Today’s topic will begin our conversation about heart disease in dogs and cats. I need to get some basics out there so that everyone understands what we’re dealing with. This is the introduction to the heart of the dog and cat.
The canine and feline hearts are essentially the same as the human heart (along with most all mammals). The basic pathway for blood to circulate in the body is a set of two loops. The heart has a left and right side, each responsible for one of those loops. The right loop takes blood back from other areas of the body through veins and pumps it to the lungs, where it is filled with oxygen and then returned to the heart. The left side of the heart takes blood from the lungs and pumps it out to the rest of the body through arteries.
Each side of the heart has 2 chambers: atrium and ventricle. There’s a valve between the atrium and ventricle on each side that prevents blood from flowing backwards through the circuit. The atria are smaller and have thinner walls. Their job is to push extra blood into the second chamber. The second (lower) chamber on each side is called the ventricle. The left and right ventricle are quite different from one another. The right ventricle doesn’t have to pump blood very far — just to the lungs — so it’s got thinner muscle walls and is a bit weaker. The left side is much stronger, with thicker muscle, because it pumps blood out to the whole body. We say that the left side of the heart is a high-pressure loop, while the right is a low-pressure loop. Each ventricle pumps blood out through another valve (aortic or pulmonic) that stops blood from flowing backwards. There is a single major blood vessel that takes blood from each ventricle and begins the pathway to the lungs or body.
The images below labels all of the parts of the heart and show the route of blood flow. You’re looking at the heart as if you were staring at the chest of someone facing you. Your left side is actually the right side of the heart, and your right side is actually the left side of the heart. Because the right side of the heart handles low-oxygen blood, it’s colored blue. The left side handles oxygen-rich blood, so it’s colored red. Arteries carry blood away from the heart and veins carry blood back to the heart. In general, arteries carry high-oxygen blood and veins carry low-oxygen blood. There are two exceptions to that rule that I’ll explain in a moment.
While nearly every artery and vein has a specific name, there are a few that attach to the heart that are really important. When blood is returning to the heart from the body, it arrives in the cranial(superior) and caudal(inferior) vena cava. The blood then goes into the right atrium. The atrium fills up, then contracts, which pushes blood through the tricuspid valve into the right ventricle. Once the right ventricle is full of blood, it contracts. The tricuspid valve closes, which stops blood from going back to the atrium and forces it to go through the semilunar (pulmonic) valve into the pulmonary arteries. These arteries are carrying low-oxygen blood out to the lungs. That’s the first exception to the above rule. In the lungs, the blood gives up carbon dioxide and picks up oxygen. High-xygen blood then returns to the heart in the pulmonary veins. That’s the second exception. The left atrium fills, then contracts. Blood is pushed through the mitral valve into the left ventricle. Once the left ventricle is full, it contracts. This closes the mitral valve and forces blood out of the aortic semilunar valve into the aorta. The aorta is that big arching red vessel that comes out of the top of the heart. It then runs down along the spine, giving off arteries all over the place to feed blood to all of the distant part of your body. Blood gives up oxygen to the body tissues, picks up carbon dioxide, and then makes its way back to the right side of the heart by way of the veins. The image below shows the flow of blood through the heart. If you start with the blue arrow going to the heart from upper or lower body, you’ll start the journey as I just described it.
Now that you’ve got an idea about how blood flows in the body, we can talk a little about how the heart does what it does. The heart is a big muscle made up of a special kind of muscle fiber. Cardiac muscle cells are all linked together in a special web that passes on the signals to contract from cell to cell. The heart has a special group of cells called the sinoatrial node. The SA node is also known as the pacemaker. It sets the rate at which the heart beats, and the signals it passes along to the rest of the heart are carried by a special network of fibers. The way the signals move from place to place in the heart causes the chambers of the heart to beat in a very specific order.
When the heart is beating as it’s supposed to, it generates some noise. The heartbeat sounds are from the atrioventricular (mitral and tricuspid) and semilunar (aortic and pulmonic) valves closing. They snap closed in a way that reminds me of shaking out a big towel or sheet.
We have a way to tell what order the heart chambers are contracting in, and whether the electrical signals are normal. It’s called an EKG (electrocardiogram. This is the little spike graph that everyone is familiar with – it tells us exactly what order the heart’s nerves are conducting impulses and how the heart muscle is responding.
That should give us enough of a start on the structure function of the heart. Thanks for slogging through another long anatomy and physiology lesson! Next week, we’ll start talking about the things that go wrong in various types of cardiac disease. I intend to start with cats, then move on to dogs.