Circulatory system of the horse
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The circulatory system of the horse consists of the heart, the blood vessels, and the blood.


Anatomy


Heart

The equine heart is a muscular pump that circulates blood throughout the body. It is more glenoid in shape than the human heart and consists of four chambers: the left and right atria, and the left and right ventricles. The average adult horse has a heart, although it can be more than twice this size. The heart grows until the horse is 4 years of age, although it can increase slightly in size as a response to conditioning. Heart size does not necessarily correlate to the size of the horse.
. Accessed July 2007.
Circulatory capacity is partially determined by functional mass of the heart and spleen. Once the oxygen has entered the bloodstream it must be transported to working muscle and waste products removed. The equine cardiovascular system is hugely compliant with a heart rate range from 20 to 240 beats per minute and a splenic red cell reserve able to double packed cell volume and oxygen delivery during maximal exercise. However, studies on
Thoroughbred The Thoroughbred is a horse breed best known for its use in horse racing. Although the word ''thoroughbred'' is sometimes used to refer to any breed of purebred horse, it technically refers only to the Thoroughbred breed. Thoroughbreds are c ...
s have shown that the proportion of skeletal muscle exceeds 50% of body weight, and so the energetic capacity of the muscular system far exceeds the capacity of the cardiovascular system to deliver oxygen.


Blood and blood vessels

Blood is made up of red blood cells (
erythrocytes Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
) and white blood cells (
leukocytes White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from mult ...
), as well as plasma. Produced in
bone marrow Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It is composed of hematopoietic ce ...
, red blood cells are responsible for carrying oxygen to tissue and removing carbon dioxide, all via
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
. White blood cells are used for defense against
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
s in the
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...
. Plasma suspends the blood cells, contains
clotting factor Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism o ...
s, and contributes to the greatest volume of blood. The heart and blood vessels contain approximately of blood in a horse, which equates to about 76 mL/kg (1.2 oz/lb).Giffin, James and Tom Gore, DVM. ''Horse Owner's Veterinary Handbook, Second Edition''. Howell Book House. New York, NY. Copyright 1998


Spleen

The spleen removes damaged red blood cells from circulation. It also holds extra blood cells, releasing them during exertion to increase blood volume and the amount of oxygen transported to tissues.


Frog

The
horse hoof A horse hoof is the lower extremity of each leg of a horse, the part that makes contact with the ground and carries the weight of the animal. It is both hard and flexible. It is a complex structure surrounding the distal phalanx of the 3rd digit ...
contains a structural component known as the "frog", which covers the deeper structure of the hoof known as the digital cushion, a vessel-filled tissue. When the horse places weight on a leg, the ground pushes upward on the frog, compressing it and the underlying digital cushion. This results in squeezing blood out of the digital cushion, which then helps to pump it back up the leg, helping the heart to work against gravity.


Pulse

The average pulse is 28–45 beats per minute (bpm) in a mature horse, but it can reach more than 250 bpm during maximum exertion. Depending on cardiovascular fitness and the horse's response to exercise, this drops significantly within 15–30 seconds after the horse stops galloping. A two-year-old horse may have a slightly faster pulse, and a 2–4-week-old foal normally has a pulse between 70 and 90 bpm. Heart rate may also increase when the horse is excited, overheated or suffering severe dehydration, has a fever, has an infection or
sepsis Sepsis, formerly known as septicemia (septicaemia in British English) or blood poisoning, is a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. This initial stage is follo ...
, has experienced a great deal of blood loss, has advanced heart or lung disease, or is in shock. In these cases, the resting heart rate may be above 80 in an adult animal. When the heart rate is below 20 bpm, the horse may be
hypothermic Hypothermia is defined as a body core temperature below in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe h ...
, or have pressure on the brain, heart disease, or collapsed circulation.


Determining heart rate

Heart rate may be determined with a stethoscope, placed just behind the left
elbow The elbow is the region between the arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and the media ...
of the animal. The pulse may also be felt when taken on an artery close to the skin, most commonly the facial artery located on the lower jaw just behind the cheek. The radial pulse may be taken right behind the back of the knee. The digital pulse is taken on the inside of the pastern, right below the fetlock. It is usually very faint and difficult to find, although certain problems, such as
laminitis Laminitis is a disease that affects the feet of ungulates and is found mostly in horses and cattle. Clinical signs include foot tenderness progressing to inability to walk, increased digital pulses, and increased temperature in the hooves. Sever ...
, will make it quite strong.


