The role of haemoglobin in the transport and combination of oxygen

2021-05-16 11:38 PM

O2 molecules are loosely and reversibly attached to the heme portion of haemoglobin. When PO2 is high, as in pulmonary capillaries, O2 binds to haemoglobin, but when PO2 is low, as in tissue capillaries, O2 is released from haemoglobin.

Haemoglobin's role in oxygen transport

Normally, about 97 percent of the oxygen transported from the lungs to the tissues is bound to the haemoglobin in red blood cells. The remaining 3% is transported as soluble in plasma and blood cells. Therefore, under normal conditions, almost all oxygen is transported to the tissues by haemoglobin.

Haemoglobin's role in oxygen transport and incorporation

The reversible combination of oxygen and haemoglobin

O2 molecules are loosely and reversibly attached to the heme portion of haemoglobin. When PO2 is high, as in pulmonary capillaries, O2 binds to haemoglobin, but when PO2 is low, as in tissue capillaries, O2 is released from haemoglobin. This is the basis for most O2 transport from the lungs to the tissues.

 

Figure. Oxygen-Haemoglobin (HbO2) dissociation graph

The figure shows a graph of Oxy-haemoglobin dissociation, which shows a gradual increase in the ratio of haemoglobin to O2 as blood PO2 increases, a ratio known as haemoglobin saturation. Since blood leaving the lungs and entering the systemic arteries normally has a PO2 of about 95 mm Hg, it can be seen from the graph that: under normal conditions, O2 saturation in the systemic arteries accounts for an average of 97 %. In contrast, in venous blood, PO2 retrieved from peripheral tissues is normally about 40 mm Hg, and the average haemoglobin saturation is 75%.

The maximum amount of oxygen that can combine with Haemoglobin in the blood

The blood of a normal person contains about 15 grams of haemoglobin in every 100 millilitres of blood, and each gram of haemoglobin can carry a maximum of 1.34 millilitres of O2 (1.39 millilitres when haemoglobin is in its pure chemical form, impurities such as : methaemoglobin will reduce O2 transport. So: 15 multiplied by 1.34 equals 20.1 That means: on average 15 grams of haemoglobin in 100 ml of blood can carry a total of about 20 ml of O2 if the saturation of haemoglobin is 100%. This is usually expressed as 20% by volume. The normal human oxy-haemoglobin dissociation graph can also be shown as a volume percentage of O2, as shown on the scale to the right of the figure, instead of a percentage of haemoglobin saturation.

Amount of oxygen dissociated from haemoglobin as systemic arterial blood passes through tissues

Normally, the total amount of O2 bound to haemoglobin in systemic arterial blood (with a saturation of 97 %) is about 19.4 ml per 100 ml of blood, as shown in the figure. After passing through the capillaries in the tissues, the average amount of O2 is reduced to 14.4 ml (PO2 PO2 40 mm Hg, 75% haemoglobin saturated). Thus, under normal conditions, about 5 ml of O2 is transported from the lungs to the tissues by every 100 millilitres of blood flow.

Figure. Effect of PO2 in the blood on the amount of Oxygen bound to Haemoglobin

The transport of oxygen increases markedly during heavy work

During heavy exercise, muscle cells use O2 at a rapid rate, so that: under extreme circumstances, it is possible to cause interstitial fluid PO2 to drop from the normal 40 mm Hg to as low as 15 mm Hg. At such a low O2 pressure, only 4.4 ml of O2 is bound to haemoglobin in every 100 ml of blood, as shown in Figure 41-9. Hence: 19.4 – 4.4; or 15 ml, which is the actual amount of O2 delivered to the tissues for every 100 millilitres of circulating blood, which is more than three times more O2 than the normal amount of O2 per volume of blood transported through the tissues. Remember that cardiac output can increase 6-7 times normal in well-trained marathon runners. Thus, multiplying the increase in cardiac output (6-7 times) by the increase in O2 transported per blood volume (3 times) gives a 20-fold increase in cardiac output. O2 to the tissues.

Coefficients used

The percentage of blood that delivers its O2 as it passes through the tissue capillaries is called the utilization factor. The normal value of this factor is about 25 %. It is clear from the previous discussion that 25% of the haemoglobin that has been attached to oxygen releases its O2 to the tissues. During exertion, the total body utilization factor can be increased up to 75 - 85 %. In areas with very slow blood flow or very high metabolic rates, a utilization factor of close to 100% has been observed, all the required O2 has been delivered to the tissues.