Many of the cardiac conditions diagnosed in animals are also diagnosed in people – that’s because our cardiac anatomy is very similar. We have highlighted the diseases by congenital and acquired causes, as well as the irregular heart rhythms (arrhythmias) that are commonly diagnosed in animals.
In order to understand the diseases and conditions discussed, a simple understanding of the anatomy and the physiology of the heart is necessary. Although you may have forgotten much from biology class, many of these terms will likely be familiar.
The best way to think of blood flow through the heart is to picture a red blood cell that is coming from the lungs with lots of oxygen. This blood cell will enter the left atrium from the pulmonary veins (the blood vessels that carry blood from the lungs to the heart). This red blood cell will stay in the left atrium (which is essentially a storage chamber) until the mitral valve opens. This allows blood to fill the left ventricle, which is the pumping chamber of the heart. Once the left ventricle is filled, the mitral valve will close and seal. This prevents any back-flow of blood across the mitral valve. Once the left ventricle begins to contract (squeeze), the aortic valve will open and the oxygenated red blood cell will travel out to the body through the aorta.
After blood has circulated to the tissues of the body delivering oxygen and essential nutrients, blood returns to the heart into the right atrium. The main blood vessels that connect to the right atrium are called the vena cava (in veterinary species, we call them the cranial and caudal vena cava). The right atrium is like the left and acts as a storage chamber. Once the tricuspid valve opens, blood flows into the right ventricle. When the right ventricle is filled, the tricuspid valve closes (and seals) and the ventricle begins to contract, pushing blood across the open pulmonic valve into the pulmonary artery. The pulmonary arteries will bring deoxygenated blood back to the lungs to pick up new oxygen and the circuit repeats itself. It is important to note that in a normal heart, there is no mixing of blood from the left and right side.
It is also important to note that the left side is dominant (larger) as it does more work – it needs to pump blood all over the body while the right side only needs to pump blood to one place – the lungs. Generally the right side of the heart will be smaller with a lower pressure in animals.