12. Which of the following is NOT a stimulus for breathing?  

4. Tracheal obstruction is life-threatening.

41. The placenta, a vitally important metabolic organ, is made up of a contribution from mother and fetus. Which portion is from the fetus?        

37. The result of polyspermy in humans is ________.        

34. A pregnant woman urinates more often than usual because the uterus compresses the bladder, her metabolic rate and blood volume are increased, and she must also dispose of fetal metabolic wastes.

21. Erection of the penis results from ________.        

10. The dartos and cremaster muscles are important to the integrity of the male reproductive system. Which of the following best describes the role they play?        

40. The loading and unloading of O2 are facilitated by a change in the shape of red blood cells (RBC).

46. Which of the following is NOT a reason for O2 delivery?

25. The basic difference between spermatogenesis and oogenesis is that ________.