Wednesday, October 3, 2007
I am not usually one to brag, but I am pretty good at holding my breath. You may have heard that pearl divers can hold their breaths for minutes at a time; my all-time record makes that seem like a fleeting moment. Yes, at one point I did not take a single breath for over 9 months.
Of course, I am being a little gratuitous here. As you may have guessed, I am talking about the time I spent in the womb, when there was no air to fill my immature lungs anyway. The cardiovascular system is at times remarkable in its simple elegance of function, and one place I think that exemplifies this is in the fetal circulation at the heart and lungs. Normally, all of the blood in our bodies must first be pumped by the heart into the lungs to be oxygenated, and then pumped into the rest of our body to distribute that oxygen. In the fetus, as I alluded to a little bit ago, there is no oxygen in the lungs because you are living in the fluid of the amniotic sac. Oxygenated blood must instead come from the mother by way of the umbilical vein. The fetal body has a unique way of separating oxygenated blood from deoxygenated blood to make sure the most vital organs can grow during crucial stages of development.
There are basically two streams of blood inside the heart: blood from the mother enters through the eustacian valve of the inferior vena cava, and goes across the heart and through a temporary valve between the right and left atria, the foramen ovale. This well-oxygenated blood can then be pumped like in an adult, going from the left heart up into the aorta and primarily heading to the oxygen-hungry developing brain via the carotid arteries. This blood also bypasses the lungs, which would only serve to remove oxygen from the blood. The other stream is oxygen poor, comes from the rest of the body into the superior vena cava, and heads down into the right side of the heart. Normally this would then go to the lungs to become oxygenated, but remember that the lungs are don't function yet. Instead, the blood goes through the ductus arteriosus, a structure that closes after birth, and enters the aorta after the carotids to go to the brain. This steers oxygen-poor blood away from the head and into the unbilical artery, returning it to the mother to be reoxygenated.
A major factor in this shunting has to do with the very high resistance of the arteries of the lungs. In the fetus, the lungs provide a huge barrier to blood flow, which means that most of the blood entering the pulmonary trunk with be diverted through the ductus arteriosus, a good thing for reoxygenation. However, at birth, when taking the that first breath, the pulmonary vascular resistance plummets and all the circulating blood is diverted to the normal pattern of entering the lungs before the systemic circulation. This might seem a little paradoxical, because normally oxygen is a potent vasoconstrictor; vessels that have a high oxygen tension will constrict as if saying "I'm fine here, go oxygenate someone else." In the fetus, the lung vascular expansion is in part due to the mechanical strain of inhalation, but also due to vasodilation mediated by oxygen. It is thought that this is in fact due to oxygen-sensitive potassium channels: fetal pulmonary vasoconstriction may be mediated by inhibiting calcium-sensitive potassium channels. Likewise, the ductus arteriosus is kept open by circulating prostaglandin E2, generated due to the relatively hypoxic, or low oxygen, state. When the newborn begins breathing on its own, this effect will stop (as long as it isn't premature) and the ductus will close.
That shift of blood flow will normally mean the end of the fetal circulation: a large return of blood from the lungs will close the valve to the foramen ovale, and the ductus arteriosus will constrict into a ligament, the end result being that the right side of the heart pumps oxygen poor blood to the lungs and that reoxygenated blood is then returned to and pumped out from the left side of the heart to the body. So, given that I had an impressive bypass tract to leach oxygen from my mom, perhaps I was cheating a little when I held my breath all that time in the womb; nonetheless, with such an elegant fetal circulation, I remain impressed.