Describe the carriage of oxygen in the blood, including total oxygen delivery per minute.
Better answers divided oxygen carriage into that bound to haemoglobin and that carried in the
dissolved form. A reasonable amount of detail on the haemoglobin structure and its binding of
oxygen was expected, including an explanation of co-operative binding and the oxygen carrying
capacity of haemoglobin. Better answers mentioned Henry’s law in the description of dissolved
oxygen, along with an estimation of the amount of oxygen that is normally in the dissolved form.
It was expected that answers include the equation for oxygen delivery, a brief description of the
components of that equation and the normal value, which a large number of candidates omitted.
Total DO2 = Cardiac output (L/min) × Oxygen content (ml/L)
- Cardiac output = HR × SV
- Where SV is influenced by preload, afterload and contractility.
Oxygen content = (sO2 × ceHb × BO2 ) + (PaO2 × 0.003), where:
- ceHb = the effective haemoglobin concentration
- i.e. concentration of haemoglobin species capable of carrying and releasing oxygen appropriately
- PaO2 = the partial pressure of oxygen in arterial gas
- 0.003 = the content, in ml/L/mmHg, of dissolved oxygen in blood
- Henry's law states that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid;
- Ergo the amount of oxygen dissolved in is proportional to its partial pressure, e.g for a PaO2 of 100 mmHg the oxygen content is 0.003 × 100 = 3ml/L
- BO2 = the maximum amount of Hb-bound O2 per unit volume of blood
- normally 1.39 of dry Hb, or closer to 1.30 in "real" conditions
- sO2 = oxygen saturation:
- determined by the sigmoid oxygen-haemoglobin dissociation curve
- Sigmoid shape of the curve comes from the positive cooperativityof oxygen binding
- Once an oxygen molecule is bound to it, the oxygenated subunit increases the oxygen affinity of the three remaining subunits
- This is because of a conformational change produced by each subunit binding oxygen, which mediates the transition from the T (tense, deoxygenated) state to the R (relaxed, oxygenated) state
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Pittman, Roland N. Chapter 4 - Oxygen Transport; in "The Circulatory System and Oxygen Transport." (2011).
Willem G. Zijlstra. "Misconceptions in reporting oxygen saturation." Anesthesia & Analgesia 105.6 (2007): S5-S9.
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