Carriage of gasses in the blood

Oxygent content is given by $$C_aO_2 \text{ (in ml/L)} = (1.34 \cdot \overbrace{ceHb}^{\text{[Hb] excl. COHb/metHb}} \cdot SaO2) + 0.03 \cdot P_aO_2$$Obeys Henry's law - dissolved concentration \(\propto\) partial pressure

Carried as: 99% bound to Hb, remainder dissolved. Poorly water soluble, coefficient 0.003, so content drastically reduced in anaemia.

Tension (mmHg): 95 (arterial), 40 (venous)

Content (mL/L): 200 (arterial), 150 (venous), \(\Delta\) -50

Hb dissociation: Sigmoid due to cooperative binding.

• Each Hb has 4 haem binding sites
• DeoxyHb is in T-state, with low O2 affinity
• When an O2 binds to haem, it tugs on the attached histidine residue \(\to\) progressive conformational change to R state \(\to\ \uparrow\)O2 affinity

The p50 is the PaO2 at which sO2 is 50%.

Curve is left shifted (increased affinity, poor offloading) by: metHb (left shifts remaining functional Hb), COHb, FHb, \(\uparrow\)pH, \(\downarrow\)PaCO2, \(\downarrow\)2,3DPG, \(\downarrow\)temperature

and right shifted (decreased affinity, good offloading) by: sulfHb,  \(\downarrow\)pH, \(\uparrow\)PaCO2, \(\uparrow\)2,3DPG, \(\uparrow\)temperature

The right-shift induced by rising PaCO2 is called the Bohr effect.

These features of Hb facilitate binding of oxygen in the pulmonary capillary (low PCO2, high pH) and offloading in the periphery, especially in hypoxic conditions (high PCO2, low pH).

Obeys Henry's law - dissolved concentration \(\propto\) partial pressure

Carried as: Bicarbonate (80%), dissolved gas (10%), carbamino compounds (10%).

Tension (mmHg): 40 (arterial), 45 (venous)

Content (mL/L): 480 (arterial), 520 (venous), \(\Delta\) +40

Hb dissociation: Linear (no cooperative binding). Binds mostly to Hb, also a little to albumin and globulin, as carbamino groups. Transport as HCO3 is also facilitated by Hb via the Hamburger effect:

1. CO2 diffuses into erythrocyte \(\xrightarrow{\text{carbonic anhydrase}}HCO_3^- + H^+\)
2. Proton buffered by Hb
3. Bicarbonate antiported with Chloride

Relationship with PaO2 - "The Haldane effect": OxyHb has lower affinity for CO2 than DeoxyHb, so as PaO2 rises, CO2 is displaced. Facilitates binding of CO2 in venous blood and offloading of CO2 in the pulmonary capillary.