Blood is the medium of transport for 02 and COa.
I. Transport of Oxygen
Oxygen is transported from the lungs to the tissues through the plasma and RBC of the blood. 100ml of oxygenated blood can deliver 5 ml of Oa to the tissues under normal conditions.
(i) Transport of oxygen through plasma: About 3% of Oz is carried through the blood plasma in a dissolved state.
(ii) Transport of oxygen by RBC: About 97% of Oa is transported by the RBCs in the blood. Haemoglobin is a red coloured iron containing pigment present in the RBCs. Each haemoglobin molecule can carry a maximum of four molecules of oxygen. Binding of oxygen with haemoglobin is primarily related to the partial pressure of Oa. At lungs, where the partial pressure of Oa (oxygen tension) is high, oxygen binds to haemoglobin (purplish-bluish-red in colour) in a reversible manner to
form oxyhaemoglobin (bright red in colour). This is called oxygenation of haemoglobin.
Hb + 402. Hb(02)4
At the tissues, where the partial pressure of 02 is low, oxyhaemoglobin dissociates into haemoglobin and oxygen. The other factors that influence binding of oxygen with haemoglobin are the partial pressure of C02, the hydrogen ion concentration (pH) and the temperature.
(iii) Oxygen - haemoglobin dissociation curve: It explains the relation between percentage saturation of haemoglobin and partial pressure of oxygen. A sigmoid curve is obtained when percentage saturation of haemoglobin with 02 is plotted against the pOa. This curve is called 'oxyhaemoglobin dissociation curve' and is highly useful in studying the effect of factors such as pC02, H+ concentration, temperature, etc., on the binding of Oa with haemoglobin. In the alveoli, where there is a high pOa, low pCOa, lesser H+ concentration (high pH) and the lower temperature, the factors of are all favourable for the formation of oxyhaemoglobin. In the tissues where low p02, high pCOa, high H+ concentration (low pH) and higher temperature exist, the conditions are of favourable for dissociation of oxygen from oxyhaemoglobin. Under these conditions, oxygen dissociation curve shifts away from the Y-axis (to the right). The effect of pC02 and H+ concentration on the oxygen affinity of haemoglobin is called Bohr Effect (increase of carbon dioxide in the blood and decrease in pH results in the reduction of the affinity of hemoglobin for oxygen).
(iii) Oxygen - haemoglobin dissociation curve: It explains the relation between percentage saturation of haemoglobin and partial pressure of oxygen. A sigmoid curve is obtained when percentage saturation of haemoglobin with Oz is plotted against the pOa. This curve is called 'oxyhaemoglobin dissociation curve' and is highly useful in studying the effect of factors such as pCOa, H+ concentration, temperature, etc., on the binding of Oa with haemoglobin. In the alveoli, where there is a high pOa, low pC02, lesser H+ concentration (high pH) and lower temperature, the factors that are for all favourable for the formation of oxyhaemoglobin. In the tissues where low pOa, high pC02, high H+ concentration (low pH) and higher temperature exist, the conditions that are of favourable for dissociation of oxygen from oxyhaemoglobin. Under these conditions, oxygen dissociation curve shifts away from the Y-axis (to the right). The effect of pCOa and H+ concentration on the oxygen affinity of haemoglobin is called Bohr Effect (increase of carbon dioxide in the blood and decrease in pH results in the reduction of the affinity of hemoglobin for oxygen).
NOTE: A rise in pCOa and fall in pH decreases the affinity of haemoglobin for oxygen. On the other hand a fall in pCOa and rise in pH, increases affinity of haemoglobin for oxygen. This clearly indicates that 02 gets bound to haemoglobin at the lung surface and gets dissociated at the tissues.
NOTE: At pOa of lOOmmHg. typical in the lungs haemoglobin is saturated to about 97%. At a pOa of 40mm Hg which is common in tissues during rest time, haemoglobin is about 75% saturated. It means oxyhaemoglobin gives away only about 22% oxygen only to 'resting tissues'. It means, only about 1/5^ of the oxygen from blood is unloaded in the tissues. The remaining 4/5* is in the form of 'Reserve' in the blood itself. If the tissues such as skeletal muscles are involved in vigorous exercise, there is more'unloading tension' in the oxyhaemoglobin and so more oxygen is given away rapidly (up to 62% at 20mmHg.of p02 and the percent of saturation of haemoglobin is only 35%). As mentioned above in a 'resting person', haemoglobin always carries about 70% oxygen (which is still available to tissues, for the asking). Hence oxyhaemoglobin ensures supply of oxygen for survival for 4-5 minutes after the stopping of heart or when breathing is interrupted.
