**3. Oxygen utilization**

The amount of oxygen in the arterial blood depends upon the inspired oxygen and the pulmonary gas exchange. It depends on the arterial blood gas partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2). The units of partial pressures may cause some confusion. Three types of units are in use. One unit is kiloPascal (kPa) equivalent to 7.5006 mmHg (or Torr). It can be measured also in %, when 100% corresponds to atmospheric pressure of 760 mmHg (Torr), i.e 1% corresponds to 7.6 Torr (mmHg). The arterial hemoglobin saturation (SaO2 ) is measured in %. The normal value is about 95%. A typical oxygen carrying capacity of the blood is 19.4 ml of O2 per dl of blood with 19.1 ml O2/dl carried by hemoglobin and only 0.3 ml O2/dl dissolved in plasma (Cope, 1991). It should be noted that the oxygen delivery to the tissues is by diffusion and the hemoglobin acts as a buffer to maintain plasma's oxygen which is extracted by the tissue.

A typical averaged value for adult cerebral blood flow (CBF) is 47.7 ml/100 ml/min (Frackowiak et al., 1980) corresponding to total oxygen delivery 9.25 ml O2/100 ml/min. (Cope, 1991). Typical oxygen consumption of the adult brain is 4.2 ml O2/100 ml/min (Frackowiak et al., 1980). CBF, cerebral blood volume (CBV) and cerebral oxygen extraction (COE) are significantly greater in grey matter compared to white matter in normal human adults (Lammertsma et al., 1983; cope, 1991). The CBF and the cerebral blood volume (CBV) of grey matter in normal human adults is approximately 2.5 times that of white matter, while the cerebral oxygen extractions (COE) are 0.37 and 0.41 for grey and white matter respectively (Lammertsma et al., 1983; cope, 1991). Only part of the arterial oxygen which arrives in the brain is absorbed and utilized. The fraction which is utilized, known as the oxygen extraction fraction (OEF), is defined as

$$\text{OEF} = (\text{SaO}\_2 - \text{SvO}\_2) / \text{SaO}\_2$$

where SaO 2 and SvO2 are the arterial and venous oxygen saturations respectively.

According to Derdeyn et al (Derdeyn et al, 2002) the EOF, measured in their normal control subjects, was 0.41±0.03. Assuming SaO2 equal to 0.95 the SvO2 will be, using Eq. (6), equal to 0.56±0.03. In the brain tissue the absorption of the oxygenated and deoxygenated hemoglobin is mostly venous. Assuming a 75% venous contribution, the brain tissue regional oxygen saturation (rSO2) in the frontal region will be about 66±3%, a mean value which is observed in experiments. With a decrease of CBF there is a bigger demand for oxygen and EOF will increase (Kissack et al, 2005). Accordingly, with an increase of CBF the OEF will decrease.
