Most vertebrates ride on red cells—except icefish in icy seas
Red blood cells, or erythrocytes, are the commonest blood cells and vertebrates’ main oxygen delivery vans. Human adults make about 2.4 million per second; each mature disk holds roughly 270 million hemoglobin molecules and sheds its nucleus for cargo space. Crocodile icefish are the known exception, dissolving oxygen in plasma instead.
Erythrocyte, the formal name, joins Greek words for red and hollow vessel. These are the most numerous cells in blood and the chief way vertebrates get oxygen to their tissues. They load up in the lungs, or the gills in fish, and unload while squeezing single file through capillaries. Their membrane of proteins and lipids gives them the flexibility and toughness to survive that journey again and again.
A mature human red cell is a bendable disk dimpled on both faces. During development it throws out its nucleus and organelles to make room for hemoglobin, so it is essentially a bag of pigment, holding about 270 million hemoglobin molecules. Adults make roughly 2.4 million new ones every second in the bone marrow. Each lives about 100 to 120 days, completing a lap of the body in about a minute, before macrophages recycle its parts. The 20 to 30 trillion red cells make up about 84 percent of all human cells and 40 to 45 percent of blood volume.
Hemoglobin's heme groups contain iron that grips oxygen temporarily and lets it go in the tissues. Hemoglobin also carries a share of carbon dioxide back, although most of that waste returns as bicarbonate dissolved in plasma. Colour tracks the chemistry: oxygen-rich blood is scarlet, oxygen-poor blood a dark burgundy, and plasma on its own is straw-coloured. Pulse oximeters read that colour shift to measure oxygen saturation. Carbon monoxide binds hemoglobin even more eagerly, producing a vivid red compound, which is why a flushed, confused patient reading 100 percent may actually be poisoned.
Nearly every vertebrate relies on these cells. The only known exception is the crocodile icefish family, Channichthyidae, which lives in cold, oxygen-rich water and carries oxygen simply dissolved in its blood, though broken hemoglobin genes survive in its genome. In medicine, donated red cells are processed and stored in blood banks as packed red cells for transfusion.
Source: Red blood cell
