Red blood cells pick up oxygen in the lungs. Blood travels away from the heart and lungs through the arteries. Red blood cells drop off oxygen to the cells through tiny tubes called capillaries. Blood then returns to the heart through the veins and the cycle begins again.
Answer: So they can fertilize the egg.
Explanation:
The first trimester is when the embryo attaches to the placenta by the umbilical cord and the placenta make contact with the uterus.
