Final answer:
The placenta acts as an endocrine gland as one of its early functions. It is involved in nutrient and waste exchange, gas exchange, and hormone production, but it cannot completely block all potentially toxic substances and does not change in thickness to facilitate the passage of nutrients and air.
Step-by-step explanation:
When considering the structure and function of the placenta, it is important to note that the correct statement within the provided options is:
B) As one of its early functions, the placenta acts as an endocrine gland.
The placenta is a vital component in fetal development and performs a variety of roles, including nutrition and waste exchange, gas exchange, and hormone production. Contrary to the belief that optimal circulation is achieved when a woman is lying on her back, this position can actually cause the weight of the uterus to compress the inferior vena cava, potentially reducing blood flow to the placenta. Additionally, while the placenta does act as a barrier to some substances, it is not completely effective at keeping out all toxic substances, such as those in cigarette smoke. The placenta does not thin to allow easier passage of air and nutrients; it maintains a structure optimized for the exchange of substances between mother and fetus.