biology blood capillary cells education haemoglobin human laboratory life microscope microscopy oxygen red red blood cells science slide smear tiny abstract background blood cells body carbon dioxide cell count circulation concept corpuscle design disease erythrocyte fill graphic health hemoglobin illness iron macrocyte medicine megaloblast megalocyte microscopic New original platelets render sickness style texture transfusion unique vector analyse backdrop bacteria blood analysis cleaning clusters contamination culture disinfect germs health care immune immune system immunity infection microbes sample slides swab tissue unclean virus color colour medical probiotics system blue green hi res white