Breathing and Exchange of Gases: common doubts, answered
The questions students ask most often about Breathing and Exchange of Gases, each with a short answer. For the full chapter, read the Breathing and Exchange of Gases notes.
Human respiratory system
Read this section in the notes →What is the difference between the conducting zone and the exchange zone of the respiratory tract?
The conducting zone runs from the external nostrils up to the terminal bronchioles, and it only carries, filters and warms air without any gas exchange. The exchange zone consists of the alveoli and their ducts, where oxygen and carbon dioxide actually move across the thin membrane between air and blood.
Why does the trachea not collapse?
The trachea is supported by incomplete cartilaginous rings, which keep the tube open for air to pass. The same kind of rings support the bronchi and the initial bronchioles. The rings are incomplete, so food can pass down the oesophagus behind the trachea. The trachea divides into the right and left primary bronchi at the level of the fifth thoracic vertebra.
What does the epiglottis do?
The epiglottis is a thin elastic cartilaginous flap that covers the glottis, the opening of the larynx, during swallowing. This prevents food from entering the windpipe. The larynx itself is the sound box, since it produces sound, and it lies at the upper end of the trachea.
Steps of respiration
Read this section in the notes →What are the steps of respiration in humans?
Respiration involves five steps: breathing, or pulmonary ventilation, which draws in atmospheric air and releases carbon dioxide-rich air; diffusion of oxygen and carbon dioxide across the alveolar membrane; transport of the gases by the blood; diffusion of oxygen and carbon dioxide between the blood and the tissues; and the use of oxygen by cells for catabolic reactions, which releases carbon dioxide and is called cellular respiration.
Mechanism of breathing
Read this section in the notes →How does inspiration take place in humans?
Inspiration occurs when the diaphragm contracts and flattens, increasing the volume of the thoracic chamber in the antero-posterior axis, while the external intercostal muscles lift the ribs and sternum, increasing the volume in the dorso-ventral axis. The lung pressure falls below atmospheric, so air rushes in.
Is expiration active or passive in normal breathing?
In quiet breathing, expiration is passive. The diaphragm and intercostal muscles relax, the thoracic volume decreases and the lung pressure rises above atmospheric pressure, so air flows out. Only forced expiration needs the help of abdominal muscles and internal intercostal muscles.
Respiratory volumes and capacities
Read this section in the notes →What is the difference between vital capacity and total lung capacity?
Vital capacity is the maximum air that can be breathed out after a deepest inspiration, equal to IRV + TV + ERV. Total lung capacity is the volume present in the lungs after the deepest inspiration, equal to vital capacity plus residual volume. So the residual volume is included in TLC but never in vital capacity.
What are tidal volume, inspiratory reserve volume and expiratory reserve volume?
Tidal volume is the air moved in or out during a normal breath, about 500 mL. Inspiratory reserve volume, 2500 to 3000 mL, is the extra air that can be breathed in after a normal inspiration. Expiratory reserve volume, 1000 to 1100 mL, is the extra air that can be forced out after a normal expiration.
What is residual volume and why can't we breathe it out?
Residual volume is the air that remains in the lungs even after a forceful expiration, about 1100 to 1200 mL. It cannot be expelled voluntarily. Functional residual capacity is the sum of expiratory reserve volume and residual volume, which is the air left after a normal expiration.
What is the difference between tidal volume and minute ventilation?
Tidal volume is the amount of air in a single normal breath, about 500 mL. The volume breathed in or out per minute is the product of this and the breathing rate, which is 12 to 16 per minute, so a healthy person handles about 6000 to 8000 mL of air each minute. Hence the second quantity is larger.
Exchange of gases
Read this section in the notes →Why does oxygen move from the alveoli to the blood?
Oxygen moves because of a partial pressure gradient. The pO₂ is 104 mm Hg in the alveoli but only 40 mm Hg in the deoxygenated blood, so oxygen diffuses into the blood. Carbon dioxide moves the opposite way, from blood at 45 mm Hg to the alveoli at 40 mm Hg. Gases diffuse from higher to lower partial pressure.
Why does CO₂ diffuse effectively with such a small pressure gradient?
Carbon dioxide is about 20 to 25 times more soluble in the membrane and fluids than oxygen is. This is why a gradient of only 5 mm Hg, from 45 to 40, is enough for a large amount of carbon dioxide to cross the diffusion membrane. The membrane itself is made of three very thin layers.
Transport of oxygen
Read this section in the notes →How is oxygen carried in the blood?
About 97 per cent of oxygen is carried by red blood cells as oxyhaemoglobin, and the remaining 3 per cent is dissolved in plasma. Each haemoglobin molecule can bind a maximum of four oxygen molecules. The binding depends mainly on the pO₂, and the dissociation curve of oxyhaemoglobin is sigmoid.
What factors favour the formation of oxyhaemoglobin?
High pO₂, low pCO₂, lower H⁺ concentration, that is higher pH, and a lower temperature favour oxyhaemoglobin formation, and these are the conditions in the alveoli. In the tissues the conditions are opposite, so oxygen is released from haemoglobin there.
Transport of carbon dioxide
Read this section in the notes →How is carbon dioxide transported in the blood?
About 70 per cent is carried as bicarbonate, 20 to 25 per cent as carbamino-haemoglobin bound to haemoglobin, and about 7 per cent dissolved in plasma. The enzyme carbonic anhydrase speeds up the conversion between carbon dioxide and bicarbonate. So haemoglobin is not the main carrier of carbon dioxide.
Regulation of respiration
Read this section in the notes →Which part of the brain regulates breathing?
The respiratory rhythm centre in the medulla oblongata is mainly responsible for the rhythm of breathing. The pneumotaxic centre in the pons can moderate the function of the rhythm centre by reducing the duration of inspiration. A chemosensitive area near the rhythm centre senses carbon dioxide and hydrogen ions.
Is low oxygen the main stimulus for breathing?
No. The chemosensitive area responds mostly to increases in carbon dioxide and hydrogen ions, which signal the rhythm centre to adjust breathing. Receptors in the aortic arch and carotid artery also sense these changes. The role of oxygen in regulating the respiratory rhythm is quite insignificant.
Disorders of the respiratory system
Read this section in the notes →What is emphysema?
Emphysema is a chronic disorder in which the alveolar walls are damaged, so the surface area for gas exchange decreases. Cigarette smoking is a major cause. It differs from asthma, which is an inflammation of the bronchi and bronchioles that causes difficulty in breathing with wheezing.
What is occupational respiratory disorder?
It is a lung disorder seen in workers in industries such as grinding or stone-breaking, where much dust is produced. The body's defences cannot clear the dust completely, so long exposure causes inflammation and then fibrosis, or scar tissue, leading to serious lung damage. Using protective masks reduces the risk.
Lumi is not affiliated with or endorsed by NCERT. The official NCERT textbooks are free to read and download from NCERT's own website, ncert.nic.in. These notes and simulations are original work by Lumi (Aikolumi Software Pvt Ltd), © 2026, shared under CC BY-NC 4.0: copy, print, share and adapt them for any non-commercial use, with credit to Lumi and a link to lumineet.com.