0610 - Oxygen Delivery to Tissues Flashcards
understand the shape of the Hb saturation curve and know which factors modify it; appreciate resting and max. O2 consumption rates; be aware of where O2 and CO2 are metabolised; understand the O2 partial pressure in mitochondria; grasp how diffusion rates change along capillary; be aware of how CO2 is removed from periphery; recognise the central role of EC in CO2 removal; and be able to state the differences between O2 and CO2 saturation curves.
What shape is the Hb saturation curve, and why is it this shape?
Sigmoid shape. This is because of O2’s affinity for Hb – as soon as one is bound, it is much easier to bind the others.
What factors modify the Hb saturation curve?
Temperature – colder = left shift, warmer = right shift.
pH (lactate) – Acidosis = right shift, alkalosis = left shift
CO2 partial pressure – decrease = left shift, increase = right shift
What is a resting O2 consumption rate?
250mL/min for 70kg person
3.3mL/min/kg
What is a normal maximum O2 consumption rate?
3.5L/min for 70kg person
(45mL/min/kg)
This is around 14 times more than the resting consumption rate
Where are O2 and CO2 metabolised?
In the mitochondria:
O2 used by mitochondrial cytochrome c oxidase – the last enzyme in the electron transport chain.
CO2 is produced by mitochondrial dehydrogenases as part of the TCA cycle.
How does Mitochondrial PO2 (PmO2) differ from artery and capillary PO2?
It is lower than PcO2, and significantly lower than PaO2. In addition to poor O2 solubility in cytoplasm, actual PmO2 also depends on distance from the capillary (diffusion) and levels of activity (Oxygen demand).
PmO2 can go from 5.0kPa resting to 0.5kPa under strenuous exercise.
How and why do O2 diffusion rates change across the capillary?
Diffusion rates drop significantly, from a steep concentration gradient (easy unbinding), to a much flatter one (harder to unbind what’s left). 3 factors:
1 – Hb dumps O2 due to lower Partial Pressure in local tissue.
2 – Bohr Effect - Lower pH in local tissue due to anaerobic metabolic activity, carbonic anhydrase, and (potentially) altitude training (though 2,3-DPG formation).
3 – Higher Partial Pressure of CO2 in local tissue due to aerobic metabolic activity.
How is CO2 removed from the periphery?
Predominantly as intracellular Bicarbonate (65%), but also binds to Hb (20%), and dissolved in cytoplasm (4%). Extracellularly it can be dissolved (6%) or as bicarbonate (5%).
Total HCO3- is 70%
Why is 90% of CO2 transported in the erythrocytes?
Predominantly Carbonic Anhydrase, which converts dissolved CO2 into Carbonic Acid, which dissociates into HCO3- and H+, keeping a CO2 concentration gradient. CO2 binding to Hb is also important.
Some HCO3- leaves the cell due to chloride shift (CL- HCO3- antiporter).
What is the difference between the O2 and CO2 saturation curve?
CO2 saturation curve is much steeper than O2 – binding and unbinding takes place over a much smaller range of partial pressures (i.e. comparatively little difference between arterial and venous). This is predominantly because there is always more CO2 than O2 in the cell.
