The beings called red corpuscles (also known as red blood cells) are cells that are found in the human bloodstream and carry oxygen from cell to cell. Red blood cells are red as they contain haemoglobin, a protein that has iron built into it. This allows the red corpuscle to transport oxygen from the lungs through the blood to the cells.
In the real world, a red corpuscle is little more than a circular sac of haemoglobin and structural proteins, about 7 micrometres across, that lacks a nucleus or any other complex structure associated with haemoglobin. This causes the red corpuscle to cave in on itself into a flexible doughnut shape where the hole doesn't quite go all the way through. This allows the red corpuscle to carry oxygen as efficiently as possible.
Red blood cells are depicted in Once Upon a Time... Life as pink- or red-coloured humanoids that contain a pouch on their back, which they carry oxygen and carbon dioxide in. Red corpuscles are born en masse at a rate of 200 million per second within the bone marrow, depicted as a single stem or "master" cell differentiates into an infant red corpuscle that goes down a slide with their fellows into the blood stream. Immediately and non-stop for 100 to 120 days, they will spend their entire lives walking through the bloodstreams, picking up oxygen in the alveoli and turning salmon, exchanging it for carbon dioxide and turning deep red at various cells, and going back to the lungs to exchange the CO2 for more oxygen and become salmon again. As weeks pass, they age and become slower and less flexible, accumulating various iron objects like keys and scissors. When they can no longer go on, they head to the spleen to drop off their scrap iron to be recycled and then lay down in their deathbeds where they are absorbed and their proteins, iron, and even personality reincarnated into new corpuscles.
Although thousands of characters in the series are red corpuscles, most of them are mere "extras" that never have more than one line of dialogue in the series.
Some red corpuscles feature in a minor role, including seniors, teenagers, and even infants. None of them are ever named in the series, and they hardly ever have more than one line.
He is tall, both fat and strong, and always there to protect his friends. Despite his strength, he is kind and soft at heart. He also often frequently expresses a strong dislike towards carrying carbon dioxide through the veins. He may or may not be based on the archetypical character Jumbo, who appears both in this series and in all the others.
She is the only important female red corpuscle in the series. She is short and thin, and her hair is short and made of "lumps". She is timid but is kind, and often expresses curiousity about how the body she travels in actually works. Unlike the other red blood cells, Globin is not apparently based on any character from Once Upon a Time... Man or Once Upon a Time... Space.