Red blood cells (RBCs), scientific name erythrocytes (from ancient Greek erythros 'red' and kytos 'hollow vessel', with -cyte translated as 'cell' in modern usage), also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cell and the vertebrate's principal means of delivering oxygen (O2) to the body tissues—via blood flow through the circulatory system. Erythrocytes take up oxygen in the lungs, or in fish the gills, and release it into tissues while squeezing through the body's capillaries.
There is an immense size variation in vertebrate red blood cells, as well as a correlation between cell and nucleus size. Mammalian red blood cells, which do not contain nuclei, are considerably smaller than those of most other vertebrates.
Mature red blood cells of birds have a nucleus, however in the blood of adult females of penguin Pygoscelis papua enucleated red blood cells (B) have been observed, but with very low frequency.
Scanning electron micrograph of blood cells. From left to right: human red blood cell, thrombocyte (platelet), leukocyte.
Two drops of blood are shown with a bright red oxygenated drop on the left and a darker red deoxygenated drop on the right.
A blood cell, also called a hematopoietic cell, hemocyte, or hematocyte, is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Together, these three kinds of blood cells add up to a total 45% of the blood tissue by volume, with the remaining 55% of the volume composed of plasma, the liquid component of blood.
Red and white human blood cells as seen under a microscope using a blue slide stain
The darker red blood syringes have deoxygenated blood, whereas the brighter red have oxygenated blood.
Artificially colored electron micrograph of blood cells. From left to right: erythrocyte, thrombocyte, leukocyte.