Natural Blood Doping
Circulatory capacity has been shown to be positively related to athletic performance in the thoroughbred and the equine heart and spleen are important contributors to this circulatory capacity. While there is a lot of literature available about heart size and its relationship to athletic performance in thoroughbreds, not much has been written about the value of the spleen. The spleen is very well suited to transabdominal ultrasonography, since it normally lies immediately adjacent to the left body wall of the mid and caudal abdomen, and usually medial to the liver in the left cranial abdomen. Testing the hypotheses that spleen volume can be calculated from transcutaneous ultrasonographic measurements in live standing horses; and that the spleen volume is proportional to blood volume, Navas de Solis, et al have released a paper which outlines both a technique to measure the equine spleen and a modified formula to calculate the size of the equine spleen. Of course the measurement of the equine spleen is an integral part of our Sales Select test.
The importance of the splenic red blood cell reserve in the horse was documented in a series of studies utilizing splendectomized saddle horses. The spleen of the horse can store up to one third of the total red blood cells that the horse has and this stored reserve can be mobilized by exercise, stress and excitement. Since oxygen is carried through the blood stream in red blood cells, the number of these cells can affect performance. Effectively, when required the thoroughbred can naturally blood dope itself by contracting the spleen, dramatically increasing the total red blood cell volume. The total red blood cell volume was found to be dependent on the weight of the exsanguinated spleen, but independent of the body weight of the horse. It was further found that the equine spleen acts as a cardiovascular reserve, modifying the heart rate response to submaximal exercise by increasing the circulating red cell volume. It was also suggested that the splenic cardiovascular reserve may assist in ventricular filling at high heart rates. Hanson et al (Heart and Spleen mass related to blood parameter changes after epinephrine injection - Hanson, et al 1995 Eq. Vet. J) reported that a major cause for the differences in blood volume parameters between breed types appears to be due to splenic mass, with thoroughbreds as a breed having some of the highest values. This is unsurprsing in many ways as the thoroughbred has been selected for performance. It has also been reported that the storage capacity of the spleen increases with training.
There is debate in literature as to which component of splenic blood is more important in the development of the high VO2Max seen in racehorses. Some evidence suggests that the overall increase in circulating blood volume is important in determining venous return and Starling mechanisms of the heart and, therefore, elevation of aerobic performance in horses. There is also evidence to suggest that the increase in circulating red cells and Heamatocrit (HCT - the volume percentage (%) of red blood cells in blood), independent of volume, are more important to VO2Max. Data from Kearns, et al (Relationship between body composition, blood volume and maximal oxygen uptake - Kearns, et al 2002) suggests the latter – that an increase in red cell volume and HCT may be more important in the relationship with VO2Max than volume increase of splenic blood in general.