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Lesson 3: The Circulatory System Assignment: Read Chapter 4 in the Textbook Read and Study the lesson discussion Complete the Lesson 3 worksheet Objectives: After you have completed this lesson, you will be able to List blood components and explain the functions of blood. Identify the basic structures of mammalian and reptilian hearts. Trace the flow of blood through the heart and body, identifying the parts of blood vessels and their structural significance. Use knowledge of the heart function and control to explain the clinical significance of the electrocardiogram; heart sounds, including heart murmurs; and blood pressure. Discuss the clinical significance of the academic material in this lesson. Identify common animal circulatory system disorders. The Circulatory System In an article titled, "Cardiovascular System: The Heart and Vessels," found at PetEducation.com, doctors Foster and Smith write: Throughout history, people have believed the heart plays a vital role in the body. The ancients supposed it was the seat of the spirit, the center of happiness, and in control of both the emotions and the intellect. Even today, we place the heart at the root of our emotions when we speak of being heartbroken or brave at heart. It is true that the heart plays an essential, life- giving role in the animal's body, but the mystery of what function it actually performs has been solved. The heart is the pump that drives the cardiovascular (circulatory) system. By circulating blood throughout the body, the cardiovascular system functions to supply the tissues with oxygen and nutrients, while removing carbon dioxide and other metabolic wastes. As oxygen-rich blood from the heart flows to the tissues of the body, oxygen and other chemicals move out of the blood and into the fluid surrounding the cells of the body's tissues. Waste products and carbon dioxide move into the blood to be carried away. As blood circulates through organs such as the liver and kidneys, some of these waste products are removed. Blood then returns to the lungs, receives a fresh dose of oxygen, and gives off carbon dioxide. Then, the cycle repeats itself. This process of circulation is necessary for continued life of the cells, tissues, and ultimately the whole organism. Up and down the evolutionary ladder, there are different forms of cardiovascular systems with different levels of efficiency, but they all perform this same basic function. Heart Anatomy In the article titled "Cardiovascular System: The Heart and Vessels," found at PetEducation.com, doctors Foster and Smith write: The cardiovascular systems of mammals, birds, amphibians, reptiles, and fish are all slightly different. The following is an overview of the main components of the mammalian system—the heart and blood vessels. The heart is composed of cardiac muscle that differs slightly from the skeletal and smooth muscle found elsewhere in the body. This special type of muscle adjusts the rate of muscular contraction, allowing the heart to maintain a regular pumping rhythm. The main parts of the heart are the chambers, the valves, and the electrical nodes. There are two different types of heart chambers. The first is the atrium (plural is atria), which receives blood returning to the heart through the veins. The right atrium pumps blood to the right ventricle, and then the left atrium pumps blood into the left ventricle. This blood is then pumped from the atrium into the second chamber, called the ventricle. The ventricles are much larger than the atria and their thick, muscular walls are used to forcefully pump the blood from the heart to the body and lungs. The valves found within the heart are situated between the atria and ventricles, and also between the ventricles and major arteries. These valves are opened and closed by pressure changes within the chambers, and act as a barrier to prevent backflow of blood. The characteristic "lub-dub, lub-dub" heart sounds heard through the stethoscope are the result of vibrations caused by the closing of the respective valves. There are two different electrical nodes, or groups of specialized cells, located in the cardiac tissue. The first is the sinoatrial (SA) node, commonly called the pacemaker. The pacemaker is embedded in the wall of the right atrium. This small patch of tissue experiences rhythmic excitation and the impulse rapidly spreads throughout the atria, causing a muscular contraction and the pumping of blood from the atria to the ventricles. The other node, the atrioventricular (AV) node, relays the impulse of the SA nodes to the ventricles. It delays the impulse to prevent the ventricles from contracting at the same time as the atria, thus giving them time to fill with blood. The cycle of contraction of the heart muscle is called a heartbeat, the rate of which varies greatly between organisms. A general rule of thumb is the smaller the animal, the more elevated the heart (pulse) rate. A vessel is a hollow tube fortransporting something, like a garden hose transports water. A blood vessel is a hollow tube for transporting blood. There are three main types of blood vessels: arteries, capillaries, and veins. These main blood vessels function to transport blood through the entire body and exchange oxygen and nutrients for carbon dioxide and wastes. The arteries carry blood away from the heart, and are under high pressure from the pumping of the heart. To maintain their structure under this pressure, they have thick, elastic walls to allow stretch and recoil. The large pulmonary artery carries unoxygenated blood from the right ventricles to the lung, where it gives