BIRTH 291 



not well denned, and then blood from the right ventricle flows through the 

 lungs. The ductus arteriosus may remain open for a matter of years and 

 may require a ligature. 



The right and left auricles in the fetus are connected by the foramen 

 ovale, and the path of the circulation through the auricles has been a matter 

 of some discussion. Oxygen-laden blood enters the right auricle from the 

 inferior vena cava, and blood carrying carbon dioxide from the upper body 

 and head enters the right auricle by means of the superior vena cava. While 

 there must be some mixing of these two currents within the right auricle, 

 some evidence shows that the oxygenated blood tends to pass through the 

 foramen ovale into the left auricle, thence into the left ventricle, and thus is 

 evenly distributed to the arteries of the head and the body. On the other 

 hand, the carbon-dioxide-laden blood passes directly into the right ventricle 

 and out through the ductus arteriosus, which joins the descending aorta after 

 the branches to the head region are given off. Therefore the head region 

 would get more oxygen, since it receives oxygenated blood directly from the 

 left ventricle, before the blood becomes mixed with the carbon-dioxide-laden 

 blood from the right ventricle. The foramen ovale closes by the time of birth 

 by a flaplike valve action. Nonclosure produces blue babies. Partial closure 

 is fairly common with no deleterious effects. 



Muscular contractions of the uterus continue after birth, and the placenta 

 is expelled. The separation of the placenta from the maternal circulation is 

 an event with profound effects. The supply of progesterone, estrone, and 

 gonadotropic hormone provided by the placenta and circulating in the ma- 

 ternal blood is cut off abruptly. The chorionic gonadotropic hormone of the 

 placenta normally functions during pregnancy to maintain the corpus luteum 

 in the ovary. The concentration of this hormone is very high during the first 

 few months of pregnancy, and then drops off. Thus during this time the 

 hormone insures a supply of progesterone and some estrone from the corpus 

 luteum, which is necessary for the pregnant state of the uterus. Later in 

 pregnancy the placenta secretes enough progesterone and estrone so that the 

 uterus is no longer dependent upon the corpus luteum. In any case, loss of 

 chorionic gonadotropic hormone would be expected to result in a regression 

 of the corpus luteum and a resumption of the normal ovarian cycle. 



The estrogens of the placenta appear to inhibit the pituitary secretions 

 during pregnancy and together with progesterone they maintain the large 

 uterus and cause growth of the mammary glands. The withdrawal of these 



