Zamzam is pregnant during the year she is taking human physiology. At each check-up, she expects to be told that her blood pressure has increased. Yet into the final weeks of her pregnancy, her blood pressure is unchanged from its pre-pregnancy value. Why did she expect her blood pressure to increase? Propose some explanations for why it did not. (Hint: Sometimes women adopt healthier habits during pregnancy, there are large blood-filled sinuses in the maternal portion of the placenta, and there are many hormonal effects that are unique to pregnancy.)