Patent Ductus Arteriosus A Leftto Right Shunt

During fetal life, the lungs are collapsed, and the elastic compression of the lungs that keeps the alveoli collapsed keeps most of the lung blood vessels collapsed as well. Therefore, resistance to blood flow through the lungs is so great that the pulmonary arterial pressure is high in the fetus. Also, because of low resistance to blood flow from the aorta through the large vessels of the placenta, the pressure in the aorta of the fetus is lower than normal—in fact, lower than in the pulmonary artery. This causes almost all the pulmonary arterial blood to flow through a special artery present in the fetus that connects the pulmonary artery with the aorta (Figure 23-4), called the ductus arterio-sus, thus bypassing the lungs. This allows immediate recirculation of the blood through the systemic arteries of the fetus without the blood going through the

Head and upper

Head and upper

Figure 23-4

Patent ductus arteriosus, showing by the intensity of the pink color that dark venous blood changes into oxygenated blood at different points in the circulation. The right-hand diagram shows back-flow of blood from the aorta into the pulmonary artery and then through the lungs for a second time.

Figure 23-4

Patent ductus arteriosus, showing by the intensity of the pink color that dark venous blood changes into oxygenated blood at different points in the circulation. The right-hand diagram shows back-flow of blood from the aorta into the pulmonary artery and then through the lungs for a second time.

lungs. This lack of blood flow through the lungs is not detrimental to the fetus because the blood is oxygenated by the placenta.

Closure of the Ductus Arteriosus After Birth. As soon as a baby is born and begins to breathe, the lungs inflate; not only do the alveoli fill with air, but also the resistance to blood flow through the pulmonary vascular tree decreases tremendously, allowing the pulmonary arterial pressure to fall. Simultaneously, the aortic pressure rises because of sudden cessation of blood flow from the aorta through the placenta. Thus, the pressure in the pulmonary artery falls, while that in the aorta rises. As a result, forward blood flow through the ductus arteriosus ceases suddenly at birth, and in fact, blood begins to flow backward through the ductus from the aorta into the pulmonary artery. This new state of backward blood flow causes the ductus arte-riosus to become occluded within a few hours to a few days in most babies, so that blood flow through the ductus does not persist. The ductus is believed to close because the oxygen concentration of the aortic blood now flowing through it is about twice as high as that of the blood flowing from the pulmonary artery into the ductus during fetal life. The oxygen presumably constricts the muscle in the ductus wall. This is discussed further in Chapter 83.

Unfortunately, in about 1 of every 5500 babies, the ductus does not close, causing the condition known as patent ductus arteriosus, which is shown in Figure 23-4.

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Essentials of Human Physiology

Essentials of Human Physiology

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