24/09/2026
Most horse owners, riders — and even practitioners — probably don’t think about the spleen very often. In fact, when we think about horse biology, the spleen is probably one of the last organs that comes to mind.
It sits quietly under the ribs, getting on with its everyday jobs.
But in the horse, it has a rather extraordinary role.
The spleen is part of the immune system, but it also acts as a large reservoir for red blood cells.
Your horse doesn’t make a fresh supply of red blood cells when it starts working. Those cells are already there, held within the spleen and ready to be released when the body needs them.
And there is a good reason for having them available.
Working muscles need energy, and the harder they work, the more energy they require. For sustained exercise, much of that energy comes from aerobic metabolism, which depends on a steady supply of oxygen.
Red blood cells carry oxygen through the body using haemoglobin. So putting more red blood cells into circulation increases the amount of haemoglobin available to transport oxygen to the tissues that are working.
The mechanism behind this is beautifully simple.
As the horse becomes more active, the sympathetic nervous system responds and adrenaline and noradrenaline rise. The spleen contracts, squeezing its stored red blood cells into the bloodstream.
And the equine spleen has quite a reserve.
Depending on the conditions and how strongly the system is stimulated, a horse can mobilise a very substantial proportion of its stored red blood cells — in some studies, around half of the circulating red-cell volume.
That gives the horse a rapid, temporary increase in its oxygen-carrying capacity without having to produce new cells.
When the work is over and the sympathetic response settles, the spleen relaxes again. The red blood cells are gradually redistributed, and the blood returns towards its resting concentration.
Nothing has been magically created or removed. The horse has simply moved its own resources to where they are needed, and then put them back again.
And the spleen has plenty of other work to do while all of this is happening.
It filters the blood, helps remove old or damaged red blood cells, recycles useful components such as iron, and plays an important role in immune function.
So that rather unassuming organ tucked away under the ribs is doing quite a lot.
An extraordinary organ — with a particularly clever trick up its sleeve.