Lung

The lungs are a pair of spongy, air-filled organs in the chest that serve as the primary organs of respiration. Their main function is gas exchange: oxygen from inhaled air passes into the bloodstream, and carbon dioxide waste passes out of the blood to be exhaled.

• Alveoli are the tiny air sacs in the lungs where gas exchange occurs. They are lined by thin membranes and surrounded by capillaries; oxygen diffuses across alveolar walls into the blood, and CO2 diffuses from blood into the alveoli to be exhaled.
• Right vs left lung: the right lung has three lobes (upper, middle, lower), while the left lung has two lobes (upper and lower) and a small cardiac notch to accommodate the heart.
• Breathing is driven by the diaphragm muscle and chest muscles: when the diaphragm contracts (moves downward), it creates negative pressure that draws air into the lungs (inhalation); when it relaxes, the lungs recoil and air is pushed out (exhalation).

• Respiratory physiology questions may ask about the exchange of gases - e.g., what happens in the alveoli, or why inhaling air with low oxygen impairs gas exchange (less O2 to diffuse into blood). Remember the role of alveoli in gas exchange.
• An exam may test knowledge of lung structure: for instance, asking which lung has a middle lobe (answer: the right lung) or what membrane surrounds the lungs (the pleura).
• Control of breathing is another angle: a classic point is that rising CO2 levels (and the corresponding drop in blood pH) are the primary drivers stimulating breathing, more so than low O2 levels.