Transport of Oxygen

Blood is responsible for the transportation of gases between the lungs and body tissues. Oxygen is transported bound to hemoglobin as it does not dissolve well in water present in plasma i.e. 97% of oxygen is transported with hemoglobin and the remaining 3% in plasma. Whereas carbon dioxide reacts easily with water to form bicarbonate ions and thus around 70% is transported as bicarbonates, 20-25% is bound to hemoglobin, and the remaining 5-10% is dissolved in plasma. 

Transport of Oxygen

The oxygen binds to the hemoglobin to form oxyhemoglobin. It is a reversible reaction in which hemoglobin present on RBCs and containing four atoms of iron binds with oxygen in a reversible reaction. 

Hb              +           2O₂                 ⇌                Hb-O₄
Deoxyhaemoglobin      Oxygen                          Oxyhaemoglobin

As oxygen has a higher affinity for hemoglobin only the 3% of oxygen dissolved in the plasma gets directly diffused into the tissues whereas the remaining 97% of oxygen needs other factors like an increase in temperatures to dissociate from the Hb and get unloaded into the tissues. 

Relationship between Haemoglobin and Partial Pressure of Oxygen

The partial pressure of oxygen or pO₂ is the most important factor that determines the binding of oxygen with hemoglobin i.e. high pO₂ higher the binding and low pO₂ lowers the binding. When all the hemoglobins present completely binds with oxygen, the hemoglobin is said to be fully saturated whereas if there is free hemoglobin remaining then the hemoglobin is said to be partially saturated. It means that the saturation level of hemoglobin with oxygen is expressed as its percent saturation. And this percent saturation is depicted by the oxygen-hemoglobin dissociation curve or oxygen dissociation curve. 

This curve is a sigmoid-shaped curve with the following characteristics;

1. The partial pressure of oxygen determines the saturation of hemoglobin i.e. high pO₂ high Hb-O₂ binding and 100% saturation, whereas low pO₂ low Hb-O₂ binding, and partial saturation.
2. In resting condition i.e. at around pO₂ of 40 mmHg the Hb is 75% saturated. It means that when a person is at rest only 25% of the available O₂ is unloaded into the tissues.
3. At around 60 to 100 mmHg pO₂ the blood takes an almost full load of oxygen i.e. it gets saturated up to 90%. This is the reason why at higher altitudes people can perform well.
4. In 20 mmHg Hb saturation drops at around 35% O₂. That means between 40 mmHg to 20 mmHg Hb saturation drops from 75% to 35% due to large quantity dumping of oxygen. This is seen in active tissues as they require large quantities of oxygen. 
5. Each 100 mL of oxygenated blood provides 5 mL of gaseous O₂ to the tissues.

Factors Affecting the Affinity of Haemoglobin for Oxygen

There are certain factors other than partial pressure that affects the Hb-O₂ affinity. These factors determine whether the oxygen-hemoglobin dissociation curve will shift towards the right i.e. lower affinity of O₂-Hb or the curve will shift towards the left i.e. higher affinity of O₂-Hb. These factors are;

1. pH: A decrease in pH as acidity increases results in the dissociation of O₂ from Hb at any given pO₂. The saturation becomes less and the curve shifts to the right, this is termed as Bohr Effect. Similarly, an increase in pH results in the shifting of the curve towards the left as the affinity of O₂ for Hb increases. 
2. pCO₂: As carbon can also bind with Hb, an increase in their concentration will result in a decrease in HB-O₂ binding. This results in the shifting of the curve towards the right. Also when CO₂ enters the blood it gets converted into carbonic acid, which due to the activity of an enzyme carbonic anhydrase present in RBC gets converted into bicarbonate and hydrogen ions. An increase in hydrogen ion concentration decreases the pH shifting the curve toward the right. Similarly, a decrease in the concentration of carbon dioxide in the blood will shift the curve toward the left.
3. Temperature: An increase in temperature promotes the dissociation of O₂ from Hb (one of the factors that promote the unloading of oxygen into tissues due to heat released by cellular metabolism) thus, shifting the curve towards the right as seen in fever. Lower body temperatures as in the case of hypothermia promote the binding of O₂ with Hb and the curve shifts towards the left.

FAQs Transport of Oxygen in Blood

Q1: Which has more affinity for oxygen fetal Hb or adult Hb?

Answer: 

Foetal Hb due to structural difference. 

Q2: What will be the percentage of saturation if only 2 molecules of O₂ bind with Hb?

Answer: 

50% as Hb can bind 4 molecules of O₂ at a given time.

Q3: Which cell in our body doesn’t respire?

Answer: 

RBCs don’t respire.

Q4: Other than temperature produced during cellular metabolism, what are the factors contributed by the cells of a tissue that promotes the dissociation of O₂ from Hb?

Answer: 

Cellular metabolism results in the production of acidic substances and CO₂ as waste that promotes the dissociation of O₂ from Hb.