Transport of Gases in the Blood

Transport of Oxygen

Oxygen is carried in the blood in two forms - dissolved in plasma and bound to haemoglobin in Red Blood Cells (RBCs).
Only about 1.5% of oxygen is carried dissolved in plasma.
The majority of oxygen, about 98.5%, is carried in RBCs bound to haemoglobin. When haemoglobin binds with oxygen, it forms oxyhaemoglobin.
Haemoglobin and oxygen have a positive cooperative relationship, meaning as more oxygen binds to haemoglobin, the higher the affinity for even more oxygen to bind. This binding is reversible.
The oxygen-haemoglobin binding is influenced by partial pressure. In the alveoli, where partial pressure of oxygen is high, haemoglobin readily binds to oxygen. In tissues, where the partial pressure of oxygen is low, haemoglobin releases oxygen.
The binding and unbinding of oxygen and haemoglobin can be displayed in an oxygen dissociation curve.

Carbon Dioxide is also carried in the blood in three ways - dissolved in plasma, attached to haemoglobin, and as bicarbonate ions.
Around 7% of carbon dioxide is transported dissolved directly in blood plasma.
Almost 23% of carbon dioxide is carried in RBCs attached to haemoglobin, forming carbaminohaemoglobin.
The majority of carbon dioxide, around 70%, is carried in the blood as bicarbonate ions. Carbon dioxide reacts with water inside RBCs to form carbonic acid which quickly dissociates into hydrogen ions and bicarbonate ions.
The reaction is catalysed by the enzyme carbonic anhydrase found within the RBCs.
The bicarbonate ions diffuse out of RBCs into the plasma. This process is facilitated by the chloride shift - the movement of chloride ions into the RBCs as bicarbonate ions exit to keep the electric charge balanced.

The Bohr effect is a physiological phenomenon where an increase in the concentration of carbon dioxide or a decrease in pH results in a reduced affinity for oxygen by haemoglobin.
This change in pH caused by increased levels of carbon dioxide changes the shape of the haemoglobin molecule making it harder for oxygen to bind and easier for it to be released. This allows oxygen to be delivered to the tissues that need it most, such as exercising muscles.
Therefore, the Bohr effect is a mechanism by which more oxygen is released to the tissues under conditions of high metabolic activity, high carbon dioxide concentration and lower pH.
The Bohr Effect causes a shift in the oxygen dissociation curve to the right. This shift indicates that more oxygen is released at the tissues for a given partial pressure of oxygen.