Biology · Breathing and Exchange of Gases · NEET
NCERT gives four boundaries. Dorsally (at the back) it is formed by the vertebral column (backbone). Ventrally (in front) by the sternum (breastbone). Laterally (on the sides) by the ribs. On the lower side (floor) by the dome-shaped diaphragm. Learn these in order because NEET has asked which structure forms which wall.
Air-tight means the chamber is sealed and no outside air can leak into the space around the lungs. Because the box is closed, when its volume increases or decreases, that change is fully passed on to the lungs. If it were not sealed, air would sneak in and the volume change would not reach the lungs, so breathing would fail.
The thoracic cavity is the whole chest box holding the lungs, heart and other organs. The pulmonary cavity is the space inside the lungs that holds air. NCERT's key line: 'any change in the volume of the thoracic cavity will be reflected in the lung (pulmonary) cavity.' So thoracic volume change comes first, and lung volume follows.
Lungs have no muscle of their own to expand. NCERT says 'we cannot directly alter the pulmonary volume.' Instead, muscles change the size of the thoracic box (diaphragm and intercostals), and because the box is air-tight, the lungs are forced to follow. This indirect control is the whole reason the air-tight design matters.
The diaphragm is the floor (lower side) of the thoracic chamber. It is a dome-shaped sheet of muscle. When it contracts it flattens and moves down, which increases the up-down (dorso-ventral) volume of the chest. It is a muscle of the wall, but position-wise it is the floor, not the side or back.
The lungs sit in the sealed thoracic box and are held stretched open by a negative (below-atmospheric) intrapleural pressure that pulls outward on the lung walls. Because of this sub-atmospheric pull, the elastic lungs never fully collapse and some air (residual volume) always stays inside — this is a favourite NEET point.
Lungs do not collapse between breaths and some air always remains in the lungs which can never be expelled because
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Dorsal: vertebral column. Ventral: sternum. Lateral: ribs. Lower side (floor): dome-shaped diaphragm.
The dome-shaped diaphragm forms the floor (lower side) of the thoracic chamber.
Because the box is sealed, any change in thoracic volume is fully passed on to the lungs. Since lungs cannot change their own volume, this indirect control is what makes breathing possible.
They change together. NCERT states any change in the volume of the thoracic cavity is reflected in the pulmonary (lung) cavity — increase thoracic volume and lung volume increases too.
