Well, different organisms require different things. A single celled bacteria does not need a lot at all compared to a plant or animal. One cell can support a bacrerium. An animal needs many cells to carry out the functions necessary for it to live.
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Oxytocin causes contractions during pregnancy, it also is the reason why mothers feel "motherly" towards their children after childbirth as there are high levels on oxytocin in the mother's blood.
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