The spleen of the horse is shaped like the blade of a scythe. It is situated on the left side of the body behind the stomach with the broad base uppermost in the loin area and the apex pointing downwards. It has a soft consistency and can actively contract due to muscle fibres incorporated in the tissue.

In the horse the spleen is an important reservoir of blood; it acts like a sponge in which large quantities of blood can be stored. At times of great physical exertion and/or excitement it contracts so that a large amount of the red blood cells are expelled into the systemic circulation. At maximal exertion the haematocrit can therefore climb to over 60%. Thus the horse can dramatically increase its capacity to transport oxygen (without blood doping!) to improve the supply to the working muscles. As well as its function as blood store, the spleen is also able to filter out and break down old red blood cells. It also contains a dense network of lymphatic tissue in which a large number of lymphocytes are harboured, making the spleen an important part of the immune system.