Answer
Alan had spent his one year sabbatical at an elevated location (above 10,000 feet).
During that time he slowly acclimated to the low atmospheric pressure and to the low partial pressure of oxygen at high altitude. One result of his successful adjustment was that his hematocrit had risen above normal values ( 45%-52% for men at sea level). After Alan returned to seal level ( or near it) his medical exam showed that his hematocrit was still elevated-- above 53%. This showed that Alan's blood had not re-acclimated to low altitude conditions ( e.g. Chicago, 586 feet). The re-acclimation process takes place over about two months during which time erythropoietin levels fall, production of erthrocytes by bone marrow decreases, and polycythemia decreases .Mature red blood cells die after 120 day, and some young erythrocytes die without the extra stimulation of high EPO levels. The end results are that hematocrit percentage and hemoglobin levels slowly return to normal. Apparently, Alan's medical exam was done soon after return from high altitude, and his blood was still re-acclimating to the conditions of low altitude--atmospheric pressure 760 mm Hg, partial oxygen pressure 152 mm Hg . Over the next month or so Alan's hematocrit will return to normal levels.
Work Step by Step
At sea level, Alan will slowly acclimate to sea-level conditions as hiscentral and peripherial receptors sensed that atmospheric pressure and partial pressures of oxygen and carbobdioxide had returned to normal seal level values.. Cosequently his kidneys and liver would cease putting out increased levels of erythropoietin(EPO) and his blood volume would return to normal as he retained more water in his blood. The latter process would reduce and evewntuaslly babolish his polycythemia.. Erytrocytes have a lifespan of only 12o days so natural die off of excess RBCs would also help to reduce RBC count and hematocrit. Other processes in the acclimation process including the repositioning of the oxygen dissociation curve, the decrease in muscular mitochondria and the fall of myoglobin level are part of re-acclimation. It will take place at different rates in different individuals. Some people experience a rapid decline in hematocrit in 10 to 40 days after returning to sea level. On the other hand some people report the beneficial effects of altitude acclimation up to two years after returning to sea level. In any case Alan's elevated hematocrit will not last . It will slowly fall as he completes his re-acclimation
