Oxygen delivery and transport

Oxygen flux is a term used to describe delivery of 02 to the tissues. An understanding of how the Po2 changes according to the location in the body is, therefore, useful when considering how the mitochondrial 02 supply is achieved. It can be represented by the 02 cascade. 

Alveolus Ventilation Pao2 = [Fio2(PAtm - -Ph2o)] - (Paco2/R) 

The multiplier 10 is used because Cao2 is measured inml.dl-1 whereas CO is measured in l.min-1. The 02 content of the blood is calculated using a specific equation that depends mainly on haemoglobin concentration, [Hb] and saturation (Sats). 

The number 1.34 is known as Huffiier s constant. It describes the volume of 02 (ml) that can combine with each 1 g Hb. In vitro, its value is 1.39 but this becomes 1.34 in vivo because abnormal forms of Hb such as carboxyhaemoglobin and methaemoglobin are less able to carry 02. 

This curve demonstrates the relationship between oxygen delivery (Do2) and oxygen consumption (Vo2). The latter is normally around 200 ml.min-1 and you should demonstrate that it is not affected until delivery falls to below approximately 300 ml.min-1, which is known as critical Do2. When 02 delivery is less than this, consumption becomes supply dependent. Above the critical value, it is termed supply independent. 

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