Heart of a Horse

Basics of the Equine Heart

The cardiovascular system is a dynamic one, designed to transport water, oxygen, carbon dioxide, energy fuels, electrolytes, hormones and other metabolic products throughout the body [1]. At the center of the system is a muscular pump known as the heart. In Thoroughbreds, the average weight of the heart is 1% of mass, or approximately 9-11 pounds [1]. Weight may vary depending on training status, size of horse or breed. (Note that in comparison, a human heart weighs somewhere in the range of 0.5 – 0.8 pounds, and is about the size of your fist [3])!

Like the human heart, the equine heart has 4 chambers – the left and right atria (top half) and the left and right ventricles (bottom half). Blood with fresh oxygen from the lungs flows into the left atria via pulmonary veins. When the left atrium contracts, this oxygenated blood is pumped to the left ventricle. Left ventricular contraction sends the blood out to the tissues in the body through the aorta [1,2]. Blood carrying carbon dioxide returns to the right atria from the body via both the superior and inferior vena cava, then upon atrial contraction, to the right ventricle. The right ventricle pumps this blood through the pulmonary artery to the lungs, where carbon dioxide is exchanged for oxygen [1,2].

The atria are smaller and less muscular than ventricles, but they function to help fill the ventricles with blood [2]. Valves between the atria and ventricles prevent backflow of blood [2] (see diagram). The left side of the heart ensures oxygenated blood reaches the entire body [1]. The left ventricle pumps against the back pressure of arterial circulation [2], and is thus larger than the right [2]. The left ventricle is often referred to as the “pressure pump,” whereas the right ventricle is referred to as the “volume pump” [2]. The right side of the heart pumps deoxygenated blood returning from the body to the lungs to exchange carbon dioxide for fresh oxygen. Then, the cycle repeats.

Diagram from [1], p. 130.

The heart beats without external nerve supply because the heart muscle generates the electrical impulse responsible for contraction “on site” [2]. This occurs via the action of a built-in pacemaker, known as the sinoatrial (SA) node [2]. A coordinated series of electrical conductions regulate contraction of the atria to ensure filling of the ventricles, followed by contraction of the ventricles themselves.

Cardiac output is a measure of blood that flows out of the left side of the heart per minute, and it depends on heart rate and the volume of blood pumped out of the heart with each beat [2]. Resting cardiac output for a horse is about 25 liters per minute, which is what a human can achieve only at maximum exercise effort [2]! Elite equine athletes can increase cardiac output up to 300 liters per minute during exercise [2]. Increased cardiac output allows a greater proportion of blood to circulate to working muscles and heart, helping to sustain work, while diverting blood from the digestive tract, kidney and other organs [2].

Blood volume is the combined volume of plasma and cells in the blood [1]. It is a factor in the measure of cardiac output, and it can change during exercise or excitement [1, 2]. Horses have the ability to increase their blood volume from about 40 liters at rest to 50 liters during exercise, due in part to contraction of the spleen and diversion of fluids from other areas of the body [1, 2]. Splenic contraction does not occur in humans, but in your horse, it results in a greater number of red blood cells available to carry oxygen to working muscles. It has been speculated that this phenomenon contributes to the great athleticism of horses.

It is important to know what is normal for your horse. At rest, a horse’s normal heart rate is usually in the range of 30-40 beats per minute (bpm) [4]. Maximal heart rates have been recorded in the range of 215-245 bpm when working at peak exercise [1]. To put this in perspective, consider that the American Heart Association describes typical resting heart rate in healthy humans as between 60-80 bpm, and maximum heart rate was traditionally estimated using the formula, 220 – age in years. Thus, the average adult has a maximum heart rate considerably lower than the average equine! Unusually elevated heart rates in horses at rest may indicate pain, stress or excitement. It is wise to know how to take your horse’s pulse. Take the pulse several times over the course of a few days to get an idea of what is normal for your horse, and seek medical help if you have questions or concerns.

The horse is a unique and phenomenal athlete with incredible capability. You may now have new respect for how much “heart” your horse really has!

References
1. Evans, DL. The cardiovascular system: anatomy, physiology and adaptations to exercise and training. In: Hodgson, DR, Rose, RJ, editors. The Athletic Horse: Principles and practice of equine sports medicine. Philadelphia, PA: Saunders; 1994, p. 129-144.

2. Marlin, D, Nankervis, K. The cardiovascular system. In: Equine Exercise Physiology. Oxford, UK: Blackwell Science, Ltd; 2002, p.55-72.

3. MacDonald, M. Your Body: The Missing Manual. Sebastopol, CA: Pogue Press, 2009.

4. Pavord, T, Pavord, M. Knowing your horse. In: The complete equine veterinary manual. Cincinnati, OH: F&W Publications, 2007, p. 8-35.