off carbon dioxide and receives oxygen. The aorta is the largest artery. It carries oxygenated blood from the left ventricle to the body. The arteries branch and eventually lead to capillary beds. The capillaries make up a network of tiny vessels with extremely thin, highly permeable walls. They are present in all of the major tissues of the body and function in the exchange of gases, nutrients, and fluids between the blood, body tissues, and alveoli of the lungs. At the opposite side of the capillary beds, the capillaries merge to form veins, which return the blood back to the heart. The veins are under much less pressure than the arteries and therefore have much thinner walls. The veins also contain one-way valves in order to prevent the blood from flowing the wrong direction in the absence of pressure. The pulmonary vein returns oxygenated blood from the lungs to the left atria. The vena cava returns blood from the body to the right atria. The blood that is returned to the heart is then recycled through the cardiovascular system. Mammalian and avian (bird) hearts have four chambers—two atria and two ventricles. This is the most efficient system, as deoxygenated and oxygenated bloods are not mixed. The right atrium receives deoxygenated blood from the body through both the inferior and superior vena cava. The blood then passes to the right ventricle to be pumped through the pulmonary arteries to the lungs, where it becomes oxygenated. It returns to the left atrium via the pulmonary veins. This oxygen-rich blood is then passed to the left ventricle and pumped through the aorta to the rest of the body. The aorta is the largest artery and has an enormous amount of stretch and elasticity to withstand the pressure created by the pumping ventricle. The four-chambered heart ensures that the tissues of the body are supplied with oxygen-saturated blood to facilitate sustained muscle movement. Also, the larger oxygen supply allows these warm-blooded organisms to achieve thermoregulation (body temperature maintenance.) Reptiles and amphibians are occasional visitors to the veterinary office. These two types of animals have a three-chambered heart (which consists of two atria and one ventricle.) Drs. Foster and Smith comment on these unique three-chambered hearts: Blood leaving the ventricle passes into one of two vessels. It either travels through the pulmonary arteries leading to the lungs or through a forked aorta leading to the rest of the body. Oxygenated blood returning to the heart from the lungs through the pulmonary vein passes into the left atrium, while deoxygenated blood returning from the body through the sinus venosus passes into the right atrium. Both atria empty into the single ventricle, mixing the oxygen-rich blood returning from the lungs with the oxygen-depleted blood from the body tissues. While this system assures that some blood always passes to the lungs and then back to the heart, the mixing of blood in the single ventricle means that organs are not getting blood saturated with oxygen. This is not as efficient as a four-chambered system, which keeps the two circuits separate, but it is sufficient for these cold-blooded organisms. The heart rate of amphibians and reptiles is very dependent upon temperature … the higher the temperature, the faster the heart rate. Fish possess the simplest type of true heart—a two-chambered organ composed of one atrium and one ventricle. A rudimentary valve is located between the two chambers. Blood is pumped from the ventricle through the conus arteriosus to the gills. The conus arteriosus is like the aorta in other species. At the gills, the blood receives oxygen and gets rid of carbon dioxide. Blood then moves on to the organs of the body, where nutrients, gases, and wastes are exchanged. There is no division of the circulation between the gills and the body. That is, the blood travels from the heart to the gills, and then directly to the body before returning to the atrium through the sinus venosus to be circulated again. The heart rates of fish fall within the wide range of 60—240 beats per minute, depending upon species and water temperature. The fish's heart rate will be slower at lower temperatures. Watch this video which identifies the parts of a sheep’s heart. Watch this video which begins with dissection of a sheep’s heart and also contains very good explanations of the parts of the heart and their functions. Now try to identify the parts of a sheep's heart. Common Circulatory System Disorders Another article obtained from PetEducation.com, titled "Mitral Valve Insufficiency: A Cause of Heart Failure" by Race Foster, DVM, and Holly Nash, DVM, MS makes some interesting comparisons between heart disease in people and in dogs: In people, heart disease usually involves the arteries that supply blood to the heart muscle. In some cases they harden, losing their elasticity and the ability to respond to blood pressure differences as they distribute blood to the cardiac muscle. In other cases, possibly due to diet or genetics, the arteries may become obstructed with a plaque that builds up internally on the artery walls. This causes the muscles of the heart to receive less than adequate amounts of blood. Starved for oxygen, the result is usually a heart attack. In dogs, arteriosclerosis (the hardening of the arteries), plaque formation, and heart attacks are all very rare. However, heart disease is very, very common. Canine heart failure simply means that the muscles 'give out.' This is usually caused by one chamber or side of the heart being required to do more than it is physically able to do. It may be that excessive force is required to pump