Transport of Oxygen and Carbon Dioxide

This section details the mechanisms involved in the transport of oxygen and carbon dioxide in vertebrates, with a specific focus on the chloride shift. Understanding this process is crucial for comprehending how efficient gas exchange occurs.

Oxygen Transport

Oxygen is primarily transported in the blood bound to hemoglobin within red blood cells. Hemoglobin has four subunits, each capable of binding one molecule of oxygen. The binding of oxygen to one subunit increases the affinity of the remaining subunits for oxygen, creating a cooperative binding effect. This results in a sigmoidal oxygen-hemoglobin dissociation curve.

Factors Affecting Oxygen Binding

• Partial Pressure of Oxygen (PO₂): Higher PO₂ increases oxygen binding.
• Partial Pressure of Carbon Dioxide (PCO₂): Higher PCO₂ decreases oxygen binding (Bohr effect).
• pH: Lower pH (more acidic) decreases oxygen binding (Bohr effect).
• Temperature: Higher temperature decreases oxygen binding.
• 2,3-Bisphosphoglycerate (2,3-BPG): Increased levels of 2,3-BPG decrease oxygen binding. 2,3-BPG is produced in red blood cells and binds to hemoglobin, shifting the oxygen-hemoglobin dissociation curve to the right.

Carbon Dioxide Transport

Carbon dioxide is transported in the blood in three main forms:

• Dissolved CO₂ (about 7%): CO₂ dissolves directly in the plasma.
• Carbaminohemoglobin (about 23%): CO₂ binds to hemoglobin, forming carbaminohemoglobin.
• Bicarbonate Ions (about 70%): CO₂ diffuses into red blood cells and combines with water to form carbonic acid (H₂CO₃). Carbonic acid then dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-). Bicarbonate ions are transported out of the red blood cells into the plasma.

The Chloride Shift

The chloride shift is a crucial mechanism that facilitates the efficient transport of carbon dioxide from the tissues to the lungs. It involves the exchange of chloride ions (Cl^-) between red blood cells and plasma.

Mechanism of the Chloride Shift

1. Carbon Dioxide and Chloride Relationship: When CO₂ is produced in the tissues, it reacts with water to form carbonic acid (H₂CO₃). Carbonic acid then dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-). The hydrogen ions bind to hemoglobin, and the bicarbonate ions diffuse into the plasma.
2. Chloride Movement: To maintain electrical neutrality, chloride ions (Cl^-) move from the plasma into the red blood cells. This movement is driven by the electrochemical gradient. The plasma becomes more positive due to the loss of chloride ions, and the red blood cells become more negative due to the gain of bicarbonate ions and the loss of positive ions (H⁺ bound to hemoglobin).
3. Facilitated Diffusion: The movement of chloride ions into the red blood cells is facilitated by chloride transporters.
4. Reverse Process in the Lungs: In the lungs, the process reverses. Bicarbonate ions diffuse back into the red blood cells, and chloride ions move out of the red blood cells and into the plasma. This allows for the efficient removal of CO₂ from the blood.

Table summarizing the Chloride Shift

Location	Process	Effect on Chloride Concentration
Tissues	CO2 production and formation of bicarbonate ions	Cl- moves into red blood cells
Lungs	Bicarbonate ions diffuse into plasma and Cl- moves out of red blood cells	Cl- moves into plasma

Importance of the Chloride Shift

The chloride shift is vital for the following reasons:

• Increased CO₂ Carrying Capacity: By moving chloride ions into red blood cells, the chloride shift helps to maintain a concentration gradient that drives the diffusion of carbon dioxide from the tissues into the blood.
• Efficient CO₂ Removal: The chloride shift ensures that a significant amount of CO₂ is transported in the blood, facilitating its removal from the body.
• Maintenance of Electrochemical Gradient: The movement of chloride ions helps to maintain the electrochemical gradient necessary for the transport of other ions, such as potassium, which is important for maintaining the proper function of red blood cells.