The heart is the muscular organ responsible for circulating blood through the body by rhythmic contraction. In the horse, the heart sits within the thoracic cavity, flanked by the lungs, and beats in a regular cycle of contraction (systole) and relaxation (diastole) that drives oxygenated blood to the muscles and returns deoxygenated blood to the lungs via the pulmonary circuit.
A horse’s heart is notably large relative to its body mass — an average adult horse’s heart weighs around 3.5 to 4.5 kilograms (approximately 8 to 10 pounds), compared to 0.3 kilograms in a human. Elite racehorses may carry hearts weighing 5 kilograms or more; the legendary Thoroughbred Secretariat was posthumously found to have a heart estimated at twice normal weight. Heart size is heritable and is one of the factors that distinguishes athletically superior horses at the population level. The left ventricle, which pumps blood to the body at systemic pressure, is particularly developed in fit horses.
Resting beats per minute at rest in a healthy adult horse range from 28 to 44 bpm, lower than the normal human range of 60 to 100 bpm. During intense exercise, a horse’s heart rate can reach 200 to 240 bpm. Deviation from normal resting rate — particularly a rate above 60 bpm at rest — can indicate pain, fever, or cardiovascular compromise; abnormal rhythm detected by a veterinarian via auscultation is one diagnostic indicator for conditions including colic. Heart health is monitored as part of routine veterinary examination throughout the horse’s physical assessment.