Oxygen Transport in Blood (original) (raw)
Last Updated : 4 Jun, 2026
Oxygen is transported in the blood mainly by binding with haemoglobin present in red blood cells. Blood plays an essential role in the transport of respiratory gases between the lungs and body tissues. Since oxygen is only slightly soluble in the water present in blood plasma, approximately 97% of oxygen is transported in combination with haemoglobin as oxyhaemoglobin, while the remaining 3% is transported dissolved directly in plasma.
Oxygen is transported in human beings mainly through haemoglobin present inside red blood cells. Oxygen binds reversibly with haemoglobin to form oxyhaemoglobin. Haemoglobin is an iron-containing respiratory pigment composed of four polypeptide subunits, each containing a haem group with an iron atom capable of binding one oxygen molecule.
Hb + 4O2 ⇌ Hb(O2)4
Deoxyhaemoglobin Oxygen Oxyhaemoglobin
- In this reversible reaction, deoxyhaemoglobin combines with oxygen to form oxyhaemoglobin.
- Since haemoglobin has a reversible affinity for oxygen, oxygen remains bound to haemoglobin in the blood and is released into tissues under suitable physiological conditions such as low partial pressure of oxygen, increased carbon dioxide concentration, decreased pH, and increased temperature.
- These conditions promote the dissociation of oxygen from haemoglobin and facilitate oxygen delivery to actively respiring tissues.
Haemoglobin Protein in Blood
- Haemoglobin is an iron-containing respiratory protein present in red blood cells that plays an essential role in the transport of respiratory gases in the body.
- It transports oxygen from the lungs to various tissues and also helps carry a portion of carbon dioxide from the tissues back to the lungs for removal during exhalation.
- Haemoglobin consists of four polypeptide subunits, each containing a haem group with an iron atom capable of binding one oxygen molecule.
- Therefore, one molecule of haemoglobin can transport a maximum of four oxygen molecules. In addition to gas transport, haemoglobin also helps maintain the acid-base balance of blood by buffering hydrogen ions
Oxygen-Carrying Capacity of Haemoglobin
- Haemoglobin is highly important for oxygen transport in the blood because each haemoglobin molecule can bind reversibly with up to four oxygen molecules.
- This high oxygen-carrying capacity allows efficient transport of oxygen from the lungs to body tissues where it is required for cellular respiration and energy production.
- The binding and release of oxygen by haemoglobin are influenced by factors such as blood pH, temperature, and the partial pressure of carbon dioxide.
- These factors regulate the affinity of haemoglobin for oxygen according to the metabolic needs of tissues.
- The efficient functioning of haemoglobin is therefore essential for maintaining normal respiration, metabolism, and overall physiological activities in the body.
Oxygen Dissociation Curve
The partial pressure of oxygen (pO2) is the most important factor that determines the binding of oxygen with haemoglobin. A high pO2 increases the binding of oxygen with haemoglobin, whereas a low pO2 decreases the binding. When all haemoglobin molecules are completely bound with oxygen, haemoglobin is said to be fully saturated. If some haemoglobin remains unbound, it is said to be partially saturated. The percentage saturation of haemoglobin with oxygen is represented by the oxygen-haemoglobin dissociation curve, also known as the oxygen dissociation curve.
Relationship Between Haemoglobin and Partial Pressure of Oxygen
The oxygen-haemoglobin dissociation curve is sigmoid or S-shaped in appearance and possesses the following characteristics:
- The partial pressure of oxygen determines the degree of haemoglobin saturation. High pO2 promotes greater binding of oxygen with haemoglobin and results in nearly complete saturation, whereas low pO2 reduces oxygen binding and causes partial saturation.
- Under resting conditions, when the pO2 is approximately 40 mmHg, haemoglobin remains nearly 75% saturated. This indicates that about 25% of oxygen carried by haemoglobin is released to the tissues under normal resting conditions.
- At a pO2 between approximately 60 and 100 mmHg, haemoglobin becomes almost fully saturated, reaching about 90–100% saturation. This ensures efficient oxygen transport from the lungs to body tissues.
- At a pO2 of approximately 20 mmHg, haemoglobin saturation decreases to nearly 35% because a large amount of oxygen is released into actively metabolizing tissues that require increased oxygen supply.
- Each 100 mL of oxygenated blood delivers approximately 5 mL of oxygen to body tissues.
Factors Affecting the Affinity of Haemoglobin for Oxygen
Certain factors other than the partial pressure of oxygen influence the affinity of haemoglobin for oxygen. These factors determine whether the oxygen-haemoglobin dissociation curve shifts toward the right, indicating decreased affinity of haemoglobin for oxygen, or toward the left, indicating increased affinity of haemoglobin for oxygen. The major factors affecting oxygen-haemoglobin affinity include pH, partial pressure of carbon dioxide, and temperature.
1. pH
- A decrease in pH, which indicates increased acidity, promotes the dissociation of oxygen from haemoglobin at a given partial pressure of oxygen.
- As a result, the oxygen saturation of haemoglobin decreases and the oxygen-haemoglobin dissociation curve shifts toward the right. This phenomenon is known as the Bohr effect.
- Conversely, an increase in pH increases the affinity of haemoglobin for oxygen and shifts the curve toward the left
2. pCO2
- An increase in the partial pressure of carbon dioxide decreases the affinity of haemoglobin for oxygen and shifts the oxygen-haemoglobin dissociation curve toward the right.
- Carbon dioxide combines with water in red blood cells to form carbonic acid, which is rapidly converted into bicarbonate ions and hydrogen ions by the enzyme carbonic anhydrase.
- The increase in hydrogen ion concentration lowers the pH of blood and promotes the release of oxygen from haemoglobin. Conversely, a decrease in carbon dioxide concentration shifts the curve toward the left.
3. Temperature
- An increase in temperature promotes the dissociation of oxygen from haemoglobin and facilitates the unloading of oxygen into metabolically active tissues.
- As a result, the oxygen-haemoglobin dissociation curve shifts toward the right, as observed during fever and increased muscular activity.
- Conversely, lower temperatures increase the affinity of haemoglobin for oxygen and shift the curve toward the left, as seen in hypothermia.
Factors Affecting the Affinity of Haemoglobin for Oxygen