Heart Rate and Blood Volume
The horse’s basal heart rate typically lies in the range of 30-40 beats/minute. When required, that rate can increase dramatically and rapidly to a maximal rate of about 250 beats/minute. An estimated 32 ounces (1 liter) of blood gets ejected from the heart with each contraction. During exercise, the volume of blood pumped with each heartbeat can increase by almost 50%. Thus, to meet tissue demands for oxygen and nutrients during exercise, the heart not only increases its frequency of contraction but also the volume of blood pumped with each beat. In total, the horse’s body houses about 14 gallons (50 liters) of blood.
By comparison, a human athlete’s heart beats about 60 times each minute, and maximal heart rates approach 220 beats/minute. Thus, a horse’s heart rate increases eightfold during exercise while elite human athletes can only increase their heart rate by about four and a half times resting values. Fun fact: The hummingbird, however, beats them both with resting and active heart rates of 120 and 1,200 beats/minute, respectively!
This rapid change in heart rate occurs because of the type of electrical “wiring” that causes the heart muscles to contract. Horse have what is referred to as a type B heart, in that the electrical conduction system is quite diffuse throughout the heart muscle, allowing rapid increases in contractility (i.e., blood pumping ability). This serves horses well as flight animals that need to be able to evade predators in, well, a heartbeat.
Horse lovers almost universally emphasize how large their horses’ hearts are—much like how big a fisherman’s latest catch was that got away. In the equine enthusiast’s defense, horse’s hearts are indeed quite large. The average horse’s heart weighs approximately 1% of its body weight, so a standard 1,000-pound horse has a heart weighing a whopping 10 pounds. The actual heart weight will vary depending on breed and other individual factors (e.g., fitness).
One of the largest measured horse hearts in history is believed to be that of the Thoroughbred racing legend Secretariat. Although the organ wasn’t weighed during the “cosmetic” autopsy, the veterinarian did intonate that it was one of the largest he had ever seen: an estimated 21 pounds. While this is miraculous, the largest mammalian heart found on land belongs to the African elephant, weighing 26-46 pounds, and the award for largest heart of any living creature belongs to the blue whale at 440 pounds. A human heart, by comparison, weighs in at only 10 ounces.
The equine heart’s size and function is central to the animal’s athletic prowess, giving horses the ability move at high speeds when necessary and the endurance to cover long distances.