Circulatory system of the horse

Produced in bone marrow, red blood cells are responsible for carrying oxygen to tissue and removing carbon dioxide, all via hemoglobin.

When the horse places weight on a leg, the ground pushes upward on the frog, compressing it and the underlying digital cushion.

In cases of support-limb laminitis, the constant pressure on the frog prevents the refilling/emptying required for good circulation in the foot, likely leading to ischemic damage to the laminae.

The capillary refill time is determined by pressing a finger against the horse's gums for about 2 seconds, so that a white "thumbprint" is left.

If it takes longer for the gum color to return, the horse may be experiencing dehydration or in severe cases shock.

However, no work has correlated this to a horse's oxygen uptake (VO2Max) and the test has not been a good predictor of future athletic ability.

[7] On the other hand, the Pearson correlation coefficient has been found to provide a link between oxygen uptake and echocardiographic measures.

Large hearts have been found in four major Thoroughbred lines, all descendants of Eclipse: Princequillo, War Admiral, Blue Larkspur and Mahmoud.

[10] Many outstanding race horses such as Eclipse and Secretariat were noted for being excellent broodmare producers but generally failed to produce male offspring with the ability of their sires, thus the theory that the gene was carried only on the x chromosome meant that stallions with large hearts could only pass on the trait via their daughters.

[11] The Heart Score, using electrocardiography, was developed over 40 years ago to describe the correlation between the QRS (intraventricular conduction time) complexes and the performances of several elite versus average racehorses with the belief that a large heart correlated to athletic ability.

[2] However, the X-Factor theory was never scientifically peer-reviewed and studies on the ECG protocol used, indicate that the Heart Score has little correlation to future racing ability.

[7] In addition, the gene(s) associated with cardiovascular dimensions and athletic performance have not been identified, nor has its mode of inheritance been determined; the condition may be influenced by multiple genetic factors.