The horse’s heart is a pump that supplies all its muscles and organs with oxygen andnutrients and carries away waste products.

The heart is divided into two sides. The left side takes oxygenated blood from the lungs and supplies muscles and vital organs, such as the brain and kidney.

The de-oxygenated blood then returns to the right side to be pumped to the lungs, where it is replenished with oxygen. Each side has a muscular pumping chamber, the ventricle, and a collecting chamber, the atrium.

With each beat, a typical adult horse pumps approximately one litre of blood out of each side of the heart – the stroke volume. With a resting heart rate of 30-40 beats a minute, this means the total blood pumped in one minute is 30-40 litres.

The heart can increase the quantity of blood being pumped around the body to meet the demands of exercise in two ways.

  • Increase the amount of blood pumped with each beat. When exercise begins, adrenaline makes the heart muscle contract more forceful, sending more blood around the body with each beat.
  • Increase the heart rate. In Thoroughbreds, the resting heart rate is between 28 and 36 beats a minute, while the maximum heart rate is between 225 and 240 a minute. The huge range of heart rate is the most important mechanism for increasing cardiac output during exercise, particularly as work intensity increases.

    Just as muscles for movement become stronger with training, so, does the heart muscle. Research has shown that the amount of heart muscle, and the width and area of the left heart chamber, increased dramatically with training.

    Increased heart volume allows it to hold more blood at rest, and during exercise, while increased muscle also allows the more powerful “fit” heart to eject more of this blood with each beat.

    New technologies enable us to measure the horse’s heart rate both at rest and during exercise. This information can provide a helpful insight into a horse’s fitness and potential performance.