Biology · Breathing and Exchange of Gases · NEET
Both are correct, and NEET has asked it BOTH ways. In 2016 the question stem said 'some air can never be expelled' and the answer was the negative intrapleural pressure pulling on the lung walls. In 2017 the stem said 'do not collapse even after forceful expiration' and the answer was Residual Volume. Read the stem: if it stresses the outward pull or air-tight chamber, pick negative intrapleural pressure; if it stresses the trapped leftover air, pick Residual Volume. They describe the same phenomenon from two angles.
The lungs sit inside the air-tight thoracic chamber, wrapped by two pleural layers with fluid between them. The pressure in this pleural space is slightly LESS than atmospheric pressure (that is why we call it 'negative'). This lower pressure acts like a gentle suction that keeps pulling the lung walls outward, so the lungs stay stretched open and follow the chest wall instead of shrinking on their own.
Because the airways and alveoli would need to be fully squeezed shut to empty completely, but the negative intrapleural pressure keeps them held open. So even after the strongest forced expiration, about 1100-1200 mL of air stays behind. This trapped air is the Residual Volume (RV). It keeps gas exchange going between breaths, so oxygen keeps moving into blood even while you are not actively inhaling.
Expiratory Reserve Volume (ERV, about 1000-1100 mL) is EXTRA air you CAN force out after a normal breath out. Residual Volume (RV, about 1100-1200 mL) is air you can NEVER force out, no matter how hard you try. ERV is movable, RV is trapped. RV is the one that stops collapse; ERV is not.
If air leaks into the pleural space (a condition called pneumothorax), the negative intrapleural pressure is lost. Without that outward suction, the elastic lung tissue recoils and the lung on that side collapses. This proves that it is the negative intrapleural pressure that normally keeps the lung expanded. NEET expects you to know the pressure is the cause, and collapse is what happens when it is gone.
Lungs do not collapse between breaths and some air always remains in the lungs which can never be expelled because
Lungs are made up of air-filled sacs the alveoli. They do not collapse even after forceful expiration, because of:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because negative intrapleural pressure in the air-tight thoracic chamber pulls the lung walls outward, and a trapped Residual Volume of about 1100-1200 mL always keeps the alveoli inflated.
Residual Volume (RV) is the air that stays in the lungs even after the most forceful expiration. Its average value is 1100-1200 mL and it can never be breathed out.
No. Negative intrapleural pressure is the outward pull that keeps lungs stretched open; residual volume is the leftover trapped air. They are two sides of the same reason lungs never collapse.
Residual Volume prevents collapse. Expiratory Reserve Volume, Tidal Volume and Inspiratory Reserve Volume can all be moved out, so they are not the cause.
It is slightly below atmospheric pressure (negative), which is why it acts like a suction that keeps the lungs expanded against the chest wall.
