Anemia Teaching Simulation
[ For Medical Physiology students- References West 9th ed. Ch6, (Ch8 if necessary), Guyton Medical Physiology 12th ed. look up selected cardiovascular items/terms as needed ]
last revised 20 Feb. 2012

(a continuation of the Anemia Teaching Simulation)

VIIb. Some explanatory notes


How far has the hematocrit dropped?
HCT drops progressively because red cell production is zero and red cell destruction continues at a constant rate. This results in a progressive decrease in red cell mass and blood hemoglobin content and therefore progressively lowers O2 carrying capacity.


How much effect has this drop had on 1) the PO2 2) the O2 content?
While there is less hemoglobin available all of it that is still present gets well-oxygenated. Therefore PO2 (mmHg) does not drop. O2 content (i.e. how much O2 is carried per ml of blood), on the other hand, does drop simply because there is less hemoglobin present to bind it.


Examine the ventilation response. Why has it not risen in response to the O2 content hypoxia?
While there is a developing tissue hypoxia due to the low arterial O2 content the respiratory O2 chemoreceptors sense PO2, not content, and thus are perfectly "happy" They thus do not increase ventilation.


Estimate what has happened to O2 extraction by the tissues (O2A-O2V).
[O2A - O2V] narrows progressively from its original 0.04 ml/ml value because arterial content (O2A) drops progressively but the tissues continue to consume O2 at a near normal rate.


Note the response of cardiac output.
It rises dramatically and progressively. Tissue hypoxia causes peripheral vasodilation (note the rise in vascular ease of flow - COND). Since (CO = COND * AP) cardiac output depends on both driving force (arterial pressure - AP - remains constant) and ease of flow (COND - rises) cardiac output progressively rises. This is in a sense adaptive as it delivers more blocks of the O2 poor blood to the tissues.


Why does the model expire?
The model dies of high output heart failure (note left & right hypertrophy, systemic edema in the autopsy). The heart can not meet the increasing demands on it for still higher flows.


Note that there are other major aspects here not discussed including issues of the role of the HCT drop in increasing flow (check out viscosity - VISC) and fluid shifts [check out fluid loss (PV) due to protein (HGB) and other osmotic particle (RCM) loss ].


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