## Oxygen Content of Blood

The theoretical maximum oxygen carrying capacity is 1.39 ml O2/g Hb but direct measurement gives a capacity of 1.34 ml O2/gHb.

The oxygen content of blood is the volume of oxygen carried in each 100 ml blood. It is calculated by:

(Ojcarried IwHh} + (Ojill solution)

where:

SO2 = percentage saturation of Hb with oxygen Hb = Hb concentration in grams per 100 ml blood PO2 = partial pressure of oxygen.

For a normal male adult the oxygen content of arterial blood can be calculated.

Given arterial oxygen saturation (SaO2) = 100%, Hb = 15 g/100 ml, and arterial partial pressure of oxygen (PaO2) = 13.3 kPa, then oxygen content of arterial blood (CaO2):

Similarly the oxygen content of mixed venous blood can be calculated. Given normal values of mixed venous oxygen saturation ( ) = 75%, and venous partial pressure of oxygen ( ^^ ) = 6 kPa, so: CvO^ = 15.1 + 0.1 = 15.2 nil/100 nil

Oxygen delivery (

) and oxygen uptake (

Oxygen delivery is the amount of oxygen delivered to the peripheral tissues, and is obtained by multiplying the arterial oxygen content (Ca02) by the cardiac output ( Q ). For Ca02 = 20.1 ml/100 ml and Q =5.0 1/min, mcygpn delivery (DO,} = KMbml/min.

Oxygen uptake is the amount of oxygen taken up by the tissues that can be calculated from the difference between oxygen delivery and the oxygen returned to the lungs in the mixed venous blood. The oxygen return is given by the product of mixed venous oxygen content ( 1 ) and cardiac output. For Cv0i " 15'2 ml/100 ml and Q =5.0 1/min:

oxygen return = 760 ml/niin.

Thus

### To summarize

The primary goal of the cardiorespiratory system is to deliver adequate oxygen to the tissues to meet their metaboHcrequnements, a balance between V°' and

The balance between oxygen uptake by the body tissues, and oxygen delivery to them, is assessed by:

The oxygen content of mixed venous blood

CvO,

, which is normally about 15 ml/100 ml

• The oxygen extraction ratio, which is the ratio of to expressed as a percentage. Normally the extraction ratio is about 25% but can double to 50% if tissue demand increases

Both of the above indices are dependent on mixed venous saturation (

SvO,

), and cardiac output.

Increased tissue demand due to exercise or disease, is normally compensated for by increased oxygen delivery. This has to be mediated by increasing cardiac output, since the ability to increase SaO2 and Hb is limited. However, under extreme conditions (severe exercise, sepsis, malignant hyperthermia) tissue demand can

increase 12-fold (requiring a 3 of up to 3000 ml 02/min). Cardiac output can usually be increased by a maximum of up to seven times, and thus under extreme conditions, tissue demand can outstrip the body's capacity to increase delivery. In such a case falls, extraction ratios increase and tissue hypoxia ensues.

This susceptibility towards tissue hypoxia will be greatly increased in conditions where cardiac output is limited or compromised.