Thorax
Diaphragm
How to spot it. A broad domed sheet of muscle separating the chest from the abdomen.
What it does. The principal muscle of breathing: contracting flattens it, enlarging the chest and drawing air in.
Where it sits. Pierced by the aorta, oesophagus and inferior vena cava.
NEET points
Class 11 · Unit V · Human Physiology · Breathing and Exchange of Gases
- Inspiration: the diaphragm contracts and flattens while the external intercostals lift the ribs and sternum. Thoracic volume rises, intrapulmonary pressure falls below atmospheric, and air moves in.
- Expiration at rest is passive — the diaphragm and external intercostals simply relax.
- NCERT volumes worth memorising: tidal volume about 500 mL, inspiratory reserve 2500–3000 mL, expiratory reserve 1000–1100 mL, residual volume 1100–1200 mL.
- Vital capacity = inspiratory reserve + tidal + expiratory reserve. Residual volume is the one a spirometer cannot measure, so any capacity that includes it cannot be measured directly either.
Chapter numbering moved when NCERT rationalised its books in 2023 and the NTA has revised the NEET syllabus since, so this maps by unit and title. It is a study aid, not a syllabus.