Beware of Bad Blood: Insights on the Blood Reserve in the Spleen

Red blood cells must be soft and flexible in order to effectively flow through the blood vessels, especially the capillaries. Horse blood cells, which are about half the size of human blood cells, measure 5-6 microns (1 micron is 1/1000th of a millimeter) in diameter. How flexible the red blood cells are is crucial because horse capillary vessels are 3-6 microns wide. This means that because of the narrowness of a horse’s capillary vessels, the red blood cell must squeeze and change shape one by one in order get through the capillaries. If they are not flexible, there will be an increase in pressure and friction, which can lead to a complete blockage [1].

Horses’ blood counts are higher during exercise when their spleens dump extra red blood cells into circulation to increase oxygen carrying capacity. Consequently, blood viscosity rises and in turn causes the friction against the inside of the blood vessels to surge. Many of these red blood cells stored in the spleen are echinocytes—stiff and spiky red blood cells—which hinder blood flow and increase blood viscosity even more than normal red cells [2]. These cells are older, stiffer, spiky-shaped cells which are stored in the spleen and released only in response to hypoxia [3]. Releasing these spiky blood cells sharply reduces the efficiency of oxygen delivery to muscles and organs; this “undoubtedly raises the risk of significant injury when horses work intensely, and could produce pathophysiologic conditions related to exertion, such as exercise-induced pulmonary hemorrhage (EIPH), exertional myopathy (tying-up syndrome), and certain orthopedic diseases.” [2].

Normal red blood cells are biconcave (satellite dish shaped) which gives them greater surface area to exchange O2 and CO2 and the critical ability to squeeze through spaces smaller than the cell itself. Stiff and spiky echinocytes however choke blood flow in a similar fashion to sickled cells in human sickle cell disease. The degree to which echinocytosis occurs is dependent on the horse’s workload. Because stiff spiky echinocytes increase blood viscosity and choke off blood flow, high levels of these spiky echinocytes during periods of intense exercise can cause pulmonary vascular shunting, pulmonary hypertension, and hypoxemia [2].

The Only Way to Know is to Test the Blood

Blood viscosity is a rich measurement that can be used to characterize individual horses. High surges in blood viscosity during exercise can be detected by sampling hot blood and shipping to the lab for testing. The Equine Blood Viscosity Test reports both the systolic and the diastolic blood viscosity scores for each blood sample we receive at our lab. This enables us to provide data for both the thickness and the stickiness of each horse’s blood specimen. We will be able to detect if your horse is having a viscosity problem because of echinocytosis and recommend protocols for treatment.

1. Weiss DJ, Evanson O, Geor RJ. Filterability of equine erythrocytes and whole blood: Effects of haematocrit, pore size and flow rate. Comparative Haematology International 1994; .4:11-6.
2. Boucher JH. Exercise-induced Echinocytosis, in Equine Sports Medicine 1989; Lea & Febiger: Philadelphia, 43-52.
3. Catalani G, Dottavio ME, Rasia M. Acute training in racing horses at two different levels of effort: A haemorheological analysis. Clin Hemorheol Microcirc 2007; 37:245-52.