Blood pressure

Although blood pressure may vary greatly between animals, the average blood pressure for a standing horse is 120/70 
mmHg A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly pascals. It is denoted mmHg or mm Hg. Although not an SI ...
. An indirect measurement of blood pressure may be taken with a cuff placed around the middle coccygeal artery at the base of the tail, or above the digital artery. It is usually taken to monitor circulation during surgery. Direct blood pressure measurements, via catheterization of an artery, provide a more accurate measurement, and are preferred for anesthetic monitoring.


Gums

The gums of the horse can offer good clues to its circulatory health. Another way to see if the circulatory system is running correctly is by pressing a finger on the gum; the pink color should return in 2 seconds. The owner can assess the gums by lifting the upper lip with one hand, while holding the head still (via halter) with the other.


Gum color

* Pale pink: the healthy color of gums, indicates good circulation. May brighten slightly after strenuous work due to an increase in circulation. * Very pale pink: due to contracted capillaries. May indicate anemia, fever, or blood loss. * Pale blue, gray, or whitish: indicative of anemia (low red blood cell count). May indicate severe shock or illness. A serious sign; often the best option is to call the veterinarian. * Yellow with a tinge of brown: indicates
jaundice Jaundice, also known as icterus, is a yellowish or greenish pigmentation of the skin and sclera due to high bilirubin levels. Jaundice in adults is typically a sign indicating the presence of underlying diseases involving abnormal heme meta ...
and liver failure. Very serious and the veterinarian should see the animal immediately. * Yellowish: a bright yellow tinge may occur if the horse has high levels of beta-carotene in its diet, such as horses that eat a good deal of
alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, and silage, as w ...
hay Hay is grass, legumes, or other herbaceous plants that have been cut and dried to be stored for use as animal fodder, either for large grazing animals raised as livestock, such as cattle, horses, goats, and sheep, or for smaller domesticat ...
. Does not indicate any serious problem. * Dark red: indicates enlarged capillaries, due to poisoning or severe dehydration. The veterinarian should be contacted immediately.


Capillary refill time

The capillary refill time is determined by pressing a finger against the horse's gums for about 2 seconds, so that a white "thumbprint" is left. After releasing, it should take no longer than 2 seconds for the gum color to return to normal. If it takes longer for the gum color to return, the horse may be experiencing shock.


Cardiovascular capacity

Measurements of heart size do not appear to correlate directly with racing speed, stride length, or stride frequency. However, the ability of the body to pump blood can help identify athletic potential in an unproven horse. There is a hypothesis that measurements of a horse's heart at rest are directly related to the same horse's cardiac function during exercise. Therefore, attempts have been made to take resting measurements of horses using an electrocardiograph (ECG). This has led to the development of the "heart score", which measures the QRS interval. However, no work has correlated this to a horse's oxygen uptake (VO2Max) and the test has not been a good predictor of future athletic ability.Echocardiography and electrocardiography as means to evaluate potential performance in racehorses, (Lightowler, et al J. Vet Sci 2004) On the other hand, the Pearson correlation coefficient has been found to provide a link between oxygen uptake and echocardiographic measures. There is also evidence that maximal oxygen consumption and heart size are more important predictors of performance for horses that run longer distances because their energy consumption is mainly aerobic.


"X factor"

