Oxygen Transport in Blood

Oxygen is transported in the blood by binding to hemoglobin. 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.

In this article, we will cover how oxygen is transported in blood, in detail.

The oxygen binds to the hemoglobin to form oxyhemoglobin. It is a reversible reaction in which haemoglobin present on RBCs (or red blood cells) 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. 

Haemoglobin is a protein found in red blood cells that is essential for transporting oxygen from the lungs to the rest of the body and returning carbon dioxide from the tissues back to the lungs. It consists of four subunits, each containing an iron atom that can bind to one oxygen molecule. Haemoglobin also plays a role in maintaining the acid-base balance of the blood.

Oxygen-Carrying Capacity of Hemoglobin

Hemoglobin is vital for oxygen transport in the blood, with each molecule capable of binding up to four oxygen molecules. This binding capacity ensures efficient oxygen delivery from the lungs to various tissues. Several factors, including blood pH, temperature, and carbon dioxide levels, can affect hemoglobin’s ability to release oxygen. This efficient oxygen transport system is essential for sustaining cellular respiration and overall metabolic activities in the body.

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.

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.

Also Read: Transport of Carbon Dioxide in Blood

The transport of oxygen in the blood is a complex yet efficient process primarily facilitated by hemoglobin. Oxygen binds to hemoglobin in the lungs, forming oxyhemoglobin, and is released to tissues based on various physiological factors. The oxygen-hemoglobin dissociation curve illustrates how oxygen affinity changes with partial pressure, pH, carbon dioxide levels, and temperature. Understanding these mechanisms is crucial for appreciating how the body maintains oxygen supply under different conditions, ensuring proper cellular function and overall metabolic activity.

Also Read:

• Exchange Of Gases Notes CBSE Class 11
• Regulation Of Respiration
• Facts About Blood
• NCERT Notes Class 11 Biology Chapter 14 – Breathing and Exchange of Gases

How is Oxygen Transported in the Body?

Oxygen is transported in the body primarily by binding to hemoglobin in red blood cells, forming oxyhemoglobin.

How is Oxygen Transported in Human Body Sequence?

Oxygen is inhaled into the lungs, binds to hemoglobin in red blood cells, and is transported through the bloodstream to tissues where it is released.

How O2 and CO2 are Transported in the Blood?

Oxygen binds to hemoglobin in red blood cells, while carbon dioxide is transported as bicarbonate ions, carbaminohemoglobin, and dissolved CO2 in plasma.

How is Oxygen Transported in Human Beings?

Oxygen is transported by binding to hemoglobin in red blood cells and carried through the bloodstream to tissues for cellular respiration.

What are the Factors Affecting the Affinity of Haemoglobin for Oxygen?

The affinity of hemoglobin for oxygen is affected by pH levels, partial pressure of carbon dioxide (pCO2), temperature, and the presence of 2,3-Bisphosphoglycerate (2,3-BPG).