Reduced oxygen content and oxygen delivery to tissues
- CaO2 (ml/L) = 1.34 x SaO2 x [ceHb] + 0.03 x PO2
- 99% of O2 normally bound to Hb
- \(\therefore \ \downarrow\)Hb \(\to \ \downarrow\downarrow\)CaO2
- DO2 = CaO2 x cardiac output
- Fall in DO2 \(\to \downarrow\)O2 delivery to organs \(\to\) anaemic hypoxia
Compensatory increase in cardiac output
- 2 mechanisms: loss of viscosity and SNS stimulation
- Loss of RBC \(\to\) less viscous blood. Decreases SVR and venous resistance \(\to\) decreases afterload, increases venous return and \(\therefore\)preload \(\xrightarrow{frank-starling} \ \uparrow\)CO
- \(\downarrow\)DO2 \(\to\) direct aortic body stimulation \(\to \ \uparrow\)SNS outflow
- Impaired O2 delivery \(\to\) lactate production in tissues \(\to\) vasodilation \(\to \ \downarrow\)MAP \(\to\) barocerecptor reflex \(\to \ \uparrow\)SNS outflow
- SNS outflow \(\to \ \uparrow\)inotropy \(\uparrow\)HR
Compensatory increase in oxygen extraction ratio
- Tissue lactate production \(\to \downarrow\)pH \(\to\) oxygen-Hb dissociation curve (OHDC) right-shift
- Right shift + low tissue O2 tension increases offloading \(\to\) increased deoxyHb
- 2,3DPG in RBC cytosol normally inhibits hexokinase
- deoxyHb readily binds 2,3DPG \(\to\) hexokinase disinhibition \(\to \ \uparrow\)2,3DPG
- Further right-shifts OHDC
EPO release
- Decreased O2 delivery stimulates HIF2 \(\to\) EPO release from peritubular cells \(\to\) erythropoiesis \(\to\) restores [Hb]
Hyperventilation
- Lactate \(\to \ \downarrow\)pH \(\to\) stimulates chemoreceptors \(\to \uparrow\)MV
- \(\uparrow\)SNS \(\to \uparrow\)MV
- \(\uparrow\)MV\(\to \downarrow\) PaCO2
- In lung, \(\uparrow\)ventilation matches \(\uparrow\)perfusion from increased cardiac output