Answer:
1) About 8.7 million.
2) They are being found everywhere.
The reproductive system has four functions: To produce egg and sperm cells. To transport and sustain these cells. To nurture the developing offspring.
Answer: Alveoli
Explanation: Alveoli of lungs are surrounded by extensive blood capillaries. The partial pressure of oxygen in the alveoli is higher than the blood entering the pulmonary capillaries, hence oxygen moves into the blood. Carbon dioxide in the lungs simultaneously diffuse out of the blood into the alveolar space.