The purpose of the circulatory system is ultimately the delivery of oxygen and nutrients tocells, with removal of waste and carbon dioxide. Oxygen delivery depends on blood flow (cardiac output) and the amount of oxygen in the blood.
Oxygen delivery ¼ cardiac output × oxygen content in arterial blood
Oxygen is carried to the tissues and delivered to cells via the capillaries, where oxygen istaken up (consumed) by cells, so that venous blood contains less oxygen (and more carbondioxide) than arterial blood. The partial pressure of oxygen in the venous blood (PvO 2 ) is,on average, ~40 mmHg (this corresponds to an oxygen saturation of ~70–75% in the venous blood).
Venous oxygen saturation (SvO 2 ) reflects oxygen supply and demand; venous oxygensaturation will decrease if there is a decrease in oxygen delivery or an increase in oxygenconsumption, because cells will extract more oxygen from the blood to meet demand.
A decrease in venous oxygen saturation below the usual value of ~70 –75% suggestsincreased oxygen extraction and an oxygen supply/demand imbalance. Increasing oxygendelivery with inotropic support, or red blood cell transfusion if the hemoglobin is low, may improve patient outcomes in sepsis.
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