Structure and Function of the Cardiovascular System in Dogs


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What Is the Cardiovascular System?

The cardiovascular system (or circulatory system) is the system responsible for circulating blood throughout the body. It consists of the heart and blood vessels, namely arteries, veins and capillaries.

Where Is the Cardiovascular System Located?

The heart is located in the chest between the right and left lungs and is contained in a very thin sac called the pericardial sac. The heart extends approximately from the 3rd to the 6th rib of the dog.

Blood vessels form a conduit system throughout the body and carry blood to all organs, tissues and cells.

What Is the General Structure of the Cardiovascular System?

The heart is the central organ that contracts rhythmically to pump blood continuously through the blood vessels. The heart consists of four chambers:

  • The right atrium. The right atrium is the collecting chamber for blood from distant parts of the body. Blood is carried back to this upper right chamber of the heart in various veins. The oxygen levels in the blood in this chamber are very low. As the right atrium contracts, blood flows through the tricuspid valve into the right ventricle.

  • The right ventricle. The right ventricle is the pumping chamber of the lower right heart. As the right ventricle contracts, it sends blood it has received from the right atrium into the pulmonary artery. The pulmonary valve sits at the opening of the pulmonary artery and prevents blood from moving backwards into the right ventricle after it contracts. The pulmonary artery carries the blood into the lungs where it picks up oxygen and gets rid of carbon dioxide. The carbon dioxide leaves the body during expiration (the action of breathing out) and oxygen is taken in during inspiration (the action of breathing in).

  • The left atrium. Blood that is high in oxygen returns to the heart from the lungs and enters the upper left chamber of the heart, the left atrium. The left atrium is a collecting chamber that sends this oxygenated blood to the left ventricle. The valve that separates the left atrium from the left ventricle is the mitral valve.

  • The left ventricle. The left ventricle is the major pumping chamber of the heart. This lower left chamber is responsible for pumping oxygen-rich blood to the rest of the body. The blood from the left ventricle enters the aorta through the aortic valve. The aorta and other arteries distribute this oxygen-rich blood throughout the body.

  • A muscular wall called the septum separates the left side of the heart from the right side of the heart.

  • Because the heart is composed primarily of cardiac muscle, a tissue that continuously contracts and relaxes, it must have a constant supply of oxygen and nutrients. The coronary arteries are the network of blood vessels that carry oxygen- and nutrient-rich blood to the heart itself.

    Arteries are strong, muscular blood vessels that carry oxygen-rich blood from the heart to various parts of the body. The wall of an artery consists of an outer coat (tunica adventitia), a middle coat (tunica media), and an inner coat (tunica intima). Small blood vessels that branch off the arteries are called arterioles.

    Veins are thin blood vessels that carry blood from various parts of the body or organs back towards the heart. Like arteries, veins have three coats, but the coats are not as thick. Because of their thin walls, veins are very compliant, and their volume and size vary with blood pressure. Veins also contain valves, which allow blood flow in only one direction, towards the heart. The valves stop blood from flowing backward towards the organs. Small blood vessels that lead from the capillaries to the larger veins are called venules.

    Capillaries are the smallest of all blood vessels. Capillaries are so small, that in many instances only a few red blood cells can pass through the center of the capillary at a time. Capillaries usually lie between the arterioles and venules. Capillary walls act as a membrane that allows various substances to travel between the blood and the tissues. These substances include oxygen, carbon dioxide, water, electrolytes (e.g. sodium, potassium), nutrients, and minerals. The capillaries are the site of the greatest exchange of material between the blood and the tissues of the body.

    What Are the Functions of the Cardiovascular System?

    The circulatory system transports to the tissues and organs of the body the oxygen, nutritive substances, immune substances, hormones and chemicals necessary for normal function and activities. It also carries away waste products and carbon dioxide, helps to regulate body temperature, and helps to maintain normal water and electrolyte balance.

  • What Are the Common Diseases of the Cardiovascular System?

