During labor, the placenta and uterus excite the muscles of the uterus to engage in contractions.

Prostaglandins

Explanation:

Prostaglandins are a class of lipids, that are secreted in the event of an injury, tissue damage or contraction. They're said to occur in almost every human organ. Contraction simply put, is when the inner muscles, the uterine begins to tighten and release at intervals during or before child-birth. So prostaglandins in this case helps to cause these contractions to occurr. it does this by also softening the cervix, making it ready for contraction.

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b) the heart is a pump made of muscle and that is difficult to grow in the lab.