Biology · Breathing and Exchange of Gases · NEET
The pleura is a DOUBLE-layered membrane. NCERT clearly states the lungs are covered by a double layered pleura, with pleural fluid between them. So there are two membranes (outer and inner) and a fluid-filled space in between. Marking it as single-layered is a common mistake.
The OUTER pleural membrane is in close contact with the thoracic (chest) wall lining, and the INNER pleural membrane is in contact with the lung surface. Easy check: inner is closer to the inside organ (lung), outer is closer to the outside wall (thorax). In advanced terms the inner one is called visceral pleura and the outer one is called parietal pleura, but NCERT itself only uses the words inner and outer.
NCERT gives one clear job: it reduces friction on the lung surface. The lungs expand and shrink thousands of times a day. Without the slippery fluid the two membranes would rub against each other and the chest wall, causing pain and damage. The fluid lets the lung glide smoothly during inspiration and expiration.
The two pleural layers, held together by the thin fluid film, keep the lung stuck to the thoracic wall. So when the thoracic cavity volume changes, the lung is pulled along and its volume changes too. NCERT says any change in the volume of the thoracic cavity is reflected in the lung (pulmonary) cavity. This coupling is why we can breathe, since we cannot directly change lung volume ourselves.
No. Pleural fluid is OUTSIDE the lung, in the space between the two pleural membranes, and its job is to reduce friction. Surfactant is INSIDE the alveoli and lowers surface tension so alveoli do not collapse. NCERT (Class 11) mentions the pleural fluid for friction; do not swap the two roles in an exam.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The pleura is the double-layered membrane that covers each lung, with pleural fluid between its two layers.
It reduces friction on the lung surface, letting the lungs glide smoothly as they expand and contract during breathing.
The inner pleural membrane is in contact with the lung surface; the outer one is in contact with the thoracic wall lining.
In the narrow space between the two layers of the pleura, that is, between the inner (lung side) and outer (chest wall side) membranes.
By linking the lung to the thoracic wall, any change in thoracic cavity volume is passed on to the lungs, which is essential because we cannot change lung volume directly.
