Which one among the following is the correct nature of the substance filling up the pleural cavity, the space between the lungs and the chest ?

Correct — A, (a) Liquid. The pleural cavity holds a thin film of watery fluid, and it is fluid rather than air or fat that makes breathing possible. The lung is wrapped in a double membrane. The VISCERAL pleura clings to the lung's own surface; the PARIETAL pleura lines the inside of the chest wall and the upper surface of the diaphragm. The gap between the two layers is the pleural cavity, and in a healthy adult it is not really a space at all — the two membranes lie almost in contact, separated by only a few millilitres of serous pleural fluid, on the order of ten to twenty millilitres in all. That small quantity does two jobs. It LUBRICATES, so that the two surfaces slide over one another without friction through some twelve to twenty breaths a minute for a lifetime. And it COUPLES the lung to the chest wall by surface tension, exactly as a film of water between two glass plates lets them slide but resists being pulled apart. That coupling is what makes breathing work: the diaphragm and the rib muscles move the chest wall, and the lung, which has no muscle of its own, is dragged along with it and expands. The pressure in the cavity is slightly BELOW atmospheric, which is why an injury that lets air in is serious. Air in the pleural cavity — a pneumothorax — breaks the coupling and the elastic lung springs back and collapses, even though the chest wall is still moving. Too much fluid, a pleural effusion, is equally disabling because it presses the lung shut. Both conditions are the clinical proof that the normal content is a small amount of liquid and nothing else.

Why the others are wrong

• (b)Dry air — Air in the pleural cavity is a medical emergency, not the normal state. It is called a pneumothorax and the lung on that side collapses, because the film of fluid that held the lung against the moving chest wall has been replaced by a compressible gas. The confusion is understandable — the lung is full of air — but the air belongs INSIDE the airways and alveoli, not in the space around the lung.
• (c)Humid air — The same error as (b) with a detail added, and the detail is what makes it tempting: air inside the respiratory tract genuinely is warmed and saturated with water vapour by the time it reaches the alveoli. That is true of the airway, not of the pleural cavity, which lies outside the lung altogether.
• (d)Fatty muscle — There is no muscle of any kind in the pleural cavity, and 'fatty muscle' is not a tissue type. The muscles of breathing lie outside the cavity — the diaphragm below and the intercostals between the ribs — and they work on the lung indirectly, through the very fluid coupling this question is about. Fat does pad other structures in the chest, such as the mediastinum, but not the pleural space.

Breathing is a mechanical system in which the lung is a passive elastic bag. Contraction of the diaphragm and the external intercostal muscles enlarges the thoracic cavity; because the pleural fluid holds the visceral and parietal layers together, the lung must enlarge with it, pressure inside the alveoli falls below atmospheric, and air flows in. Relaxation reverses the process and the lung's own elastic recoil drives air out, so quiet expiration needs no muscular effort. The intrapleural pressure stays below atmospheric throughout the cycle. Serous membranes of the same design surround other moving organs — the pericardium around the heart and the peritoneum in the abdomen — each with its own thin film of lubricating fluid.

Key facts

• The pleural cavity is the space between the visceral pleura on the lung and the parietal pleura lining the chest wall.
• It contains a thin film of serous pleural fluid — of the order of ten to twenty millilitres in a healthy adult.
• The fluid lubricates the two membranes and couples the lung to the chest wall by surface tension.
• Intrapleural pressure is normally slightly below atmospheric pressure.
• Air entering the cavity causes a pneumothorax and collapse of the lung; excess fluid is a pleural effusion.
• The lung contains no muscle; inspiration is driven by the diaphragm and the intercostal muscles.
• Quiet expiration is passive, driven by the elastic recoil of the lungs and chest wall.
• The pericardium around the heart and the peritoneum in the abdomen are serous membranes of the same design.

Common traps

• Confusing the air inside the lungs with the contents of the space around them.
• Assuming a 'cavity' must be filled with gas; several body cavities hold only a film of fluid.
• Thinking the lung expands by its own muscular effort.
• Reading the stem's gloss too quickly — 'the space between the lungs and the chest' is outside the lung, not inside it.

No directly related past PYQ was found.

• practice — not a real PYQ

  The presence of air in the pleural cavity, causing collapse of the lung, is known as which one among the following ?

  • (a)Pleural effusion
  • (b)Pneumothorax
  • (c)Emphysema
  • (d)Atelectasis of the bronchus

  Answer(b) Pneumothorax