the blood through an area, and, over time, the muscles fail. Also, in some cases, the volume of blood that must be pumped to keep up with other areas of the heart is more than the particular chamber can adapt itself to move, again leading to muscle failure. Unlike a heart attack in humans, cardiac failure in the dog is a slow, insidious process that occurs over months or years. In these cases, once symptoms are noted, they will worsen over time until the animal is placed on treatment, dies, or is 'put to sleep’. Heart failure in older dogs is usually due to problems with the mitral valve of the heart, and occurs most commonly in smaller breeds, such as Poodles, Yorkies, Pugs, Pomeranians, Lhasas, etc. As the dog ages, the mitral valve between the left atrium and left ventricle starts to fail. A heart valve is designed to prevent back flow. Blood can be easily pumped through the valve, but once the more forward chamber is filled, the valve closes to prevent blood from flowing backwards into the atrium. These valves greatly increase the efficiency of the heart as a pump, since the blood only has to be pumped once to keep it moving forward from one chamber to another chamber. No blood is spilling back, which would require additional effort to move it forward. In the case of a mitral valve disorder (referred to as mitral insufficiency), the valve ages and shrinks and thereby fails to completely close off the area on the left side of the heart between the two chambers. The left ventricle is very strong, and with mitral insufficiency, it easily forces a portion of the blood backward into the left atrium with each heartbeat. The pressure of blood within a normally functioning heart is highest in the left ventricle, as it is required to move the blood throughout the entire body. When blood flows backwards into the atrium, it elevates the blood pressure in that chamber and even further back into the lung field. There are additional complications, which increase pressures in the right side of the heart as it attempts to correctly pump blood forward into the lungs for oxygenation. Confronted with this elevated pressure within the lung field, the right side of the heart must work harder in moving the blood into the area. Symptoms: The result of mitral insufficiency is elevated blood pressure within both the lung field and right side of the heart. The symptoms caused by this condition are very predictable. When hypertension (higher than normal blood pressure) occurs within the lungs, fluid actually leaves the blood vessels and leaks into the tissues. The medical term used to describe 'fluid in the lungs' is 'pulmonary edema.' This causes the affected individual to cough in an attempt to clear the lungs of the fluid, just as you would do with a chest cold. The cough that typically occurs after exercise, excitement, or when the animal has first gotten up after sleeping is the first clinical symptom noted with mitral insufficiency. In most cases, this is controlled with diuretics that cause the animal to urinate more and thereby remove excess fluids from his lungs. As the condition progresses, the right side of the heart starts to fail because of its increased work and elevated pressures. At first, the muscles strengthen, thereby increasing the mass and thickness of its walls. Over time, however, even these 'athletic' muscles cannot keep up with the ever-increasing pressures, and they start to fail. During this latter stage of the disorder, the animal will be weaker and tire more easily, [and may] even faint. Treatment: Medications are used to strengthen and coordinate the muscles' contractions and they will help (and possibly eliminate) the clinical signs… . Diuretics (water pills)… can help remove the excess fluid that can accumulate in the lungs. Diets lower in sodium may assist in decreasing the fluid build-up. Limited exercise may be beneficial—however, consult with your veterinarian before starting your dog on an exercise program. Regardless of treatment, anatomical changes continue to occur internally so the medications will only slow down the progression of the disease. Even with therapy, the condition usually worsens over time, and in many cases, finally results in the death of the animal. Prevention: There are few things that can be done to prevent mitral valve insufficiency. Diagnosing the disease earlier in its course is very helpful in slowing the progression. Notify your veterinarian of any signs of heart failure in your dog. Keeping your dog healthy and at his ideal weight can lessen the severity of symptoms if mitral insufficiency does occur. Valves of the heart can be injured by infection; this infection can result from severe dental problems, which allow bacteria from the mouth to enter the bloodstream. Good oral health, then, is also important. Summary Based on the information from the textbook and this discussion, it is obvious that veterinarians must have a wealth of knowledge about blood and the circulatory system in order to properly care for the animals that they see every day. If veterinarians didn't know the specifics about blood and the two circulatory pathways, they simply would not be able to care for our pets effectively. Sources Cited: Kahn, Cynthia, ed. ":Musculoskeletal System Introduction." The Merck Veterinary Manual. Ninth Edition. New Jersey: Merck & Co. Inc. 2006 <http://merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/90100.htm>. Veterinary and Aquatic Services Department, Drs. Foster and Smith, Inc. "Cardiovascular System: The Heart and Vessels." PetEducation.com. 1997—2007. 28 Dec. 2006 <http://www.peteducation.com/article.cfm?cls=17&cat=1848&articleid=2951>.