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Red blood cell

Erythrocytes regulate vascular function through the modulation of oxygen delivery and the scavenging and generation of nitric oxide (NO). First, haemoglobin inside the red blood cell binds oxygen in the lungs and delivers it to tissues throughout the body in an allosterically regulated process, modulated by oxygen, carbon dioxide and proton concentrations. The vasculature responds to low oxygen tensions through vasodilation, further recruiting blood flow and oxygen carrying erythrocytes. 

Red cells are approximately 7.8 μm (1 μm = 0.000039 inch) in diameter and have the form of biconcave disks, a shape that provides a large surface-to-volume ratio. 
When blood is centrifuged to cause the cells to settle, the volume of packed red cells (hematocrit value) ranges between 42 and 54 % of total volume in men and between 37 and 47 % in women; values are somewhat lower in children.