Description: Atrial septal defect (ASD) is a form of congenital heart defect that enables blood flow between the left and right atria via the interatrial septum. The interatrial septum is the tissue that divides the right and left atria. Without this septum, or if there is a defect in this septum, it is possible for blood to travel from the left side of the heart to the right side of the heart, or vice versa. Irrespective of interatrial communication bi-directions, this results in the mixing of arterial and venous blood. The mixing of arterial and venous blood may or may not be hemodynamically significant, if even clinically significant. This mixture of blood may or may not result in what is known as a "shunt". The amount of shunting present, if any, dictates hemodynamic significance (see Pathophysiology below). A "right-to-left-shunt" typically poses the more dangerous scenario (see Pathophysiology below). The right side of the heart contains venous blood with a low oxygen content, and the left side of the heart contains arterial blood with a high oxygen content. The construction of a heart void of an ASD prevents interatrial communication by means of an uncompromised interatrial septum. This prevents the atria from regular communication with each other, and thus oxygen-rich blood and oxygen-deficient blood do not mix together improperly. During development of the fetus, the interatrial septum develops to eventually separate the left and right atria. The foramen ovale (pronounced /fɒˈreɪmən oʊˈvɑːli/) remains open during fetal development to allow blood from the venous system to bypass the lungs directly and enter the circulatory system. This is because the oxygen content of the fetal arterial system is provided by the placenta, as the lungs of the fetus are nonfunctional. A layer of tissue begins to cover the foramen ovale during fetal development, in which typically, after birth, the pressure in the pulmonary circulatory system drops, thus causing the foramen ovale to close entirely. In approximately 25% of adults, the foramen ovale does not entirely seal. In this case, elevation of pressure in the pulmonary circulatory system (ie: pulmonary hypertension due to various causes, or transiently during a cough) can cause the foramen ovale to remain open. This is known as a patent foramen ovale (PFO).
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