is present as bicarbonate ions. 
• According to the Henderson Hasselbach equation, at a pH of 7.4, the ratio of carbonic acid to bicarbonate ions is 1:20. 
• The hydrogen ion formed as a result of the dissociation of carbonic acid is neutralized by various buffering agents present in the blood. 
• The RBCs contain an enzyme called carbonic anhydrase, which catalyzes the reversible reaction between CO₂ and H₂O, forming carbonic acid (H₂CO₃) and subsequently bicarbonates. 

• The enzyme also facilitates the rapid release of CO₂ from the blood through the lungs. 
• Thus, despite the low concentration gradient (the partial pressure of CO₂ in tissues is 45 mm of Hg, while in arteriolar capillaries, it is 40 mm of Hg), carbon dioxide diffuses rapidly into the blood.