The circulatory system of the horse consists of the heart, the blood vessels, and the blood.
The circulatory system of the horse consists of the heart, the blood vessels, and the blood.
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 3.6-kilogram (7.9 lb) 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. [1] Heart size does not necessarily correlate to the size of the horse. [2]
Circulatory capacity is partially determined by functional mass of the heart and spleen. [3] 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. [4]
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 34 L (9.0 US gal) of blood in a 450 kg (990 lb) horse, which equates to about 76 mL/kg (1.2 oz/lb). [5]
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.
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.
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. [5] 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. [5]
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.
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. [5] Direct blood pressure measurements, via catheterization of an artery, provide a more accurate measurement, and are preferred for anesthetic monitoring. [6]
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.
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.
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. [7]
On the other hand, the Pearson correlation coefficient has been found to provide a link between oxygen uptake and echocardiographic measures. [8] 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. [9]
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 (8.2 kg (18 lb)), and Secretariat (estimated at 10 kg (22 lb)). 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. [10] 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. [11]
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. [12] This belief is widespread and therefore a high heart score can increase the animal's worth in some circles. [2] 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. [7] 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. [11]
An artery is a blood vessel in humans and most other animals that takes oxygenated blood away from the heart in the systemic circulation to one or more parts of the body. Exceptions that carry deoxygenated blood are the pulmonary arteries in the pulmonary circulation that carry blood to the lungs for oxygenation, and the umbilical arteries in the fetal circulation that carry deoxygenated blood to the placenta. It consists of a multi-layered artery wall wrapped into a tube-shaped channel.
The heart is a muscular organ found in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to the lungs. In humans, the heart is approximately the size of a closed fist and is located between the lungs, in the middle compartment of the chest, called the mediastinum.
Blood vessels are the structures of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away from the tissues. Blood vessels are needed to sustain life, because all of the body's tissues rely on their functionality.
Veins are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and fetal circulations which carry oxygenated blood to the heart. In the systemic circulation, arteries carry oxygenated blood away from the heart, and veins return deoxygenated blood to the heart, in the deep veins.
Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" refers to the pressure in a brachial artery, where it is most commonly measured. Blood pressure is usually expressed in terms of the systolic pressure over diastolic pressure in the cardiac cycle. It is measured in millimeters of mercury (mmHg) above the surrounding atmospheric pressure, or in kilopascals (kPa). The difference between the systolic and diastolic pressures is known as pulse pressure, while the average pressure during a cardiac cycle is known as mean arterial pressure.
The circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, that consists of the heart and blood vessels. The circulatory system has two divisions, a systemic circulation or circuit, and a pulmonary circulation or circuit. Some sources use the terms cardiovascular system and vascular system interchangeably with circulatory system.
Angina, also known as angina pectoris, is chest pain or pressure, usually caused by insufficient blood flow to the heart muscle (myocardium). It is most commonly a symptom of coronary artery disease.
In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols , , or , is the volumetric flow rate of the heart's pumping output: that is, the volume of blood being pumped by a single ventricle of the heart, per unit time. Cardiac output (CO) is the product of the heart rate (HR), i.e. the number of heartbeats per minute (bpm), and the stroke volume (SV), which is the volume of blood pumped from the left ventricle per beat; thus giving the formula:
Hemodynamics or haemodynamics are the dynamics of blood flow. The circulatory system is controlled by homeostatic mechanisms of autoregulation, just as hydraulic circuits are controlled by control systems. The hemodynamic response continuously monitors and adjusts to conditions in the body and its environment. Hemodynamics explains the physical laws that govern the flow of blood in the blood vessels.
Heart rate is the frequency of the heartbeat measured by the number of contractions of the heart per minute. The heart rate varies according to the body's physical needs, including the need to absorb oxygen and excrete carbon dioxide. It is also modulated by numerous factors, including genetics, physical fitness, stress or psychological status, diet, drugs, hormonal status, environment, and disease/illness, as well as the interaction between these factors. It is usually equal or close to the pulse rate measured at any peripheral point.
