1.6.1 The short-term effects of exercise
The short-term effects of exercise usually begin before the horse has even begun to trot. Excitement plays an important part in preparing the body to cope with the demands of exercise. The sympathetic nervous system will be activated and parasympathetic discharge reduced or abolished, resulting in a tachycardia and vasodilation of the blood vessels supplying skeletal muscle. Blood is diverted from other organs, such as the gut, by local vasoconstriction. Myocardial contractility may increase. Venoconstriction results in an increased venous return, and splenic contraction increases the packed cell volume and oxygen carrying capacity of blood.
During exercise, venous return is also aided by the pumping effect of skeletal muscle on the blood within the venous system, and the changes in intrathoracic pressure associated with respiration. Intrathoracic pressure and venous return are also affected by the alternate compression and decompression of the thorcavity by the bulk of abdominal contents moving backwards and forwards during locomotion. Cardiac output is therefore increased due to increased heart rate, preload and contractility and reduced afterload. Of these, increased heart rate is the most significantly altered from the resting value in the horse.
Marked changes in the peripheral cardiovascular system also occur during exercise to allow perfusion of active skeletal muscle and to aid thermoregulation. Although stroke volume increases little during exercise, changes in loading conditions and contractility are required in order to maintain stroke volume in the face of a much increased heart rate and vasodilation in skeletal muscle.
The horse has a tremendous capacity to increase cardiac output, which can rise to six times the resting value. This is more than many other species, including man. The huge cardiac reserve is reflected by the low resting heart rates in horses compared with animals of similar body size, such as cattle, which usually have a resting heart rate of approximately 50-100% higher than that found in horses. Many of the unusual features of the rhythm of the equine heart are related to the high vagal tone which maintains low resting cardiac output in this species.