Video narration
OXYGEN CONTENT AND OXYGEN DELIVERY How much oxygen is actually carried in our blood? The answer depends mainly on haemoglobin, not just the oxygen level measured by a blood gas. Oxygen is carried in two forms: mostly bound to haemoglobin, and a tiny amount dissolved in plasma. Arterial oxygen content, or CaO₂, is calculated using this formula: CaO₂ equals 1.34 multiplied by haemoglobin multiplied by arterial oxygen saturation, plus 0.003 multiplied by PaO₂ in millimetres of mercury. The first part represents oxygen bound to haemoglobin. The second represents dissolved oxygen. Let's take an example. If haemoglobin is 15 grams per decilitre, saturation is 100%, and PaO₂ is 100 millimetres of mercury, arterial oxygen content is approximately 20.4 millilitres of oxygen per decilitre. Notice that almost all of this oxygen is carried by haemoglobin. Dissolved oxygen contributes very little. Now consider anaemia. Even if oxygen saturation and PaO₂ are normal, reduced haemoglobin means less oxygen can be carried in the blood. What happens when blood reaches the tissues? Oxygen is extracted, so venous oxygen content is lower than arterial oxygen content. Finally, oxygen delivery, or DO₂, depends on both arterial oxygen content and cardiac output. DO₂ equals CaO₂ multiplied by cardiac output multiplied by ten, to convert litres into decilitres. Remember: oxygen saturation tells us how full haemoglobin is, oxygen content tells us how much oxygen the blood carries, and oxygen delivery tells us how much oxygen reaches the tissues each minute.