Answer:
When there is more oxygen in alveolar sac than in blood capillaries, oxygen will diffuse from alveolar sac to blood capillaries.
Step-by-step explanation:
The concept behind this is very simple. Oxygen or carbon dioxide move according to their partial pressure differences at various places. Gases always move from an area of high partial pressure to an area of low partial pressure. Pulmonary artery carries the deoxygenated blood from heart to lungs. Near the alveoli of lungs exchange of gases take place. Oxygen from the inhaled air enters into the blood capillaries and carbon dioxide from the blood capillaries move into the alveolar sac for exhalation.