In this article we will discuss about the contents of oxygen in blood of human beings.
The O2 content of the arterial and venous blood and relevant data are given in the following table:
It will be seen that about 98% of oxygen is present in the blood in chemical combination with haemoglobin and only 0.3 ml in arterial blood (about 2%) is in physical solution in water of the plasma. However, this small quantity in physical solution is responsible for tension of oxygen in the plasma.
It is the oxygen tension in the blood which is responsible for transfer of this gas from alveolar air to the venous blood across the alveolo-capillary membrane and also for transfer of this gas from arterial blood to the tissue fluid across the capillary membrane. Further, it is the tension of oxygen in plasma which controls the amount of oxyhaemoglobin present in the blood.
Haemoglobin is remarkable in that it can combine with oxygen forming a loose chemical compound known as oxyhaemoglobin which differs from oxide of haemoglobin known as methaemoglobin in the fact that the O2 in methaemoglobin is firmly fixed with the haemoglobin molecule.
The amount O2 of the haemoglobin will combine with forming oxyhaemoglobin is dependent upon the tension of oxygen in the medium where haemoglobin is placed. If the O2 tension is high a large amount of oxyhaemoglobin will be formed whereas in a medium of low O2 tension the oxyhaemoglobin will break down liberating O2 itself being converted into reduced haemoglobin.
O2 Capacity, O2 Content and Percentage Saturation of Haemoglobin:
1 gram of haemoglobin when fully saturated will combine with 1.34 ml of O2. Since the blood contains approximately 15 grams of Hb per 100 ml-the oxygen content of the blood when fully saturated will be about 15 × 1.34 = 20 ml. This is called O2 capacity of the blood. Naturally the O2 capacity depends upon the amount of haemoglobin in blood.
Oxygen content of the blood is usually determined by Van Slyke’s manometric apparatus in which 1 ml of blood is treated anaerobically with potassium ferricynide solution in the chamber of the apparatus. The pressure exerted by the liberated oxygen after absorption of CO2 on a manometer attached with the apparatus is noted and is used for calculation of the oxygen content.
If the blood is first agitated with air so that the Hb gets fully saturated with O2 the result will indicate O2 capacity.
Thus if in a particular sample of blood the O2 content = 19 volume % and the O2 capacity = 20 volume %.
The percentage saturation of Hb = 19/20 × 100 = 95%. If in another sample of blood the O2 content is 15 volume % and capacity is 20 volume %. The saturation of Hb will be 75%. If the same blood contained 10 volume of O2 per 100 ml the saturation would be 50%.
Delivery of O2 in the Tissues in Anaemia:
Table 8.9 below gives approximate relevant values of oxygen in blood in normal subjects and in a patient with anaemia (Hb content of blood = 7.5 grams of or 50 % of normal).
The first line of compensation is, therefore, effected by drawing upon the reserve volume of O2 from the mixed venous blood as a result of which the venous blood gets more unsaturated. However, in spite of the greater coefficient of O2 utilisation the tissues fail to get adequate amount of O2 as shown in the last column.
Due to increased quantity of 2, 3-DPG within the RBC in anaemia, the dissociation curve shifts to the right which favours release of oxygen in the tissues. The third method of compensation is increased cardiac output which ensures increased blood flow (and oxygen supply) to the tissues.
Oxygen Exchange in the Lungs (Fig. 8.25):
The tension of O2 in the alveolar air is 100 mm Hg and that of the dissolved O2 in the plasma of the mixed venous blood is only 40 mm Hg.
Due to the tension gradient O2 diffuses rapidly from the alveolar air to the mixed venous blood increasing the quantity and also tension of O2 in the plasma in the venous blood. Rise of O2 tension of the mixed venous blood is followed by rapid increase in O2 saturation of the haemoglobin. The oxygenated blood leaves the pulmonary capillaries in tension equilibrium with alveolar air, that is, with O2 tension of 100 mm Hg and with haemoglobin 98% saturated with oxygen.
i. The gaseous composition of blood is different in different veins of the body O2 content of blood of veins draining metabolically active tissues (e.g., muscles, heart) is low whereas the O2 content of blood of veins from skin and brain is rather high. The term mixed venous blood indicates sample of venous blood from the right heart or pulmonary artery.
ii. The actual gas exchange in the lungs takes place between the gases dissolved in alveolar fluid and plasma water.
Oxygen Transport in the Tissues (Fig. 8.25):
Oxygen tension in the tissue fluid is low and is about 40 mm Hg during ‘rest’. The arterial blood enters the tissue capillaries with an oxygen tension of 100 mm Hg and with haemoglobin 98 % saturated with oxygen. So the O2 diffuses from the plasma to the tissue fluid due to tension gradient between the two fluids. As O2 diffuses the amount of O2 in solution decrease and the tension of O2 in the arterial blood falls.
This results in desaturation of haemoglobin which gives oxygen firstly to the plasma from where it goes to the tissue space— the guiding force being the tension gradient. We know that the reaction is very rapid and so the blood leaves the tissues with oxygen in tension equilibrium with tissue fluid that is 40 mm Hg and consequent 75% saturation of haemoglobin with oxygen.