Anatomy and function of the heart and circulatory system in health and disease

animal heart diagram

Anatomy and function of the heart and circulatory system in health and disease

Heartworm disease affects the heart and lungs; therefore, a brief introduction of the anatomy and physiology of this organ system is in order. Other common heart diseases will also be discussed as they apply to an understanding of heart disease in the typical veterinary care facility. The heart should be thought of as a very simple fluid pump. The heart is divided into two halves which function the same with regards to how the heart works as a pump. The right half of the heart pumps blood to the lungs and the left half of the heart pumps blood out the aorta to the rest of the body. Each right and left pump works in two stages; a top stage (atrium), and a bottom stage (ventricle). These two stages are separated by a trap door (valve) which opens to drop the blood from the top stage to the bottom stage to be pumped out. The bottom stage does most of the pumping and the top stage serves to hold blood waiting to be pumped. Blood flows into the top part of the heart while the bottom part is pumping. When the bottom part finishes squeezing the blood into the vessels, the trap door opens between the top and bottom. The blood in the atrium falls, and is followed by additional blood which fills the entire bottom chamber or ventricle. When this chamber is full the ventricle will pump again. Pumping closes the trap door preventing any blood from escaping the way it came in. The blood is forced out the vessels making the chamber empty for the next cycle. This process occurs simultaneously on both the left and right side of the heart. The heart beat sounds are made as the trap doors slam shut when the ventricles start to beat or contract.

Blood delivers oxygen to the tissues of the body. All body cells need oxygen. When the blood cells have delivered their oxygen, they must return to the lungs for more. All blood returns to the heart through the vena cava. The vena cava (cave-like vein) is the vein that collects all the blood from the jugular veins in the neck as well as the lower body veins. The vena cava empties this blood into the right atrium. This blood is swirling around in the right atrium until the right atrium trap door (tricuspid valve) opens to drop the blood into the right ventricle. The tricuspid valve closes as the right ventricle contracts to force the blood out the main pulmonary artery into the lungs. The blood travels through the lungs to eventually surround the grape-like airbags called alveoli. It is here that the blood receives the life-giving oxygen. The oxygenated blood is then collected into larger and larger pulmonary veins. These veins empty the oxygen rich blood into the left atrium. The blood swirls around in the left atrium waiting for the trap door (mitral valve) to open. The blood then drops into the left ventricle and the mitral valve closes preventing blood from escaping back into the lungs when the ventricle contracts. The left ventricle is a very strong muscle and can pump blood at a very high pressure. The left ventricle creates the blood pressure. This pressure is high enough to squirt blood across the room if an artery is cut. The left ventricle pumps its blood out the aorta. The aorta supplies blood to the brain through the carotid arteries which branch from the aorta just as it leaves the heart. Mineral and cholesterol deposits, called plaques, in the carotid arteries may cause strokes in humans when they break free and pass to the brain. Other major branches of the aorta, supply blood to the arms or front legs first. Then, as the aorta turns to go down the center of the animal along the spine, branches of the aorta feed the digestive organs and additional branches feed the kidneys, the colon, reproductive organs and the rear legs. This blood is used by the cells of the body to provide necessary nutrients and oxygen to the tissues. The blood is then returned into the veins of the tissues to eventually be collected by the vena cava which then completes the cycle of blood circulation.

The kidneys act as blood pressure regulators. They filter the blood to remove impurities. The kidneys require very high pressure to function correctly, and if the pressure is not adequate the kidneys will conserve salt in order to cause water retention which results in higher blood pressure. The kidneys can also increase the blood pressure by making the arteries in the body smaller in diameter. This artery constriction is caused by a hormone system controlled by the kidneys. Smaller diameter vessels with the same fluid volume results in higher blood pressure, much like placing your thumb over the end of the water hose causes the water to spray farther. This can make the kidneys the enemy of the heart when the heart is ill. A poorly functioning heart will pump with less pressure to the kidneys, the kidneys compensate by increasing the blood volume through salt resorption while reducing the diameter of the blood vessels. The heart therefore, has more blood to pump through smaller diameter vessels, which makes it harder on the heart. This mechanism complicates heart disease.

The most common cause of heart disease is valve disease. This occurs because of poorly formed valves, old age or valvular infections such as those caused by dental disease. When the mitral valve is not functioning it is easy to see why blood would back flow into the lungs. This back-flow increases the blood pressure in the lungs resulting in congestion and fluid leakage into the alveoli (airsac). The back-flow also reduces the forward flow of blood into the aorta. The kidney gets less blood pressure and begins salt conservation as well as artery constriction which makes it worse on the heart. Because the heart has an 800% reserve, it can function with a bad valve for some time before the signs of congestion are seen. If we identify a heart murmur on physical examination we should evaluate the pet for the need for a low salt diet. Also, medications that block the kidney system from constricting blood vessels have been shown to increase the life of dogs with valve disease. Once congestion occurs in the lungs, drugs are given to make the pet loose this excessive water retention. These drugs are called diuretics and cause the animal to urinate more than normal which removes the excessive fluid congestion from the lungs.

Heartworms in dogs do not actually cause holes or disease of the heart. They actually block the pulmonary artery. This artery carries blood from the right side of the heart to the lungs. Because the heartworms block this blood flow, the right ventricle becomes congested with blood waiting to be pumped. When this congestion is severe it causes increased blood pressure in the vena cava. This high “caval” blood pressure will cause the abdominal organs to leak fluid. The blood cells will not leave the high pressure vessels but the water portion of the blood will leak into the abdomen. This condition of fluid accumulation in the abdomen is called ascites. Ascites is a sign of high blood pressure and right heart failure. The most common cause of ascites in the dog is heartworm disease. The lungs actually sustain the most damage from heartworms due to the irritation of the lining of the pulmonary blood vessels which will be discussed more in the following pages. Heartworm disease is a very prevalent and preventable disease of dogs, cats, ferrets and many other animals, and it is your job to make sure every pet is on heartworm prevention.

Heart disease in animals is different than most heart disease in humans. Humans have cholesterol plaques that form in the carotid arteries. These plaques can break loose to form blood clots which result in strokes. These same plaques form in the coronary arteries. The coronary arteries form a “Crown” of blood vessels that surround the heart. These vessels feed the heart muscle itself. Disease of these vessels is largely due to cholesterol plaques which block blood flow to the heart muscle. Blockage eventually leads to a total loss of blood flow to a part of the heart muscle, resulting in the death of that muscle. The dead muscle cannot contract which results in a “heart attack”. Therefore, heart attacks and strokes are caused by the same culprit, namely cholesterol plaques. These plaques take about 35 years to form and are therefore, not likely to affect our pet animals. Heart disease in humans is related to genetics, poor diet, obesity, and poor exercise habits. Fat animals are not likely to have heart disease. Actually, it is my rule of thumb that “fat animals do not die of heart disease”. If we are treating fat animals for heart disease, perhaps we should reconsider the diagnosis.



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