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Breathing and Exchange of Gases looks easy until the numbers start: four volumes, five capacities and a table of partial pressures. Most questions test whether you can add the right volumes or read a gradient the right way. This post works through both with NCERT's own values, explains oxygen and CO2 transport, and flags two misconceptions that research has found in large numbers of students.
What does the syllabus include?
The NEET (UG)-2026 syllabus lists the human respiratory system, the mechanism and regulation of breathing, exchange and transport of gases, respiratory volumes, and asthma, emphysema and occupational disorders. All of it is in NCERT Class 11 Chapter 14. Check the syllabus for your exam year.
How does breathing work?
Air moves because of pressure differences between the lungs and the atmosphere. In inspiration, the diaphragm contracts and enlarges the thoracic chamber along the antero-posterior axis, and the external intercostal muscles lift the ribs and sternum, enlarging it along the dorso-ventral axis. Lung volume rises, intra-pulmonary pressure falls below atmospheric pressure, and air flows in. In normal expiration, these muscles relax, the volume falls, pressure rises slightly above atmospheric, and air flows out. Abdominal muscles can add force. A healthy person breathes 12 to 16 times a minute.
What are the lung volumes and capacities?
NCERT's four volumes:
Tidal volume (TV): about 500 mL per normal breath.
Inspiratory reserve volume (IRV): 2500 to 3000 mL, the extra you can forcibly breathe in.
Expiratory reserve volume (ERV): 1000 to 1100 mL, the extra you can forcibly breathe out.
Residual volume (RV): 1100 to 1200 mL, left even after a forced expiration.
The capacities are sums: IC = TV + IRV; EC = TV + ERV; FRC = ERV + RV; VC = ERV + TV + IRV; TLC = VC + RV.
Worked example: lung volume sums
Take TV = 500 mL, IRV = 3000 mL, ERV = 1000 mL and RV = 1200 mL, all within NCERT's ranges.
Inspiratory capacity: 500 + 3000 = 3500 mL.
Expiratory capacity: 500 + 1000 = 1500 mL.
Functional residual capacity: 1000 + 1200 = 2200 mL.
Vital capacity: 1000 + 500 + 3000 = 4500 mL.
Total lung capacity: 4500 + 1200 = 5700 mL.
Air per minute: 500 mL × 12 breaths = 6000 mL, and 500 × 16 = 8000 mL, which matches NCERT's 6000 to 8000 mL per minute. Per hour, as NCERT's exercise asks: 6000 × 60 = 360,000 mL, or 360 litres, up to 480 litres at 16 breaths.
How do oxygen and CO2 cross the alveoli?
By simple diffusion down partial-pressure gradients. NCERT's values in mm Hg:
pO2: atmosphere 159, alveoli 104, deoxygenated blood 40, oxygenated blood 95, tissues 40.
pCO2: atmosphere 0.3, alveoli 40, deoxygenated blood 45, oxygenated blood 40, tissues 45.
So the O2 gradient from alveoli to deoxygenated blood is 104 − 40 = 64 mm Hg, while the CO2 gradient from blood to alveoli is only 45 − 40 = 5 mm Hg. CO2 still crosses easily because it is 20 to 25 times more soluble than O2. The diffusion membrane has three layers (alveolar epithelium, capillary endothelium and the basement substance between them) and is much less than a millimetre thick.
How do the blood gases travel?
Oxygen: about 97 per cent bound to haemoglobin in red blood cells and 3 per cent dissolved in plasma. Each haemoglobin carries up to four O2.
CO2: 20 to 25 per cent as carbamino-haemoglobin, about 70 per cent as bicarbonate and about 7 per cent dissolved.
Carbonic anhydrase in red blood cells speeds CO2 + H2O to H2CO3 to HCO3− + H+ at the tissues, and the reverse at the alveoli. Plotting haemoglobin saturation against pO2 gives the sigmoid oxygen dissociation curve. High pCO2, high H+ and higher temperature, as in tissues, favour release of O2.
Each 100 mL of oxygenated blood delivers about 5 mL of O2 to tissues. Combine that with the resting cardiac output of about 5000 mL per minute from NCERT's next chapter: 5000 ÷ 100 × 5 = about 250 mL of O2 per minute at rest.
Which misconceptions trip students up?
Michael and colleagues (1999) surveyed about 700 undergraduates. Nearly 90 per cent held a misconception about the relationship between arterial pO2 and haemoglobin saturation, and 66 per cent believed that raising alveolar pO2 lowers alveolar pCO2. The fixes:
The curve is sigmoid, not a straight line, so saturation is not proportional to pO2.
Each gas diffuses down its own gradient. Raising one does not push the other down.
How is breathing regulated?
The respiratory rhythm centre in the medulla sets the rhythm. The pneumotaxic centre in the pons can shorten inspiration. A chemosensitive area next to the rhythm centre responds to CO2 and H+, as do receptors in the aortic arch and carotid artery. NCERT says oxygen's role in regulating the rhythm is quite insignificant.
The dissociation curve is a shape you need to picture, not just recall. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
A practice routine
Pick new values within NCERT's ranges and recompute all five capacities.
Write the partial-pressure list from memory and compute both gradients.
Sketch the dissociation curve and mark where the alveoli and tissues sit.
Link this chapter to body fluids and circulation, which carries the gases, and to respiration in plants, which uses them.
Today, compute IC, EC, FRC, VC and TLC from a fresh set of volumes without looking at the formulas.
