899
CHAPTER TWENTY-THREE
Reproductive System
Birth Process
Pregnancy terminates with the
birth process
(parturition).
It is complex. Progesterone plays a major role in its start.
During pregnancy, this hormone suppresses uterine con-
tractions. As the placenta ages, progesterone concentration
in the uterus declines, which stimulates synthesis of a pros-
taglandin that promotes uterine contractions. At the same
time, the cervix begins to thin and then open. Changes in the
cervix may begin a week or two before other signs of labor.
Stretching of the uterine and vaginal tissues late in preg-
nancy also stimulates the birth process. This initiates nerve
impulses to the hypothalamus, which, in turn, signals the
posterior pituitary gland to release the hormone
oxytocin
(see chapter 13, p. 499), which stimulates powerful uterine
contractions. Combined with the greater excitability of the
Placenta
Liver
Lungs
Umbilical vein
(oxygen, nutrients)
Ductus venosus
Inferior
vena cava
Right atrium
Right ventricle
Foramen ovale
Left atrium
Left ventricle
Aortic arch
Aorta
Heart, brain,
upper limbs
Internal iliac
arteries
Umbilical
artery
(carbon dioxide,
wastes)
Umbilical
artery
(carbon dioxide,
wastes)
Pulmonary
trunk
Ductus
arteriosus
(most of the blood)
Trunk and
lower limbs
Superior
vena cava
Decreasing
blood
oxygen
level
FIGURE 23.26
The general pattern of fetal circulation is shown schematically. Flow arrows are shown in gray so as not to be confused with blood
oxygenation.
TABLE
23.4
|
Fetal Cardiovascular Adaptations
Adaptation
Function
Fetal blood
Has greater oxygen-carrying capacity than adult blood
Umbilical
vein
Carries nutrient-rich, oxygenated blood from the placenta
to the fetus
Ductus
venosus
Conducts about half the blood from the umbilical vein
directly to the inferior vena cava, bypassing the liver
Foramen
ovale
Conveys a large proportion of the blood entering the
right atrium from the inferior vena cava, through the atrial
septum, and into the left atrium, bypassing the lungs
Ductus
arteriosus
Conducts some blood from the pulmonary trunk to the
aorta, bypassing the lungs
Umbilical
arteries
Carry the blood containing carbon dioxide and wastes from
the internal iliac arteries to the placenta
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