Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Red blood cells, or erythrocytes, are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system. They take up oxygen in the lungs or gills and release it while squeezing through the body's capillaries. These cells' cytoplasm is rich in haemoglobin, an iron-containing biomolecule that can bind oxygen and is responsible for the blood's red color. Learn more about erythrocytes, the red blood cells, their function and significance, as well as diseases that may afflict them in the following pages.