Circulatory system Every cell in the dog’s body depends on certain compounds for survival, which have to be supplied permanently. At the same time, metabolites and synthesized substances (i.e. hormones) have to be removed. This exchange is mostly achieved via the circulatory system. Except for the lung circuit, arteries always carry oxygen-rich blood and drive it away from the heart, outwards towards the rest of the body. On their way into the periphery, the parts of the body away from the heart, the vessels branch out and reduce in size and diameter. Large body vessels lead into smaller arterioles and eventually into capillaries where the actual substance exchange with the tissue is taking place. After oxygen has been absorbed into the cells, the now oxygen-poor blood is loaded with carbon dioxide and other metabolites, and is lead through venules and veins back to the heart. From here the cycle starts anew. Heart and circulatory system The heart of the dog is shaped like a pointed cone and - and as is the case in all physically active mammals - is relatively large. The heart is divided into two sections, each containing one atrium (pre-chamber), one ventricle (chamber) and two valves which allow the blood to flow in one direction only. An expulsion cycle, where blood is pushed away from the heart, starts with the contraction of the heart muscle. The contraction of the heart muscle then pushes open the valves of the diverting vessels, and the blood contained inside both ventricles is driven into the circulatory system. Simultaneously with this movement the atria are filled with blood. When the heart muscle subsequently relaxes, blood contained in the atria is sucked into the ventricles, and from this point, the cycle starts anew. The right ventricle then pumps blood into the lungs (small circuit or pulmonary circuit) where carbon dioxide is discarded and oxygen is absorbed. From here the oxygen-rich blood flows into the left atrium and is pumped via the left ventricle into the aorta, from where it is spread throughout the body. After gas is exchanged with the body cells, oxygen-poor blood is returned to the right atrium (large circuit or body circuit). As the resistance of the body circuit is greater than that of the lung circuit, the heart muscle of the left ventricle is better developed. The heart rate of an adult dog is between 60 and 180 beats per minute. More precise figures cannot be given as size, physical condition and mental state of the dog influence the heart rate significantly. The heart rate of a puppy can be as high as 220 beats per minute. Blood The main function of the blood is transport and supply. Oxygen and nutrition can be transported to as well as metabolites, i.e. carbon dioxide and urea, which have to be redirected to the lungs or the liver respectively. The cellular part of the blood consists of red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes). Red blood cells contain hemoglobin, which binds oxygen. White blood cells repel microorganisms and foreign substances, and protect the body from infection. Platelets are part of the intrinsic process of coagulation. When an injury occurs, they seal the damaged or severed vessels, and thus prevent excessive blood loss. The fluid inside blood is called plasma. It consists mainly of water, and also contains a number of compounds produced by the body, primarily proteins and vitamins. The products mentioned above: oxygen, carbon dioxide, urea, fats, sugars, vitamins, and hormones, are found in the blood are moved via the bloodstream between organs and the different tissues of the body. Spleen The spleen of a dog is both integrated into the circulatory system and the lymphatic system. The main function of the spleen is the mechanical filtration of red blood cells, which scans for old and damaged red blood cells and removes them from circulation, if necessary. Also, some blood may be stored inside the spleen, which acts as a sponge that can release the blood on demand. In young dogs the spleen is also where red blood cells are produced. However, this function is mostly performed by the bone marrow later in life. Lymphatic system The lymphatic system is the part of your pet's defense system that protects against infectious agents. Next to the bloodstream, it is also an important method for transportation inside the body, especially for fats. The fluid that facilitates transportation is called lymph. The lymphatic system helps to identify and destroy invading microorganisms, foreign compounds as well as corrupted cells (i.e. cancer, tumor cells). Fats that are digested inside the small intestines are absorbed into the lymphatic system and thus transported into the bloodstream. The lymphatic system begins with tiny vessels, known as lymph capillaries, which end "blindly" in tissue. In contrast to the blood circulatory system, the lymphatic system is not a closed circuit. Lymph fluid is transported in one direction only, towards the heart, and pumping function is achieved through small contracting structures, the lymph hearts, which are spread throughout the system of vessels. The capillaries collect fluid from the intercellular-space, which consists mostly of fluid inside the blood, white blood cells and fats. Thus, lymphatic fluid is created and transported towards collection points, the lymph nodes. From here it reaches the anterior vena cava inside the thorax where the lymph is allowed to drain into the blood. Inside the lymph nodes the lymph is filtered and cleared of metabolites. Furthermore, a certain fraction of white blood cells, the lymphocytes, mature here. Maturation itself is a process in which a lymphatic cell makes contact with a pathogen, i.e. a bacteria, and memorizes its structure. As a result, the immune system can respond more rapidly to invasion of the memorized pathogen. In addition to vessels and lymph nodes, the lymphatic system involves a number of additional organs. These include the spleen, the thymus (which serves as the primary space for maturation of lymphocytes in young individuals before becoming obsolete in older age), and the tonsils (which intercept pathogens from the nasal cavity). Also, lymph nodes are always draining a specific areas of the body. If there is swelling in any one of these areas, it may indicate an infectious process in that region.

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