Last Updated on February 28, 2022 by Allison Price
Horses are one of the greatest natural athletes in the world. The horse’s natural ability to exercise is due to its specialized circulatory system. This system can also accommodate large oxygen requirements from the horse’s muscles.
The Equine Circulatory System consists of two main organs, the heart, and the spleen. They are connected by many vessels that deliver oxygen and nutrients to cells and eliminate any wastes or toxins.
Racehorse Phar Lap’s heart
This remarkable racehorse was discovered to have a large heart. It is now on display at the National Museum of New Zealand. (c) 2006 by A.Y. Arktos
A horse’s average heart size is approximately 1% of their body weight. This means that a horse weighing 1000 lbs will have an 8-10 pound heart. Thoroughbreds tend to have larger hearts than their bodies, while drafts have hearts that weigh 0.6%.
Heart size is often associated with athletic ability. The famous Australian racehorse Phar Lap was said to have a 13.6-pound heart, which is now on display at Australia’s National Museum of Australia.
Despite years of research, the Heart Score measurement, which measures the size of the horse’s heart, is not able to predict its athletic potential. A horse’s blood volume is approximately 8% of their bodyweight in kilograms. Therefore, an adult horse of 500 kg has 40 liters of blood.
The Horse’s Complete Circulatory System
Horses’ hearts are very similar to ours. There are four chambers and two atria, which sit over two ventricles that are separated by four valves. The right side of your heart receives blood from the body. The right atrium is filled with deoxygenated red cells (RBCs).
The blood flows into the right ventricle by passing through the tricuspid valle. Blood is pumped into the lungs through the pulmonary valve by contracting the right ventricle.
The RBCs pass through tiny capillaries around the micro-blind-ended sacks of the lungs (called alveoli) and then exhale carbon dioxide. They also attach oxygen molecules to the hemoglobin compounds within the cell.
After the blood has been fully oxygenated, it returns to its source and enters the left atrium. With its strong, muscular walls, the mitral valve is the link between the left ventricle and left atrium. Blood is pumped through the aortic valve by the strong contraction of left ventricle into the aorta, and throughout the body.
The vessels that carry oxygen and nutrients-rich blood away from the heart are called arteries. When the blood reaches the tissues’ capillaries, oxygen is released. Carbon dioxide and other cellular pollutants are then added to the bloodstream.
The veins run from the tissues to the heart. The portal vein collects blood that has passed the intestines and is then sent to the liver for processing. The kidneys filter out waste from the blood.
The Cardiac Cycle
Adult horses’ resting heart beats range from 28 to 44 beats per hour. The rate of rest increases with exercise and is an average 80 bpm for the walk, 130 at the trot, 180 at the canter, and 240 at the galloping.
Work and the cardiac cycle
Racing puts stress on the heart system because of the liters and minutes of blood circulation that is required to power a heart rate greater than 200 beats per hour.
Anemia, anxiety, fever, dehydration and anemia can all increase heart rate. When you take a heart rate ( lub-dub), the heartbeats that you hear are the sounds of the valves closing.
After the ventricles contract, the mitral and tricuspid valves close. You hear the 1 first heart sound. S1 ( lub). S2 ( dub) is the 2 second heart sound.
Two distinct sounds may be heard in the cardiac cycle at times. S3 is the sound of blood filling ventricles, while S4 can be heard during atrial contraction. The cycle that repeats itself is called S4–S1( lub),–S2( dub),–S3.
The spleen is the second organ involved in the circulatory system. The spleen is located on the left side against the horse’s body wall and plays an important part in horse’s immune system. It removes diseased or damaged white blood cells from the circulation. Horses are able to excel because of their participation in the cardiovascular system.
Relaxed horses have a spleen that is at its largest. It extends from the 9 thrib space back to the point where the hip, covering most of the left side and covering the entire abdomen. It can store approximately 30 liters blood in this state. The blood slowly flows through the spleen and then returns to the circulation.
The spleen can contract under stress or excitement, such as in a race or before the jump-off or when the “fight/flight” response kicks into effect. Up to 25 liters stored blood can be expelled into the vessels. This makes the extra RBCs more available to transport an enormous amount of oxygen to the muscles.
This can almost double the oxygen-carrying ability of the bloodstream, and increase the aerobic capabilities of horses within seconds.
