The circulatory system (heart and blood vessels) and respiratory system (lungs and airway) work together to provide oxygen throughout the horse's body, and to transport waste material from the horse's tissues. During exercise, the circulatory and respiratory systems are the mechanisms that allow the horse's musculoskeletal system (bones, connective tissue, muscles) to produce motion from energy. Without any one of these components–circulatory, respiratory, and musculoskeletal systems–performance would be impossible.

According to Robert Gilmour, PhD, a veterinarian at Cornell University, diseases of the circulatory system are the third most frequent cause for loss of performance after respiratory and musculoskeletal disorders. Though classic "heart attacks" similar to those suffered by humans are quite rare in horses, there are a number of heart problems that can cause poor performance and even death.

Signs of a heart problem might include a loss of condition, shortness of breath, slow recovery after exercise, an increased effort to breathe, a shorter period of exercise before development of fatigue, and general weakness. Fluid accumulation in the abdomen, legs, or under the skin surrounding the ribs is another indication of poor heart function. Owners or trainers who suspect a disorder should contact a veterinarian who can examine the horse's heart and determine the cause of the problem.

Cardiac arrhythmias (irregular heartbeats) are more common in the horse than in other domestic species, and atrial fibrillation, a condition in which the upper chambers of the heart may beat up to 400 times a minute, is the most common heart problem that affects performance. Though the upper chambers of the heart are contracting almost non-stop, the rapid fluttering action does not produce significant circulation of blood into the lower chambers, negatively affecting performance. Atrial fibrillation occurs relatively frequently when horses race, sometimes resolving on its own after the race, according to studies at Ghent University in Belgium. Atrial fibrillation also predisposes horses to exercise-induced pulmonary hemorrhage, or "bleeding