本题目来源于试卷: Continuity and Change D.7 Reproduction，类别为 IB生物学
[单选题]
Which two hormones are involved :tw1s+r ezz(q :)p)qci3lgye,djmq4 in0 wfx(-gp1:qo nka n;d a positive feedback loop to promote muscle coqw- kgx aod0n np:;(f1ntractions during childbirth?
A. Oxytocin and estrogen
B. Estrogen and progesterone
C. Human chorionic gonadotropin and oxytocin
D. Progesterone and oxytocin
参考答案: A
本题详细解析:
During the last third of pregnancy, progesterone levels fall, resulting in the rise of oxytocin levels. There is also a rise in estrogen levels which increases the number of oxytocin receptors on the muscle cells in the uterine wall. When oxytocin binds to these receptors, the muscle contracts, and the contraction results in the secretion of more oxytocin. This is positive feedback. Choice A is correct. Choices B and D are incorrect, as high progesterone levels prevent uterine contractions. Choice C is incorrect because human chorionic gonadotropin stimulates the ovary to produce progesterone during early pregnancy and is at lower levels during later pregnancy.
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