An increase in the concentration of red blood cells and hemoglobin in the blood, usually caused by loss of plasma or dehydration.
From Greek 'haima' (blood) + Latin 'concentrare' (to bring together). The term emerged in 20th-century medical practice to describe the physical process of blood becoming more concentrated when water content decreases.
Your body can actually become dangerously thick like syrup when you're severely dehydrated—this is why athletes are warned about hypohydration during marathons, because thickened blood strains the heart and can cause clots.
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