Oxygen transport

O2 is carried in blood in two forms:

1. Dissolved
2. Bound with haemoglobin

1. O2 dissolved

Dissolved O2 follows Henry's Law
--> Amount dissolved is proportional to partial pressure

For each mmHg of PAO2, dissolved O2
= 0.003 mL of O2 per 100 mL of blood
= 0.03 mL of O2 per L of blood

2. O2 bound to Hb

2.1. O2 capacity

O2 capacity
= Maximum amount of O2 that can be bound to Hb
= 1g of Hb can bind with 1.34mL of O2
= 1.34 x [Hb]

Normal O2 capacity
= 20.1mL of O2 per 100mL of blood
* Assuming [Hb] = 15g/dL
= mL/100mL

O2 capacity
* Does not include dissolved O2
* Does not depend on pO2

NB:

• Theoretical maximum is 1.39mL of O2 per gram of Hb
  --> But because of presence of other species of Hb, 1.34 is the traditional figure quoted

2.2. O2 saturation (Sat)

O2 saturation (%)
= O2 combined with Hb / O2 capacity
= (O2 content - dissolved O2)/O2 capacity

 

NB. West defined O2 capacity as maximum O2 bound to Hb, thus doesn't include dissolved O2.

NB. Nunn includes dissolved O2 in its definition of O2 capacity.

Functional vs Fractional

Functional saturation
= [OxyHb]/([OxyHb]+[DeoxyHb])

Fractional saturation
= [OxyHb]/[Total Hb]

NB:

• Total Hb include HbO2, DeoxyHb, MetHb, COHb
• Pulse oximeter measures HbO2 and deoxyHb, but may be calibrated to allow for MetHb and COHb

2.3. O2 concentration  (C)

O2 concentration (mL/dL) 
= 1.34 x [Hb] x Sat + 0.003 x pO2

NB:

• Remember this is NOT content
• Unit for O2 concentration is mL of O2 per 100mL of blood.
• Once the volume of blood is known, then the content of O2 can be calculated

Total oxygen content per 100mL

CaO2
= 1.34 x 15 x 0.975 + 0.003 x 100
= 19.5975 + 0.3
= 19.8975 mL O2 per 100mL blood
~ 20 mL/dL

CvO2
= 1.34 x 15 x 0.75 + 0.003 x 40
= 15.075 + 0.12
= 15.195 mL O2 per 100mL blood
~ 15 mL/dL