Circulatory system of the horse
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. 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 Thoroughbreds 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) and white blood cells (leukocytes), as well as plasma. Produced in bone marrow, red blood cells are responsible for carrying oxygen to tissue and removing carbon dioxide, all via hemoglobin. White blood cells are used for defense against pathogens in the immune system. Plasma suspends the blood cells, contains clotting factors, 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). 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 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, 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, 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 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, 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. 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. 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. 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. Section::::"X factor". The X factor theory proposes that a mutation within a gene located on the X chromosome of horses causes a larger-than-average heart. A larger-than average heart was documented in certain high-performance Thoroughbred, Quarter Horse, and Standardbred racehorses. It was first seen in Eclipse, at 6.4  kg (14 pounds). A large heart was also seen in Phar Lap (6.4 kg/14 lb), Sham (), and Secretariat (estimated at ). It is also proposed as a theory that the great producing mare Pocahontas was homozygous for the X factor. Large hearts have been found in four major Thoroughbred lines, all descendants of Eclipse: Princequillo, War Admiral, Blue Larkspur 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. 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.