  • Acquired valvular disease. Various diseases can develop during the life of a dog that affect the valves and alter the normal function of the valves. Acquired valvular disease of the mitral valve is the most common form of heart disease encountered in small and medium-sized dogs. It affects many different breeds, including cocker spaniels, Cavalier King Charles spaniels, poodles, miniature schnauzers, Chihuahuas, terrier breeds, etc. As dogs age, the mitral valve degenerates and begins to leak. The degeneration is usually progressive, and causes the left side of the heart to change as more and more blood leaks back into the left atrium. Eventually, left-sided heart failure may develop, as the workload of the heart continuously increases.

  • Heart failure. Heart failure is the inability of the heart to maintain a circulation sufficient to meet the body's needs. Congestive heart failure (CHF) occurs as a result of impaired pumping ability and is associated with water and sodium retention. Heart failure may develop on either the right or left side of the heart, and sometimes involves both sides. It may arise with severe acquired valvular disease, certain forms of cardiomyopathy, from heartworm disease, from inflammation of the heart, secondary to pericardial diseases, and tumors.

  • Dilated cardiomyopathy (DCM). Dilated cardiomyopathy is disease where the heart muscles become weak and very flabby. As the muscles weaken, the chambers of the heart enlarge in size or become dilated. This disease primarily affects the left side of the heart. Weakening of the muscles decreases the ability of the heart to pump blood to the rest of the body. In dogs, this disease occurs most often in large breeds, such as the boxer, Doberman pinscher and Great Dane. It may also occur in some smaller dogs, such as the American cocker spaniel. In cockers, the disease has been associated with a deficiency in the amino acids, carnitine and taurine. The cause in most other dogs is unknown, but may involve genetic factors.

  • Arrhythmias. Cardiac arrhythmias are disturbances in heart rate or rhythm. The electrical activity of the heart is altered, which may greatly affect the ability of the heart to coordinate the contractions of its different chambers. Arrhythmias can be mild and insignificant, or be serious enough to cause heart failure and sudden death. Arrhythmias may arise in conjunction with almost any form of heart disease, and can also develop with other diseases in the body, such as high potassium levels, low oxygen levels, infections, hormone imbalances, drugs, trauma, and organ failure.

  • Heartworm disease. Heartworm infection is disease of both domestic and wild canines, and it occurs in many areas of the United States. It is caused by a parasite (Dirofilaria immitis) that enters the body through the bite of an infected mosquito. The adult worms prefer to live in the pulmonary vessels that lead from the right heart to the lungs. Heartworm disease in the dog most often causes obstruction of the pulmonary artery, with secondary right heart failure.

  • Congenital heart defects. Several congenital heart defects occur in the dog. Aortic stenosis (narrowing of the opening of the aorta as it leaves the left ventricle) is an inherited condition that affects primarily large-breed dogs, such as the Newfoundland, rottweiler, German shepherd dog, golden retriever, etc. Pulmonic stenosis (narrowing of the opening of the pulmonary artery as it leaves the right ventricle) is seen in the English bulldog, beagle, keeshond, bullmastiff, mastiff, terrier breeds, etc. Patent ductus arteriosus (PDA) is a deformity that arises when a small communication between the aorta and the pulmonary artery (that is present before birth) fails to close after birth. This disease affects female dogs more often than male dogs, and is seen in many different breeds. Small holes in the septum of the heart may also occur. They may be located between the two atria (atrial septal defects) or between the two ventricles (ventricular septal defects). Most of these congenital disorders affect the function of the heart and may give rise to clinical signs of heart disease and eventual heart failure.

  • Infectious endocarditis. Infectious endocarditis is an inflammation of the heart caused by some sort of infectious agent, such as bacteria, protozoa, and viruses. This condition is uncommon in the dog, but may arise with bacterial or viral infections.

  • Pericarditis and pericardial effusion. Pericarditis is inflammation of the pericardium, which is the fibrous sac that encloses the heart. Pericardial effusion is the accumulation of fluid within the pericardial sac. As fluid accumulates in this sac it applies pressure to the heart and decreases the ability of the heart to pump blood. Pericardial diseases in the dog arise with infections (e.g. bacterial and fungal), tumors, blood clotting disorders, trauma, foreign bodies, kidney failure, and for unknown reasons. Pericardial effusion occurs most often in large breed dogs.

