The system that is called the circulatory system of the dog is comprised of the blood, the blood vessels and the heart. The heart of the dog is a four chambered structure that consists of the right and left upper chambers called the atria and the two lower chambers that are called the ventricles. Essentially the heart is a two sided pump, a right side and a left side. They are independently dependent, that is, the upper right chamber or atrium receives the used or deoxygenated blood from the body and them it goes through the right A/V valve or tricuspid valve into the right lower chamber of the heart or the right ventricle and then pumps it to the lungs through the pulmonary valve into the pulmonary artery. Then the left upper chamber or atrium receives the blood from the lungs and then it goes through the left A/V valve called the bicuspid or mitral valve into the lower left chamber of the heart called the left ventricle.
It is then pumped out the aortic valve through the aorta and out to the body. The two sides of the heart are separated by a muscular wall called a septum. The left side of the ventricles is much thicker than the right side due to the fact that the pressure inside the left ventricle is much greater than in the right ventricle and so it has to withstand more pressure and so it is thicker. In a fully developed heart, the blood cannot get from one side of the heart to the other without first going around the circulatory path that all the blood has to follow. The four valves that we have mentioned are in place to keep the blood flowing in only one direction. When the heart or the valves are diseased, blood can leak through the valves and flow backwards, creating more compromise in heart and circulatory function.
We have talked about the heart being a right sided pump and a left sided pump and that they are independently dependent. This means that the one side acts independently of the other, but its function is totally dependent on the function of the other side. For example, if one of the valves of the arteries, like the aortic valve, is leaking and does not seal the opening properly, then blood can flow back into the ventricle and create a flow problem for all the blood that is coming from the lungs or from the body. Over time there are two things that the heart itself tries to do to make up for the inefficient action of the leaking valves. First the heart tries to increase its rate of action in an attempt to increase the volume of blood that is being pumped. Then it tries to increase its stroke volume by filling the chambers more and so it dilates in an effort to increase its volume pumped with each stroke or contraction. These two mechanisms fail in a short time and then the heart ends up failing and the dog develops congestive heart failure or CHF.CHF results in fluid accumulation in the abdominal cavity or in the chest, depending on which side of the circulation is affected by the left or the right side of the heart that is failing.