How A Horse Heart Works
 By Winniefield Park   •   28th Apr 2019   •   334 views   •   0 comments

A horse’s heart is a large muscular mass that lies inside the chest just forward of the girth area. The heart and lungs of a horse have evolved to facilitate a large muscled animal that must run suddenly and fast to escape its predators. Although it doesn’t have the largest heart in ratio to its body mass - that record goes to the dog, it does very efficiently circulate about 42 quarts (40 liters) of blood throughout the average riding horse’s body per minute. The heart is one of the organs responsible for circulating blood - some credit has to go to the spleen and lungs too, that make horses such amazing born athletes.

The size of the heart is of course, roughly relative to body size and will weigh slightly less than 1% of its body weight. That means the heart of an average sized riding horse will be about 8 to 10 lbs. There are exceptions to that rule however. The weight of a draft horse’s heart is only about 0.6% of its body weight, and the heart of a Thoroughbred is slightly heavier than average. Secretariat had a huge heart, weighing in at over twice the size of a normal horse’s. Fitness has a bearing on heart size too. A well conditioned horse’s heart can be slightly larger than one that is unfit.

A horse’s heart is shaped roughly like a human’s heart. There are left and right chambers as in all mammals. The two lower chambers are called the ventricles and the upper chambers are the atria. Between are four valves that control the blood flow to and from the body, and between the chambers.

The job of the heart is to collect oxygen depleted blood from the body and deliver it to the lungs to be re-oxygenated. One way valves keep the blood flowing in the correct direction. Blood depleted of oxygen is pumped into the right atrium. It then flows through the tricuspid valve, so called because it has three flaps, and into the right ventricle. With a contraction of the right ventricle, the blood is pumped through the pulmonary valve and through pulmonary artery. The pulmonary artery channels the blood into the lungs where the blood disperses into the tiny capillaries. In the capillaries, the blood sheds carbon dioxide and absorbs oxygen. Once all the CO2 is shed, and the blood is filled with O2, it is pumped back into the left atrium, through the very strong mitral valve into the left ventricle. The strong contraction of the left ventricle sends the blood through the aortic valve into the aorta that branches off into arteries throughout the body. The veins then carry the O2 and nutrient depleted blood back to the heart to repeat the process.

Heart Rate
A horse’s heart beats about 28 to 44 times per minutes at rest. This depends on the horse’s condition, size, and the environment. The rate increases with activity or other stress. This is why a horse’s pulse is an indication of its health. Pain, stress and illness can increase a horse’s pulse rate. Exercise too, will increase a horse’s heart rate. At a walk, the heart rate can easily double. At a hard gallop the heart rate can increase to up to 240 beats per minute, or four beats per second. While the part of the brain called the medulla governs the heart rate, each beat is triggered by an electrical impulse. This impulse starts in specialized cells in the right atria that cause the walls of the chamber to contract. There are corresponding responses in each part of the heart that keep the beat coordinated.

You can take your horse’s heart rate with your fingers or a stethoscope. The first half of the beat is the sound of the tricuspid and mitral valves closing and the second half is the pulmonic and aortic valves closing. One of these ‘lub dub’ cycles is a heart beat. Count the ‘lub dubs’ for 15 seconds and multiply by four for an average heart rate. If the horse has heart valve problems, the beat cycle might not sound as clear, or will be delayed.
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