  • Hypertrophic cardiomyopathy (HCM). Cardiomyopathy is a disease of heart muscle. In hypertrophic cardiomyopathy the muscles of the heart become tremendously thickened. This disease most often affects the muscles of the left ventricle and septum of the heart. As the muscular walls increase in size, the size of the heart chambers become smaller, which reduces the amount of blood that can flow through the heart. This form of cardiomyopathy is rare in the dog and occurs for unknown reasons.

  • Thromboembolism. A thrombus is a blood clot that develops within the heart or a blood vessel. An embolus is a blood clot that arises in one area of the circulatory system and is transported in the bloodstream to a distant site, where it becomes lodged in a blood vessel. The most common form of this disease in dogs occurs with heartworm disease. When adult heartworms die or are killed, they break apart and float to the lungs where they lodge in the small blood vessels. This causes a severe inflammatory reaction in the lungs and may significantly decrease the functional capacity of the lungs.

  • Arteritis or vasculitis. This is an inflammation of arteries or veins that may be caused by viruses, bacteria, parasites or immune-mediated diseases. This type of inflammation is uncommon in the dog.

  • What Types of Diagnostic Tests Are Used to Evaluate the Cardiovascular System?

  • Auscultation of the heart. Auscultation is listening to the heart through a stethoscope. The valves of the heart make certain sounds as they close, which can be heard through the stethoscope. Diseased valves usually produce abnormal sounds, called murmurs. Arrhythmias may also sometimes be detected on auscultation.

  • Palpation of pulses. Pulses are caused by blood traveling through arteries after contraction of the left ventricle. They represent the beat of the heart and provide information on the regularity and strength of the beat.

  • Evaluation of mucous membrane color and capillary refill time. The color of the gums of the mouth and the time it takes for color to return after pressure is applied to the gums both provide information on the circulation of blood to the body tissues. If oxygen levels are low in the blood, then the gums appear blue or cyanotic. If the blood is anemic, the gums may appear pale. If carbon monoxide levels are high in the blood, the gums are usually very bright red. If the gums take more than two seconds to have their color return (prolonged capillary refill time), then circulation to the tissues is poor.

  • Electrocardiogram (ECG). This is the graphic recording of electrical currents generated by the heart to study the action of heart muscle. The electrocardiogram can be performed awake in most dogs. It provides information on the size of the heart chambers, the regularity and speed of the heartbeat, and defines the type of any arrhythmia present.

  • Thoracic radiography. Chest X-rays allow the silhouette of the heart to be examined. They provide information on the size and contour of the heart, the size of the chambers of the heart, and the blood vessels around the heart. Chest X-rays also provide important information about the lungs, which are often affected by heart disease.

  • Echocardiography. Echocardiography is an ultrasound study of the heart. The position and motion of the heart, heart valves and chambers of the heart are measured by the echo obtained from ultrasonic waves. The dynamics of blood flow within and around the heart can also be studied with a form of echocardiography, called Doppler color flow ultrasonography. Echocardiography is also helpful to detect pericardial effusion (fluid around the heart), tumors of the heart, and thrombi within the heart. Heartworms are also sometimes detected on echocardiography.

  • Certain laboratory tests. Laboratory tests are often performed to assess various functions of the body and circulatory system. A complete blood count, biochemistry organ profile, and urinalysis may detect abnormalities such as anemia, kidney disease, and chemical imbalances. Blood may be submitted for heartworm tests. The measurement of oxygen and carbon dioxide (also known as blood gas analysis) may be performed in blood samples from both arteries and veins. Tests for certain infectious diseases may also be indicated. If bacterial infections of the blood stream are suspected, then samples of blood may be taken for culture.

  • Cardiac catheterization and angiocardiography. With this procedure a catheter is inserted into a vein or artery and guided into the interior of the heart. A dye is injected into the catheter that shows up white on X-rays. A video X-ray is acquired as the dye travels through the different chambers and vessels of the heart. Cardiac catheterization with angiocardiography is uncommonly performed in the dog, and has been replaced in large part by echocardiography. It is most often used to detect certain congenital heart defects.

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