Transport of Blood Gases

Transport of Oxygen

• 97% of oxygen is transported in blood in combination with hemoglobin (oxyhemoglobin; Hb-O2)
• Remaining 3% is in the dissolved state in the plasma
• Each of the 4 iron atoms in hemoglobin can bind one O2 molecule reversibly
• Oxygen binding to hemoglobin is an oxygenation, not an oxidation reaction
• Oxygenation and deoxygenation are very rapid processes, taking approximately 0.01 seconds

Quaternary Structure of Hemoglobin

• Hemoglobin's quaternary structure is responsible for its sigmoid-shaped oxygen binding curve
• The structure serves for its affinity for O2 by shifting the relationship of four components (polypeptide chains)
• This helps in either O2 uptake or O2 delivery

Affinity

• When hemoglobin takes up a small amount of O2, additional uptake of O2 is facilitated
• The first heme binds with O2, increasing the affinity of the 2nd heme to combine with O2
• Oxygenation of the 2nd heme increases the affinity of the 3rd, explaining the sigmoid shape of the O2-Hb dissociation curve

O2-Hb Dissociation Curve

• The percentage of hemoglobin that is bound with O2 increases progressively as the PO2 increases
• The curve is sigmoid-shaped, with saturation not differing much between 60-100 mm Hg
• Below 60 mm Hg, the curve is very steep, with a larger release of O2 to the tissues

Myoglobin Curve

• Myoglobin curve is hyperbolic, with only one O2 molecule per one myoglobin molecule

Binding/Releasing

• Binding and releasing of O2 involves breaking or forming salt bridges between polypeptide chains
• Upon oxygenation, 2 β chains move closer, while upon deoxygenation, they move apart

Maximum Amount of O2 that can be Bound to Hb

• When blood is equilibrated with 100% oxygen, Hb is 100% saturated
• Each gram of Hb can bind 1.34 ml of O2
• Hb in 100 ml of blood can combine with nearly 20 ml of O2, when Hb is 100% saturated

O2 Saturation

• In systemic arterial blood, PO2 is about 95 mm Hg, and O2 saturation is about 97%
• In normal venous blood, PO2 is 40 mm Hg, and saturation is about 75%

Utilization Coefficient

• The fraction of blood that gives off its O2 as it passes through the tissue capillaries
• Normal utilization coefficient is 25% (1/4th), but it can rise to 75% to 85% during heavy exercise

P50 and Affinity

• P50 is the PO2 at which the Hb is half saturated with O2
• A higher P50 indicates lower affinity of Hb for O2
• The Bohr effect: when the blood becomes slightly acidic, the affinity of Hb for O2 decreases, and O2 is released

Shift of OHDC by Changes in Blood CO2 and Hydrogen Ions

• Shift of the OHDC to the right with an increase in CO2 and hydrogen ions, enhancing the release of O2 from the blood in the tissues

CO2 Transport

• CO2 diffuses from the blood into the alveoli in the lungs, reducing PCO2 and decreasing H+ ion
• This shift of the OHDC to the left and upward, allowing greater O2 transport to the tissues

Factors that Shift the Curve

• Factors that shift the curve to the right: increased CO2 and decreased pH, increased blood temperature, and increased 2,3-diphosphoglycerate
• Factors that shift the curve to the left: presence of large quantities of fetal Hb and low blood temperature