A red blood cell is a disc with a dent in each side, about seven or eight thousandths of a millimetre across, and it has thrown away its own nucleus to get that way. Every part of that description is a design decision, and each one has a reason.
Most cells do many things. A red blood cell does one: carry oxygen from your lungs to everywhere else, and help carry carbon dioxide back. Because the job is so narrow, evolution has been free to optimise for it to a degree that is rare in biology — and the result looks less like a general-purpose cell than like a purpose-built container.
The obvious shape for a bag of liquid is a sphere. A sphere is exactly the wrong answer here, for two reasons.
Surface area. Oxygen has to cross the cell's surface to get in and out, so the more surface the cell has for a given amount of contents, the faster it can load and unload. A sphere is the shape with the least surface for its volume. Pressing dents into both sides of a disc adds a great deal of surface without adding any contents — a straightforward win for a cell whose entire purpose is exchange across its own membrane.
Diffusion distance. Oxygen does not get pumped through the interior; it diffuses, and diffusion is slow over distance. In a sphere the molecules in the middle are as far from the surface as possible. In a flattened, dimpled disc no part of the interior is far from the membrane, so loading and unloading happen quickly and evenly throughout.
The shape solves both problems at once, which is why it is worth having despite being much harder to build than a sphere.
A maturing mammalian red blood cell ejects its nucleus, and along with it most of its internal machinery. That is a drastic thing to do — the cell gives up the ability to divide, to repair itself, or to make new proteins. It is a one-way trip.
Two things are bought with it:
There is a third, quieter advantage. Mammalian red blood cells also lack mitochondria, so they do not use oxygen to power themselves. A delivery vehicle that consumed its own cargo would be a poor design; this one does not touch it.
Here is the fact worth taking away. The smallest capillaries are narrower than the red blood cells that have to pass through them. This is not a rare accident; it is normal, and it happens continuously throughout your body.
The cell manages it by deforming — folding into an elongated shape to squeeze through, then springing back afterwards. What makes that possible is a flexible protein scaffold just under the membrane, a mesh that behaves like a net stretched over a frame: it holds the shape, gives when pushed, and restores it when the pressure is released.
And squeezing through is not merely tolerated — it is useful. Being forced into a tight tube presses the cell's membrane against the capillary wall, exactly where oxygen needs to cross. The narrowest, most awkward part of the journey is where the cell does its actual job.
The clearest evidence that the shape matters is what happens when it changes.
Sickle cell disease comes from a single change in the haemoglobin molecule. Ordinarily haemoglobin stays dissolved; the altered version can link up into long rigid fibres when oxygen is scarce, and those fibres distort the whole cell into a stiff, curved shape. A stiff cell cannot fold through a capillary. It jams, and others pile up behind it, so tissue downstream is starved of oxygen — which is painful and damaging. The cells are also fragile and are destroyed early, causing anaemia. Two symptoms, one cause: a cell that has lost its flexibility.
There is a twist. Carrying one copy of the sickle variant offers real protection against malaria, a parasite that spends part of its life inside red blood cells. That is why the variant remains common in regions where malaria has long been present — the same change is protective in one dose and harmful in two.
Hereditary spherocytosis makes the point from the other direction. Faults in the flexible scaffolding leave the cell unable to hold its dimpled shape, so it rounds off into a sphere. It becomes stiffer and less able to deform, the spleen removes it early, and anaemia follows. No change to the haemoglobin at all — losing the shape alone is enough to cause disease.
Because it cannot repair itself, a red blood cell simply wears out. It lasts roughly four months, spending that time in constant circulation, and is then recognised as ageing and broken down — with its iron carefully recovered and reused rather than discarded.
Replacing them is a continuous industrial operation. Your bone marrow produces millions of new red blood cells every second, for your whole life, to hold the number steady. The most numerous cell in your body is also among the most short-lived, and the system that keeps that balance never pauses.
Written for a general audience; details are simplified deliberately, and this is not medical advice. Corrections are welcome via the contact page.