Original sentence: Heart size does not necessarily correlate to the size of the horse.However, studies on Thoroughbreds have shown that the proportion of skeletal muscle exceeds 50% of body weight, and so the energetic capacity of the musclular system far exceeds the capacity of the cardiovascular system to deliver oxygen.The average adult horse has a  heart, although it can be more than twice this size.The heart grows until the horse is 4 years of age, although it can increase slightly in size as a response to conditioning
Edited sentence: As overall cardiac function is determined by a number of functions, it is not surprising that a number of variables would combine to give the best prediction of VO2max, Cardiac Output, Total Blood Volume and Stroke Volume, which are all important physiological variables to identify elite racehorses.As the horse becomes more fit the SV increase at the HR tends to decrease.Both TBV and Total Hemoglobin (THb) is related to the state of training in the thoroughbred  From this it follows that a correlations between blood-volume parameters and racing performance could be expected.CO is the product of Heart Rate (HR) x Stroke Volume (SV).Cardiac Output (CO or Q) is a measure of how much blood the heart can pump per minute.However, studies have shown that the proportion of skeletal muscle in thoroughbreds exceeds 50% of body weight, and the energetic capacity of the equine muscle far exceeds the capacity of the cardiovascular system to deliver oxygen.SV is a function of the dimensions of the cardiovascular system and thus related to Total Blood Volume (TBV).The Thoroughbred racehorse has evolved to allow it consume more oxygen per kilogram than most other large mammals.The average adult horse has a 3.6 kg (8.5 lb) heart, although it can be more than twice this size.The heart grows until the horse is 4 years of age, although it can increase slightly in size as a response to training.The superiority of the Thoroughbred cardiovascular system rests in a proportionately larger heart and spleen per unit body mass than other large mammals