The X factor theory proposes that a mutation within a gene located on the
X chromosome The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO sex-d ...
of horses causes a larger-than-average heart. A larger-than average heart was documented in certain high-performance
Thoroughbred The Thoroughbred is a horse breed best known for its use in horse racing. Although the word ''thoroughbred'' is sometimes used to refer to any breed of purebred horse, it technically refers only to the Thoroughbred breed. Thoroughbreds are c ...
,
Quarter Horse The American Quarter Horse, or Quarter Horse, is an American breed of horse that excels at sprinting short distances. Its name is derived from its ability to outrun other horse breeds in races of a quarter mile or less; some have been clocked at s ...
, and
Standardbred The Standardbred is an American horse breed best known for its ability in harness racing, where members of the breed compete at either a trot or pace. Developed in North America, the Standardbred is recognized worldwide, and the breed can trace i ...
racehorses Horse racing is an equestrian performance sport, typically involving two or more horses ridden by jockeys (or sometimes driven without riders) over a set distance for competition. It is one of the most ancient of all sports, as its basic pr ...
. It was first seen in
Eclipse An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ce ...
, at 6.4 kg (14 pounds). A large heart was also seen in
Phar Lap Phar Lap (4 October 1926 – 5 April 1932) was a champion New Zealand–bred Thoroughbred racehorse who is widely regarded as New Zealand's greatest racehorse ever. Achieving incredible success during his distinguished career, his initial ...
(6.4 kg/14 lb), Sham (), and
Secretariat Secretariat may refer to: * Secretariat (administrative office) * Secretariat (horse) Secretariat (March 30, 1970 – October 4, 1989), also known as Big Red, was a champion American thoroughbred horse racing, racehorse who is the ninth winne ...
(estimated at ). It is also proposed as a theory that the great producing mare
Pocahontas Pocahontas (, ; born Amonute, known as Matoaka, 1596 – March 1617) was a Native American woman, belonging to the Powhatan people, notable for her association with the colonial settlement at Jamestown, Virginia. She was the daughter of ...
was
homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
for the X factor. Large hearts have been found in four major Thoroughbred lines, all descendants of Eclipse:
Princequillo Princequillo (1940–1964) was a Thoroughbred racehorse conceived in France and born in Ireland. He is known for his performances in long-distance races and his successes as a sire. Background His sire, Prince Rose, stood at the Haras de Cheff ...
,
War Admiral War Admiral (May 2, 1934 – October 30, 1959) was a champion American Thoroughbred racehorse who is the fourth winner of the American Triple Crown. He was also the 1937 Horse of the Year and well known as the rival of Seabiscuit in the 'Match ...
,
Blue Larkspur Blue Larkspur (1926–1947) was a bay Kentucky-bred thoroughbred race horse. He was inducted into the National Museum of Racing and Hall of Fame in 1957, and ranks Number 100 in The Blood-Horse's top 100 U.S. thoroughbred champions of the 20t ...
and Mahmoud. Many outstanding race horses such as Eclipse and Secretariat were noted for being excellent broodmare producers but generally failed to produce male offspring with the ability of their sires, thus the theory that the gene was carried only on the x chromosome meant that stallions with large hearts could only pass on the trait via their daughters. The Heart Score, using electrocardiography, was developed over 40 years ago to describe the correlation between the QRS (intraventricular conduction time) complexes and the performances of several elite versus average racehorses with the belief that a large heart correlated to athletic ability.{{Cite journal, last = Steel, first = , J. D., last2 = Beilharz, first2 = , R. G., last3 = Stewart, first3 = , G. A., last4 = Goddard, first4 = , M., date = 1977-07-01, title = The Inheritance of Heart Score in Racehorses, journal = Australian Veterinary Journal, language = en, volume = 53, issue = 7, pages = 306–309, doi = 10.1111/j.1751-0813.1977.tb00237.x, issn = 1751-0813 This belief is widespread and therefore a high heart score can increase the animal's worth in some circles. However, the X-Factor theory was never scientifically peer-reviewed and studies on the ECG protocol used, indicate that the Heart Score has little correlation to future racing ability. In addition, the gene(s) associated with cardiovascular dimensions and athletic performance have not been identified, nor has its mode of inheritance been determined; the condition may be influenced by multiple genetic factors.


Disorders of the circulatory system

*
Anemia Anemia or anaemia (British English) is a blood disorder in which the blood has a reduced ability to carry oxygen due to a lower than normal number of red blood cells, or a reduction in the amount of hemoglobin. When anemia comes on slowly, th ...
*
Arrhythmia Arrhythmias, also known as cardiac arrhythmias, heart arrhythmias, or dysrhythmias, are irregularities in the heartbeat, including when it is too fast or too slow. A resting heart rate that is too fast – above 100 beats per minute in adults ...
*
Heart disease Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina and myocardial infarction (commonly known as a heart attack). Other CVDs include stroke, hea ...
*
Heart failure Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome, a group of signs and symptoms caused by an impairment of the heart's blood pumping function. Symptoms typically include shortness of breath, excessive fatigue, a ...


References


External links


Equine Anatomy and Physiology: Circulatory Systems
Horse anatomy