In part one of this series, we learned that the horse's cardiovascular system (heart, blood vessels, blood, and spleen) is bigger than would be expected and many of the improvements seen in a horse's fitness at the start of a preparation are due to changes in the cardiovascular system. In this article we will be exploring how the cardiovascular system reacts to the increased demands imposed by exercise.

An exercising horse needs more oxygen so that more energy is available to the working muscles. The cardiovascular system makes this happen. The increased blood flow back from the working muscles also reduces the onset of fatigue by carrying away waste products.

The ability of the heart to pump enough oxygen carrying blood to working muscles determines how long a horse can exercise.  The amount of blood the heart pumps out to the horse's body is the cardiac output. Cardiac output, measured in litres per minute, is determined by two factors:

Heart rate is measured as beats per minute (bpm) and increases directly with increasing exercise demand.  Each horse has an individual maximum heart rate which is determined by genetics, foetal development, and possibly early life experiences. Racehorses have been recorded to have heart rates of up to 240 bpm, which is an astounding four beats every second. The maximum heart rate levels begin to drop after seven years of age in the horse.

Stroke volume is the volume of blood pumped out of the heart into the circulatory system with each heart beat or stroke. Stroke volume is measured in litres and is around 1.1 L in a fit racing horse. Stroke volume is determined by:

  • Size of the heart (determined by genetics, genetics, foetal development, and possibly early life experiences) 
  • Increases in blood volume (determined by hydration and current health)
  • Increased contraction of the heart muscle in response to the increasing blood pressure (determined by fitness, genetics, and current health)

Although maximum horse cardiac output is the combination of stroke volume and heart rate (theoretically around 250l/min), most of the stroke volume increases occurs as the horse moves from walk into the trot or pace. The increase seen in cardiac output as the horse exercises harder after this is mainly due to increases in heart rate. It is not currently possible to measure stroke volume easily in the exercising horse, but heart rate monitoring with electronic equipment is very accessible. Measuring heart rate post exercise can provide important insight into your horse's cardiovascular fitness. Click on the link to this article for more information: Evaluating and monitoring fitness levels in standardbreds.

Unfortunately having a very large maximal cardiac output is not a guarantee of optimal exercise performance over racing distances. In our next article we will explore how the horse's evolutionary strategy of relying on a supersized cardiovascular system can actually limit exercise performance.