好似冇documented,不過唔奇,因為fetusBP低,用valve regulate BP 唔奇。
補充樓上,nasal circulation artery and vein 有valve,這是1980's 香港大學生理系Dr. James CC Wang 發現的。
2012-07-25 09:56:01 補充:
Valve is use to regulate blood circulation in low BP environment, the nasal circulation valve system explain why the "block" nose behave the way it is.