The blood contains many components. As mentioned, the RBCs are responsible for carrying oxygen to the tissues and carbon dioxide out. White blood cells, which are the infantry cells in the immune system’s army of soldiers, attack and kill bacteria, viruses and other pathogenic invaders.
Specialized cells called platelets aid in the formation of clots when there is damage to blood vessels, such as a laceration. These cells are made in bone marrow and then released into the circulation. These cells are suspended in plasma which is a pale yellow solution of electrolytes and proteins.
There are over 30 blood factors that have been identified in horses, seven of which are well-known. Two blood types are relevant for checking compatibility with blood transfusions.
Although blood typing is useful for determining the horse’s readiness to receive a transfusion before it occurs, most horses don’t have any antibodies that could cause an allergic reaction. Therefore, a transfusion can be given without cross-matching and without the risk of a reaction.
Assessment of the Cardiovascular System
The cardiovascular system evaluation can provide insight into how the heart functions. This can provide information about the horse’s hydration status. You must listen to more than the heart in order to fully assess the cardiovascular system.
Each heart sound should be clear and without any pauses before or after the actual beat. Murmurs are abnormal heart sounds. Because murmurs are related to the closing of the valves and heart sounds, they can indicate valvular problems.
Listening to the heart of a horse
It is difficult to listen to the heartbeat of a horse, despite its size and immense capacity. This requires concentration.
Although the heart rate can usually be heard on the left side, due to the horse’s large heart, only three out of four valves can properly evaluated on this side: The mitral, the Aortic and the Pulmonic. To assess the tricuspid, place your stethoscope to the right side.
Most commonly, murmurs can be heard in the mitral or aortic valves. Congenital heart defects can cause murmurs. Blood flow through holes or defects in the heart can lead to murmurs. An echocardiogram (ultrasound in the heart) is required to diagnose the reason for a murmur.
Regular rhythm means that there should be a consistent spacing between each beat and that the interval doesn’t change over time. Arrhythmia is a condition in which the rhythm of your heart is disrupted.
Arrhythmias are caused by disruptions in the electrical conduction, which controls the contraction and rhythm. They can only be diagnosed by an electrocardiogram (ECG), of the heart. Atrial fibrillation is the most common arrhythmia in horses. It can be identified by an irregular rhythm.
Horses with electrolyte imbalances can develop A-fib. However, it is most commonly caused by an enlargement in the atria. This condition is often secondary to valvular diseases. The heart can be returned to normal rhythm with treatment protocols that include antiarrhythmic drugs and electrical stimulation.
The facial artery, which runs along the mandible, is another location where you can measure your heart rate. You can feel the pulse with your fingers and also assess its quality. A strong pulse means that there is normal vascular volume. A weak pulse can indicate hypotension (low blood pressure).
Horses have an average blood pressure of 120/70. This is similar to humans. Horses are not hypertensive like humans. They do not become hypertensive unless they have severe kidney disease. Also, horses do NOT develop arthrosclerosis (hardening the arteries) and plaques in their vessels. Horses do not experience “heart attacks” or myocardial infarction in the same way that humans.
Horses can die suddenly from rupture of the aorta. Hypertension is uncommon, but hypotension can be seen in cases such as colic, blood loss, dehydration and other sepsis-related illnesses.
Because of the horse’s hair and pigmented skin, it is necessary to evaluate the horse’s circulatory system using the mucous membranes (gums). The mucous membranes must be pink and moist. Anemia is reflected in pale membranes. Hyperemic membranes (bright pink to purple) are associated with severe inflammation and sepsis.
As an indicator of hydration, the capillary replenish time (CRT), is used. You can push the blood from the capillaries out by pressing your finger into the gum. This will leave you with a white print. CRT is the time it takes for pink to return to the area. Normal CRT takes between 1.5 to 2.5 seconds. Dehydration is indicated by prolonged CRT.
The blood is composed of between 30-40% red blood cell, which we call the Packed Cell Volume (PCV). Anemia can cause a horse’s PCV to drop if they have lost whole blood or RBCs. A decreased level of blood water means that blood is more concentrated, which increases the relative percentage RBCs and thus the PCV (also called hemoconcentration).
The PCV is used by veterinarians to assess the horse’s hydration and monitor their response to therapy.
When you’re performing a dressage exam, taking part in a cross-country course or going on a trail ride, remember the intricate cycle of your horse’s heart. The miles of veins, arteries, and millions of RBCs that travel through the horse’s body provide the energy and oxygen required for each step.
I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!