From Fetal Circulation to Pulmonary Circulation
The development of the cardiovascular system begins to develop toward the end of the third week of pregnancy. On the Fetal Circulation ,shunts for the blood in the developing heart serve a very practical need. Initially all of the blood returns to the right atrium. However, because the lungs develop very late, the pulmonary vessels are limited in their capacity and the resulting resistance is high. Thus, the pulmonary circulation system cannot deal with all the blood. For the circulation system to remain balanced nonetheless, there are two shunts that provide shortcuts for most pulmonary circulation system.
The Fetal Circulation:
- Oxygen from the placenta travels to the umbilical vein bringing oxygen and nutrients.
- Some of the blood flows to the hepatic circulation, others bypass the liver and pass through the ductus venosus.
- The blood from the lower parts of the lower parts of the body together with the blood in the ductus venosus flows toward the inferior vena cava.
- Some of the blood goes from the right atrium goes to the right ventricle via the tricuspid valve while others pass the foramen ovale leading to the left atrium.
- From the left atrium, it goes towards the left ventricle, mixing with the poorly oxygenated blood from the lungs and then pumped towards the ascending aorta.
- From th ascending aorta, the blood is pumped to the upper parts of the body like the heart, neck, head and upper limbs.
- Then perfuse to the placenta via the two umbilical arteries.
- Meanwhile the blood that enters the right ventricle together with the poorly oxygenated blood from the head and upper extremities returns to the right side of the heart by the way of the superior vena cave then, passes through the pulmonary artery wherein 10% enters the lungs, most of the blood bypasses the lungs which is then pumped to the ductus arteriosus going to the descending aorta.
- The blood is the pumped and perfused to other parts of the fetus.
- The blood then returns to the placenta via the two umbilical arteries.
Transition after Birth
The infant takes his first breath causing the mechanical expansion of the lungs. In this way the ductus venosus and ductus arteriosus constrict as large amount of oxygen enters the young lungs. With lung inflation, the alveolar oxygen tension increases, causing reversal of the hypoxemia-induced pulmonary vasoconstriction of the fetal circulation. Clamping of the umbilical cord also increases systemic vasucalr resistance and left ventricle pressure, which further closes the ductus venosus. A decrease in pulmonary vascular resistance and an increase in systemic vasuclar resistance causes a left to right shunting and increasing PO2 which further closes the ductus arteriosus.
- Oxygen-depleted blood from the body leaves the systemic circulation when it enters the right heart, more specifically the right atrium through the superior (upper) vena cava and inferior (lower) vena cava.
- The blood is then pumped through the tricuspid valve (or right atrioventricular valve) into the right ventricle
- Blood is then pumped through the semilunar valve and i nto the left and right pulmonary arteries (one for each lung) and travels through the lungs.
- The pulmonary arteries carry out deoxgenated blood to the lungs while the pulmonary veins carry oxygenated blood to the red blood cells where they release carbon dioxide and pick up oxygen during respiration.
- The oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle.
- This blood then enters the left atrium, which pumps it through the bicuspid valve, also called the mitral or left atrioventricular valve, into the left ventricle.
- The blood is then distributed to the body through the systemic circulation before returning to the pulmonary circulation.
As changes in the cardiovascualr system at birth happens, there are some structure of the system do not function as it used to be and there are adult structures are derived from these:
- Foramen Ovale turns into Foramen Ovalis
- Umbilical Vein turns into Ligamentum teres
- Ductus Venosus turns into medial umbilical ligaments
- Umbilical arteries and abdominal ligaments turns into superior vesicular artery (which supplies the bladder)
- Ductus Arteriosum turns into Ligamentum arteriosum
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