How is oxygen transported by the circulatory system?
One of the circulation’s major functions is to carry oxygen to tissues for use in metabolism. Oxygen delivery (oxygen transport or DO.2) is the product of two factors: blood flow or cardiac output (CO) and the amount of oxygen carried in the blood, or arterial oxygen content (CaO2).
DO.2=COxCaO2
As the oxygen content of blood decreases,
oxygen deliv-ery can be maintained by a proportionate increase in CO.
Oxygen is present in the blood in two forms. It
is bound by Hb and dissolved in plasma. The oxygen content of 100 mL of blood
is described by the equation:
CaO2 = (Hb × 1.34 × SaO2) + (PaO2 × 0.0031)
where
CaO2 = arterial oxygen content in milliliters of
oxygen per 100 mL of blood
SaO2 = percent of Hb saturated with oxygen PaO2
= partial pressure of arterial oxygen.
Under normal conditions (e.g., absence of
pulmonary disease, normal Hb, physiologic shunt of 2–3%), the oxyhemoglobin
saturation is approximately 97%, and the partial pressure of dissolved oxygen
is approximately 100 mmHg. CaO2 can then be calculated as follows:
CaO2 = (15 g/dL × 1.34 × 0.97) + (100 mmHg x 0.0031)
= 19.5 + 0.31
= 20 mL
O2/100 mL blood
This calculation demonstrates that the amount
of oxygen dissolved in plasma (0.31 mL/100 mL blood) is negligible compared
with the amount carried by Hb (19.5 mL/100 mL blood).
Furthermore, because the oxyhemoglobin binding
factor (1.34) and the SaO2
(0.97) are constant under normal conditions, it can be seen that CaO2
varies almost linearly with Hb concentration. Under abnormal conditions, such
as obesity and term pregnancy, rapid falls in SaO2 during induction
of anesthesia result in immediate decreases in CaO2. The consequent
tachycardia may or may not be sufficient to maintain normal DO2.
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