Anatomy · Cells & tissues

Red blood cell

The most numerous cell in the body

Blood · formed element

The oxygen-carrying cell of the blood: a flexible biconcave disc about 7–8 micrometres across, with a thin pale centre. In humans it has thrown out its nucleus and most organelles, leaving room for haemoglobin, the iron-containing red protein that binds oxygen in the lungs and releases it in the tissues. Red cells make up about 45% of the blood's volume and are made in red bone marrow at more than two million a second, lasting around 120 days before the spleen removes them.

How to spot them

▸Round discs with a pale, thin centre, like doughnuts with the hole not quite through
▸Uniform size, about 7–8 micrometres, smaller than any white cell
▸No nucleus in mature human cells
▸Often stacked like coins in a slide, a formation called rouleaux
▸Stains pink-red in a blood smear and outnumbers the white cells by hundreds to one

What to know

A biconcave disc about 7–8 micrometres across with no nucleus

Red blood cell — the erythrocyte.

Each holds some 270–300 million copies of an iron-containing protein that binds four oxygen molecules

Red blood cell — haemoglobin.

Made in the marrow at more than two million a second, under the control of a hormone from the kidney

Red blood cell — erythropoietin (EPO) drives erythropoiesis.

Removed after about 120 days by macrophages in the spleen and liver

Red blood cell — the iron is recycled, the rest becomes bilirubin.

First seen under the microscope by Jan Swammerdam in 1658 and described in detail by Antonie van Leeuwenhoek (1658)

Red blood cell — the earliest microscopic descriptions of blood corpuscles.

A single change to one amino acid in its main protein can distort it into a crescent shape that blocks capillaries

Red blood cell — sickle cell disease, from an altered beta-globin chain.

Its surface carries the A and B antigens that define the ABO blood groups

Red blood cell — the ABO antigens are on the red cell membrane.

Key ideas

The disc shape gives a high surface-to-volume ratio for gas exchange and lets the cell fold through capillaries narrower than itself, held together by a flexible cytoskeleton including spectrin.
Because mature red cells have no mitochondria they make ATP by anaerobic glycolysis and use none of the oxygen they carry.

Quick-fire facts

•Males have about 5.4 million red cells per microlitre of blood, females about 4.8 million (OpenStax figures)
•An adult holds roughly 20–30 trillion red cells
•Red cells are produced at more than 2 million per second in the red bone marrow
•Average life span is about 120 days
•Haemoglobin is built from four globin chains each carrying one iron-containing haem group; each cell holds about 300 million molecules
•Newly released immature cells still containing some ribosomal RNA are called reticulocytes
•Erythropoietin is made mainly in the kidneys; its abuse in endurance sport is a doping issue
•Living at altitude increases the number of red cells; low counts and low haemoglobin are called anaemia
•Camel and llama red cells are oval; those of birds, reptiles and fish keep their nuclei

Drill it — 8 cards

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Sources

OpenStax — Erythrocytes (Anatomy and Physiology 2e, section 18.3)
NHS — Sickle cell disease