The blood of many animals, including humans, contains about 40% red blood cells, which bind oxygen but also impede blood flow in the blood vessels. Mathematical models suggest that 40% is optimal. It provides the highest oxygen supply to the tissues. Nature has arrived at it through evolution. However, previous theories have not explained why elite athletes perform better after (illegal) blood transfusions or erythropoietin, nor why there are animals, such as dogs and horses, that expel concentrated blood from the spleen during physical exercise. In both cases the resulting haematocrit is higher than the optimal 40%.

“Together with Dr. Stark and Prof. Schuster from the Friedrich Schiller University in Jena, Germany, we studied the optimal percentage of red blood cells in the blood (the so-called hematocrit) during extreme physical exertion limited by cardiac performance,” said Michal Šitina, M.D., head of the Biostatistics research team at the International Clinical Research Centre of St. Anne’s University Hospital in Brno.

“In our study, we mathematically described the blood oxygen supply to tissues both under resting conditions without cardiac restriction and under conditions of extreme physical exercise, when blood flow is restricted by maximal cardiac output,” described Šitina. The calculation showed that under resting conditions, the optimal hematocrit value is indeed 40%, but it rises to 60% during physical exercise. This is the value observed in horses just after a race. This work thus explained the observed findings and refined the theory of the optimal haematocrit. Limiting cardiac performance, however, even without major physical exertion, are patients with severe heart failure. “In the next theoretical study, we would like to calculate the optimal hematocrit value for just such patients,” said Šitina

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