In order to understand how this disease may affect your pet, it is important to understand how blood normally travels through the heart. As blood drains from the body back to the heart, it first enters the upper chamber on the right side of the heart referred to as the right atrium. Following the contraction of the right atrium, blood flows through the tricuspid valve and into the lower right heart chamber, called the right ventricle. Following a right ventricular contraction, blood is pumped into the pulmonary artery, which carries blood to the lungs.
In the lungs, there is an exchange of gases due to respiration, in which oxygen (O2) is incorporated into the bloodstream and the waste gas carbon dioxide (CO2) is released. Following circulation through the lungs, oxygenated blood then drains through the pulmonary veins and into the left side of the heart. As this occurs, blood enters the left atrium and is pumped through the mitral valve and into the left ventricle. Left ventricular contraction results in the propulsion of oxygenated blood into the aorta, which ultimately carries blood back to body.
In a developing fetus, the ductus arteriosus functions to shunt blood from the pulmonary artery to the aorta, bypassing non-functional lung tissue. Soon after birth, the lungs expand as breathing begins and the ductus arteriosus should close, allowing blood to flow through the now functional lung tissue. In some patients, the ductus remains open, or ‘patent.’ As a result, communication remains between the aorta and the pulmonary artery, allowing increased blood flow between these vessels, through the lungs, and ultimately back to the left side of the heart.