Insights on Dehydration and Blood Plasma

Blood is a slurry—a complex mixture of cells and plasma. The plasma component consists of water, proteins, electrolytes, clotting factors, hormones, waste products, and other molecules that are moving to and from the body’s cells and tissues. During exercise, the volume of plasma decreases for several reasons: higher blood pressure in the arteries forces plasma out of the capillary vessels and into the tissues, and at the same time, the plasma is squeezed out of the capillary vessels by the contraction of muscles. In addition, much of the water and electrolytes within the plasma are lost through the pores as sweat [1].

Reduced plasma volume is directly related to acute dehydration. The amount of fluid that can be lost during exercise is large, and it underscores the impact of dehydration on the body. In high temperatures, horses competing in endurance races have been shown to lose up to 11% of their body mass [2].

Athletes adapt to fluid loss during exercise by signaling the body to retain more water and electrolytes. This mammalian adaptation is present in both humans and horses. One study showed that human athletes who vigorously cycled two hours a day for one week were able to expand their plasma volume by more than 400 mL. Retaining water and electrolytes causes plasma volume to increase, raising the overall volume of blood pumped from the heart [1]. More plasma in the blood reduces blood viscosity (makes the blood thinner) and allows blood to flow more efficiently.

Things get more complicated, however, because exercise also increases the amount of oxygen that the muscles and organs of the body need to perform at a high level. To meet the greater demand for oxygen, the spleen squeezes out the extra red blood cells stored inside. For humans, there is an approximately 10% increase in circulating red blood cells. For horses, the increase is about 50%, which can cause an unhealthy surge in blood viscosity [3]. This is a critical difference in horses that makes them susceptible to more dangerous levels of dehydration. The composition of the blood stored in the spleen and dumped into circulation is usually between 70-80% red blood cells, most of which are stiff, rigid, and old [3,4].

Blood viscosity is the only way to test the ability of the blood to flow. It is the most direct way to check for hydration and dehydration. We offer the Equine Blood Viscosity Test to allow veterinarians, trainers and owners to collect blood samples when the horse is at rest and also immediately after exercise.

There is a delicate balance between acute dehydration and healthy hydration. Large amounts of fluid loss during exercise causes dehydration and reduces plasma volume, which in turn leads to exhaustion and tissue damage. However with training, athletes’ bodies adapt to counteract fluid loss by retaining fluid and maintaining a higher volume of plasma in the blood. The Equine Blood Viscosity Test uses a resting baseline blood specimen as a measure of the hydration status of the horse at rest. This provides insight on how well the athlete is adapted to exercise.

Testing viscosity on blood that is collected immediately after exercise (a hot blood sample) provides additional insight on how the blood-boosting impact of the spleen changes hydration and blood flow. A horse that is not well-adapted to exercise or is holding a dangerously high amount of old and rigid blood cells in reserve will be identified through screening. Such athletes have impaired circulation and compromised cardiovascular function.


1. Eichner, E.R., Sports anemia, iron supplements, and blood doping. Med Sci Sports Exerc, 1992. 24(9 Suppl): p. S315-8.

2. McKeever, K.H., et al., Plasma constituents during incremental treadmill exercise in intact and splenectomised horses. Equine Vet J, 1993. 25(3): p. 233-6.

3. Stewart, I.B. and D.C. McKenzie, The human spleen during physiological stress. Sports Med, 2002. 32(6): p. 361-9.

4. Catalani, G., M.E. Dottavio, and M. Rasia, Acute training in racing horses at two different levels of effort: A haemorheological analysis. Clin Hemorheol Microcirc, 2007. 37(3): p. 245-52.