Cyanosis is the change of body tissue color to a bluish-purple hue, as a result of decrease in the amount of oxygen bound to the hemoglobin in the red blood cells of the capillary bed. Cyanosis is apparent usually in the body tissues covered with thin skin, including the mucous membranes, lips, nail beds, and ear lobes. Some medications may cause discoloration such as medications containing amiodarone or silver. Furthermore, mongolian spots, large birthmarks, and the consumption of food products with blue or purple dyes can also result in the bluish skin tissue discoloration and may be mistaken for cyanosis. Appropriate physical examination and history taking is a crucial part to diagnose cyanosis. Management of cyanosis involves treating the main cause, as cyanosis isn’t a disease, it is a symptom.
Vital signs are a group of the four to six most crucial medical signs that indicate the status of the body's vital (life-sustaining) functions. These measurements are taken to help assess the general physical health of a person, give clues to possible diseases, and show progress toward recovery. The normal ranges for a person's vital signs vary with age, weight, sex, and overall health.
In humans, the circulatory system is different before and after birth. The fetal circulation is composed of the placenta, umbilical blood vessels encapsulated by the umbilical cord, heart and systemic blood vessels. A major difference between the fetal circulation and postnatal circulation is that the lungs are not used during the fetal stage resulting in the presence of shunts to move oxygenated blood and nutrients from the placenta to the fetal tissue. At birth, the start of breathing and the severance of the umbilical cord prompt various changes that quickly transform fetal circulation into postnatal circulation.
Exercise-induced pulmonary hemorrhage (EIPH), also known as "bleeding" or a "bleeding attack", refers to the presence of blood in the airways of the lung in association with exercise. EIPH is common in horses undertaking intense exercise, but it has also been reported in human athletes, racing camels and racing greyhounds. Horses that experience EIPH may also be referred to as "bleeders" or as having "broken a blood vessel". In the majority of cases, EIPH is not apparent unless an endoscopic examination of the airways is performed following exercise. This is distinguished from other forms of bleeding from the nostrils, called epistaxis.
Arterial stiffness occurs as a consequence of biological aging and arteriosclerosis. Inflammation plays a major role in arteriosclerosis development, and consequently it is a major contributor in large arteries stiffening. Increased arterial stiffness is associated with an increased risk of cardiovascular events such as myocardial infarction, hypertension, heart failure and stroke, the two leading causes of death in the developed world. The World Health Organization predicts that in 2010, cardiovascular disease will also be the leading killer in the developing world and represents a major global health problem.
The Bruce protocol is a standardized diagnostic test used in the evaluation of cardiac function and physical fitness, developed by American cardiologist Robert A. Bruce.
Cardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; myocardium structure; the electrical conduction system of the heart; the cardiac cycle and cardiac output and how these interact and depend on one another.
The cardiovascular examination is a portion of the physical examination that involves evaluation of the cardiovascular system. The exact contents of the examination will vary depending on the presenting complaint but a complete examination will involve the heart, lungs, belly and the blood vessels.
Doppler ultrasonography is medical ultrasonography that employs the Doppler effect to perform imaging of the movement of tissues and body fluids, and their relative velocity to the probe. By calculating the frequency shift of a particular sample volume, for example, flow in an artery or a jet of blood flow over a heart valve, its speed and direction can be determined and visualized.
Biofluid dynamics may be considered as the discipline of biological engineering or biomedical engineering in which the fundamental principles of fluid dynamics are used to explain the mechanisms of biological flows and their interrelationships with physiological processes, in health and in diseases/disorder. It can be considered as the conjuncture of mechanical engineering and biological engineering. It spans from cells to organs, covering diverse aspects of the functionality of systemic physiology, including cardiovascular, respiratory, reproductive, urinary, musculoskeletal and neurological systems etc. Biofluid dynamics and its simulations in computational fluid dynamics (CFD) apply to both internal as well as external flows. Internal flows such as cardiovascular blood flow and respiratory airflow, and external flows such as flying and aquatic locomotion. Biological fluid Dynamics involves the study of the motion of biological fluids. It can be either circulatory system or respiratory systems. Understanding the circulatory system is one of the major areas of research. The respiratory system is very closely linked to the circulatory system and is very complex to study and understand. The study of Biofluid Dynamics is also directed towards finding solutions to some of the human body related diseases and disorders. The usefulness of the subject can also be understood by seeing the use of Biofluid Dynamics in the areas of physiology in order to explain how living things work and about their motions, in developing an understanding of the origins and development of various diseases related to human body and diagnosing them, in finding the cure for the diseases related to cardiovascular and pulmonary systems.
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