Cardiovascular disease is the leading cause of death in the US and worldwide. However, according to official statistics, mortality related to cardiovascular diseases and especially myocardial infarction in premenopausal women is significantly lower then in men of the same age range. The nature of this difference is not well understood. We hypothesized that rheological properties of premenopausal female blood are optimized by a monthly blood loss to maximize gas transport. In fact, monthly bleedings result in a lower viscosity and in a greater population of young and a smaller population of old red blood cells (RBCs) in female blood. Younger RBCs are known to have higher deformability than older ones. Studying blood samples obtained from a large group of premenopausal women and age-matched men, we found that male RBCs have significantly lower deformability. This can reduce the ability of the RBC to enter and pass through small capillaries, and thus weaken oxygen delivery and waste removal. In addition, male blood has higher hematocrit and viscosity values. The Oxygen Delivery Index (ODI), calculated as a ratio of hematocrit to blood viscosity, was found to be significantly lower in male blood. Interestingly, the optimal ODI pertains to the range of hematocrit values which are much below those typical for the male blood, and overlaps with those in premenopausal female blood. Decreased oxygen delivery due to higher blood viscosity and decreased RBC deformability both contribute to the higher risk for the development of cardiovascular diseases. We suggested that regular blood donation could improve rheological properties of blood and enhance gas transport in men by increasing the amount of young RBCs, decreasing the number of old RBCs, and reducing both hematocrit and blood viscosity. This hypothesis was supported by several epidemiological studies.
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