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A quick look at alex.tan's Blood circulation through the heart writeup reveals that the correct blood flow in a normal person is as follows:

"Anterior and posterior vena cava -> right atrium -> tricuspid valve -> right ventricle -> pulmonary semilunar valve -> pulmonary artery -> lungs -> pulmonary veins -> left atrium -> bicuspid valve -> left ventricle -> aortic semilunar valve -> aorta -> arteries -> body"

Now, while I don't know what most of that means, I do know that in a person with a patent foramen ovale (around 25% of us, apparently), a percentage of a person's blood will bypass the parts in bold by taking the more convenient route through a tiny hole in the heart between the right and left atrium that was supposed to close at birth.

It may be obvious that the blood which takes this route will not have been oxygenated, having missed out its visit to the lungs. What's less obvious, however, is that the lungs have a secondary function in that they filter out air bubbles and small clots in the blood.

While this is not generally considered serious in itself, it leads to an increased risk of the bends for divers and also slightly increases the risk of stroke in those affected.

Interestingly (for me) though, New Scientist magazine reports that the patent foramen ovale could also be responsible for some aura Migraine headaches. Apparently, "7 out of 16 divers who had surgery to close the holes stopped having migraines altogether, and all but one of the other 9 reported that their migraines had become less severe and less frequent"


References
New Scientist issue 14th April 2001
alex.tan's foramen ovale
Or something's Blood circulation through the heart

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