Give an essay on the transport of respiratory gases in man ?

Blood is the medium oftransport ofrespiratory gases between lungs and tissues. It
transports 02 from lungs to tissues, and CO2 from tissues to lungs.
Transport of oxygen
Blood transports 02 in two ways (i) in chemical combination with haemoglobin (ii) in physical solution in the water ofplasma and corpuscles. Normally, about 97% of 02 is transported in combination with haemoglobin, and about 3% in dissolved state.
Haemoglobin is a red coloured iron-containing respiratory pigment found in erythrocytes. It is formed of a basic protein called globin and an iron containing portion called haem . Haemoglobin molecule consists of a globin molecule and four haem molecules. The globin molecule, in is formed of four polypeptide chains, and the haem molecule is formed of an iron atom and a porphyrin molecule. Iron occurs in ferrous (Fe I I ) state. ifaem is in active group that combines with 02 .
Haemoglobin has great affinity for 02 at high P02. In the respiratory surfaces the P02 is very high. At this high P02 it loosely and reversibly combines with 02 and forms the unstable globin oxyhaemoglobin. This is called oxygenation of haemoglobin
Hb + 02 --> Hb02 (Hb represents haemoglobin)
The High P02 at which haemoglobin gets oxygenated is known as tension of loading. Each iron atom ofit can bind with one molecule ofoxygen. So, a haemoglobin molecule can bind with as many as four molecules of 02 at a time. In that condition haemoglobin is said to be saturated or fully loaded. Saturation ofhaemoglobin takes place at the maximum tension ofloading (ie. at maximum P02).
In the tissues P02 is very low. Hence, oxyhaemoglobin dissociates and sets free 02. This is known as deoxygenation of haemoglobin. The low P02 at which haemoglobin gets deoxygenated is known as tension of unloading.
Transport of carbon dioxide.
Blood transports CO2 to respiratory surfaces in three ways: (1) as carbonic acid. (2) as carbaminohaemoglobin (3) as bicarbonates.
(i) Transport of CO2 as carbonic acid :
In this case CO2 reversibly combines with plasma water and fo s carbonic acid. On reaching the lungs carbonic acid dissociates, and releases CO2. About 5% of CO2 is transported in this manner.
CO2 + H20-> H2CO3
(ii) Transport of CO2 as carbamino haemoglobin :
In this case CO2 reversibly combines with amino (NH2) group of haemoglobin, and forms carbamino haemoglobin. On reaching the lungs carbamino haemoglobin dissociates, releasing CO2. About 10% of CO2 is transported in this manner
CO2 + Hb.NH2 --> Hb.NHCOOH