Because the heart and lungs work together to provide oxygenated blood to the whole body, heart problems and respiratory problems often go hand in hand. Heart problems can have an effect on the lungs and vice versa.
The developing heart
The heart of a fetus within the womb and that of a baby outside the womb are different in both structure and function. Both immature and mature hearts pump blood through the baby’s body; however, the source of oxygen supply for blood cells and the route by which blood is pumped around the body are different. In the womb, the oxygen comes from the mother, passes through the placenta and the umbilical cord, and then travels to the fetal heart through a blood vessel called the ductus venosus. Once it reaches the heart, the blood is pumped to the rest of the body. As there is no need for blood to pass through the lungs, the blood is diverted away form the lungs through an artery called the ductus arteriosus. After it has delivered the oxygen, the blood returns to the heart and is pumped back to the umbilical cord and placenta to be re-oxygenated.
A fetus’s blood pressure is low by mature standards and the patterns of blood flow are different from those of a baby’s heart after birth. Since oxygenation takes place outside the fetus’s body, flow to fetal lungs is minimal. Unlike the fetal heart, which is already beating and pumping blood, the lungs will not begin to function until birth. In the womb, the blood vessels in the lungs are constricted, which limits blood flow to them. Much of the blood flow is directed to the brain and to the blood vessels around the heart itself. The heart muscle, or myocardium, which is the engine that powers the pumping action of the heart, is still immature and the extra blood helps it develop the strength it will need to function in the outside world.
A full-term baby’s heart
Once a baby is born, his first breath initiates changes in the function of the heart. Suddenly filling with air, the lungs expand, the blood vessels within the lungs relax or dilate, and blood flow increases, allowing for gas exchange. Blood flow to the umbilical cord and placenta drops to nothing. The ductus arteriosus begins to close rapidly. Under normal circumstances, the ductus arteriosus is completely sealed and replaced with scar tissue in a few weeks, never to open again. At this point, the baby’s heart is functioning and circulating blood in much the same way as an adult’s heart.
With this rapid change from womb to world, premature babies are at risk for several heart complications. Their frequency and severity is related to how premature the baby is.