Carriage of Blood Gases and Haemoglobin Structure

Carriage of Blood Gases

• Blood carries oxygen and carbon dioxide.
• Oxygen transport is affected by factors like the oxygen partial pressure, pH, and temperature in different areas of the body.
• Factors also cause changes in oxygen transport in various parts of the body.
• Carbon dioxide transport is also important, particularly the bicarbonate buffer system.

Haemoglobin Structure

• Haemoglobin is a complex molecule.
• It's composed of four globin chains (two alpha and two beta).
• Each globin chain has a haem group, which contains an iron atom that binds oxygen.

Oxygenated and Deoxygenated Haemoglobin

• Haemoglobin's structure changes when oxygen binds.
• In the deoxygenated form, the binding site is narrow, restricting oxygen's access.
• Binding each oxygen molecule allows the haemoglobin molecule to relax.
• This easier access allows more subsequent oxygen molecules to bind.

Hb Saturation with Oxygen and Partial Pressure

• The relationship between oxygen saturation and partial pressure is sigmoid.
• In alveoli, the high partial pressure of oxygen leads to almost 100% haemoglobin saturation.
• Haemoglobin has a high affinity for oxygen in the alveoli.
• In systemic veins, there's a lower partial pressure of oxygen and a lower Hb saturation (around 77%).
• This allows oxygen to be released to tissues during aerobic respiration.

Effect of pH on Haemoglobin Affinity for Oxygen

• Low pH (high hydrogen ion concentration) reduces haemoglobin's affinity for oxygen.

• This means that more oxygen is released to tissues in areas of low pH.

• The curve shifts to the right.

Effect of PCO₂ on Haemoglobin Affinity for Oxygen

• High PCO₂ (high carbon dioxide) reduces haemoglobin's affinity for oxygen.

• This causes more oxygen to be released.

• The curve shifts to the right.

Effect of Temperature on Haemoglobin O₂ Affinity

• Increased temperature reduces the affinity of haemoglobin for oxygen.
• This causes a rightward shift of the dissociation curve.
• Decreased temperature increases the affinity of Hb for oxygen and shifts the curve to the left.

Effect of 2,3-DPG on Haemoglobin Affinity

• 2,3-BPG is a molecule found in red blood cells.

• It promotes oxygen release from haemoglobin, particularly in tissues with lower oxygen partial pressures.

• Levels of 2,3-BPG increase under conditions where oxygen levels are low

Foetal Haemoglobin Affinity

• Foetal hemoglobin (HbF) has a higher affinity for oxygen than adult hemoglobin (HbA).
• This enables efficient oxygen transfer from mother to foetus.

Myoglobin

• Myoglobin has a higher oxygen affinity than haemoglobin.
• Myoglobin stores oxygen in muscle and releases it when the partial pressure of oxygen is low.

CO₂ Transport in Blood

• Carbon dioxide is a waste product of metabolism.
• Most CO₂ is transported as bicarbonate.
• Carbonic anhydrase catalyses the reaction that converts CO₂ into bicarbonate.
• Bicarbonate is then transported out of the red blood cell.

Importance of Bicarbonate Ion

• The bicarbonate buffering system is crucial for maintaining a stable blood pH.
• This system can absorb or release hydrogen ions to keep pH constant.
• It's crucial for regulating many enzyme reactions and the ionization states of various substances, avoiding changes in their structure.
• Bicarbonate reabsorption in the kidneys maintains pH homeostasis.

Primary acid-base disorders

• Changes in ventilation compensate for metabolic disorders.
• Renal excretion compensates for respiratory disorders.
• The underlying cause of primary acid-base disorders is evident from the pH, PCO₂, and serum bicarbonate analysis.

Learning Outcomes

• Learning objectives focus on understanding respiration, maintaining acid-base balance and respiratory system role in that.