Respiratory Physiology: Transport of gases in blood

Percentage of Oxygen bound in blood

• 98% combines with hemoglobin, in RBC

• < 2% dissolved, in plasma (arterial blood gas)

Deoxyhemoglobin

O₂ is not bound to hemoglobin

Oxyhemoglobin

O₂ is reversibly bound with hemoglobin

Hemoglobin

• Has 4 hemoglobin chains

• Each hemoglobin chain has a single heme group with a single ferrous iron atom (Fe⁺²) in the middle (= total of 4 iron atoms)

• One or more (2,3, or 4) iron molecules in the hemoglobin molecule may reversibly bind oxygen

• One iron molecule binds one oxygen molecule

• This means each iron molecule binds 2 oxygen atoms

Allosteric effect

• Hgb binds one oxygen molecule

• Hgb changes the shape

• The change in shape makes it easier for Hgb to bind the next O₂ molecule

O₂ carrying capacity

The amount of oxygen that can be transported in the blood of an animal

Oxygen-hemoglobin dissociation curve

Shows the relationship between the fraction of Hgb bound to oxygen, and the partial pressure of oxygen in arterial blood

Left-shifted curve

• Where hemoglobin needs to pick up more oxygen (at the lungs)

• Hgb is saturated at a lower concentration of oxygen

• This implies that Hgb now has a higher affinity for oxygen (hangs on tighter)

Left-shifted curve factors

• Lower CO₂

• Higher pH

• Lower temp

• Lower 2,3 DPG

Right-shifted curve (Bohr effect)

• Where hemoglobin needs to release more oxygen (at peripheral tissues)

• Hgb is saturated at higher concentration of oxygen

• This implies that Hgb now has a lower affinity for oxygen (lets go more easily)

• Helps unload oxygen to meet the oxygen demands of tissues

Right-shifted curve factors

• Higher CO₂

• Lower pH

• Higher temperature

• Higher 2,3 DPG

Percentage of CO₂ in the blood

• 70% as bicarbonate (HCO₃^-)

• 20% as carbaminohemoglobin (does not bind to heme)

• <10% as dissolved CO₂ (arterial blood)

CO₂ equation

CO₂ + H₂O ←carbonic anhydrase→ H₂CO₃ (carbonic acid) ←→ H⁺ + HCO₃^- (bicarbonate)

Details of CO₂ equation

• The equilibrium of this equation results in 70% of CO₂ in blood being carried as HCO₃^-

• This chemical reaction occurs inside the RBC and other tissues

• It would be a slow reaction

• But the enzyme carbonic anhydrase makes conversion very fast

Haldane effect

• Oxygenation of blood displaces CO₂ from Hgb

• Where PCO₂ levels are lower (lungs)

  • oxygenated Hgb has lower affinity for CO₂

  • Hgb carries more O₂

• Where PCO₂ levels are higher (peripheral tissues)

  • Deoxygenated Hgb has higher affinity for CO₂

  • Hgb releases more O₂, carries more CO₂

Chloride shift

• Occurs when HCO₃^- is generated in a cell

• Bicarbonate ions leaves the cell down a concentration gradient

• When it leaves the cell, chloride ions enter

• Uses a protein transporter