I’ll do my best to explain this based on my layperson’s understanding. Hypervolemia is the process of expanding blood volume. Endurance training causes hypervolemia. In the early stages (2-4 weeks) the response is mainly increased plasma volume. After that increased RBC are usually observed in response to chronic endurance training. The evolutionary advantages that these afford are increased heat tolerance and increased cardiac filling and ejection volume. Both of of which have a positive effect on endurance performance.
Increased hematocrit allows more oxygen to be carried to the working muscles so directly affects endurance performance. This is why blood doping in sports is banned. The earliest method of this was to re-infuse your own previously withdrawn blood and was called blood packing. In the mid ’80s synthetic erythropoietin (a hormone that stimulates red blood cell production) was invented to help leukemia patients whose bone marrow was not producing enough red blood cells. In the sports world this allowed a more sophisticated way to boost RBC (illegally). Since synthetic erythropoietin is indistinguishable from the real McCoy it was undetectable. Overnight, erythropoietin became endurance sport’s magic bullet.
Sport governing bodies test hematocrit and do not allow athletes to compete when it is above about 50 (depends on sport). Crit levels above 50 pose a significant risk of stroke.
Crit levels can be increased by training at altitude and even sleeping in a hypoxic environment (altitude tent or room). Dehydration raises crit levels. Both these factors can push crit into the danger zone for high altitude mountaineers. When I returned from K2 I had a crit of 54.
There is a genetic mutation, polycythemia vera that causes the bone marrow to produce excessive red blood cells, boosting crit well into the 50s. Athletes with his disorder are granted special dispensation from WADA but have to take active measures to keep their crit to below the legal limit, I have trained one Olympian with this disorder and it is like being naturally blood doped.
But I have also trained some very successful World Classe athletes and alpinists with crits in the low 40s who still can climb 8000m peaks in record times and stand on Olympic podiums.
I suggest focusing on the training Sam gives you and that your hematocrit do its thing naturally.