section_path: Biology/Circulatory system
section_name: Circulatory system
paragraph_text: Circulatory system
  
  Women have lower hematocrit (the volume percentage of red blood cells in blood) than men; this is due to lower testosterone, which stimulates the production of erythropoietin by the kidney. The normal hematocrit level for a woman is 36% to 48% (for men, 41% to 50%). The normal level of hemoglobin (an oxygen-transport protein found in red blood cells) for women is 12.0 to 15.5 g/dL (for men, 13.5 to 17.5 g/dL).
  
  Women's hearts have finer-grained textures in the muscle compared to men's hearts, and the heart muscle 's overall shape and surface area also differs to men's when controlling for body size and age. In addition, women's hearts age more slowly compared to men's hearts.