Breathing and Exchange of Gases

1. Chapter at a Glance

Oxygen is used by organisms to break down simple molecules like glucose, amino acids and fatty acids to release energy for various activities. Carbon dioxide, which is harmful, is released during these catabolic reactions. So O₂ must be continuously supplied to cells and CO₂ removed. This exchange of O₂ from the atmosphere with CO₂ produced by cells is called breathing, commonly known as respiration. This chapter covers respiratory organs in animals, the human respiratory system, the mechanism of breathing, respiratory volumes and capacities, the exchange and transport of gases, the regulation of breathing, and common disorders. NEET asks about two questions every year, usually on lung volumes, partial pressures, the oxygen dissociation curve and transport of CO₂.

2. Respiratory Organs

Mechanisms of breathing vary among different groups of animals, depending mainly on their habitats and levels of organisation.

3. Human Respiratory System

• A pair of external nostrils opens out above the upper lips. It leads to a nasal chamber through the nasal passage. The nasal chamber opens into the pharynx, a common passage for food and air.
• The pharynx opens through the larynx region into the trachea. The larynx is a cartilaginous box that helps in sound production and is therefore called the sound box.
• During swallowing, the glottis can be covered by a thin elastic cartilaginous flap called the epiglottis, preventing food from entering the larynx.
• The trachea is a straight tube extending up to the mid-thoracic cavity. It divides at the level of the 5th thoracic vertebra into a right and left primary bronchus. Each bronchus undergoes repeated divisions to form secondary and tertiary bronchi and bronchioles, ending in very thin terminal bronchioles.
• The trachea, primary, secondary and tertiary bronchi and initial bronchioles are supported by incomplete cartilaginous rings.
• Each terminal bronchiole gives rise to a number of very thin, irregular-walled, vascularised bag-like structures called alveoli. The branching network of bronchi, bronchioles and alveoli makes up the lungs.
• The lungs are covered by a double-layered pleura with pleural fluid between the layers, which reduces friction on the lung surface. The outer pleural membrane is in close contact with the thoracic lining; the inner membrane is in contact with the lung surface.

The lungs are situated in the thoracic chamber, which is anatomically an air-tight chamber. It is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs, and on the lower side by the dome-shaped diaphragm. Because of this arrangement, any change in the volume of the thoracic cavity is reflected in the lung (pulmonary) cavity. Such an arrangement is essential for breathing, since we cannot directly alter the pulmonary volume.

Steps of respiration: (i) breathing or pulmonary ventilation, by which atmospheric air is drawn in and CO₂-rich alveolar air is released; (ii) diffusion of gases (O₂ and CO₂) across the alveolar membrane; (iii) transport of gases by the blood; (iv) diffusion of O₂ and CO₂ between blood and tissues; (v) utilisation of O₂ by the cells for catabolic reactions, releasing CO₂ (cellular respiration).

4. Mechanism of Breathing

Breathing involves two stages: inspiration, during which atmospheric air is drawn in, and expiration, by which alveolar air is released. Air moves because a pressure gradient is created between the lungs and the atmosphere.

• We can increase the strength of inspiration and expiration with the help of additional muscles in the abdomen.
• On average, a healthy human breathes 12–16 times per minute.
• The volume of air involved in breathing movements can be estimated with a spirometer, which helps in the clinical assessment of pulmonary functions.

5. Respiratory Volumes and Capacities

6. Exchange of Gases

Alveoli are the primary sites of exchange of gases. Exchange also occurs between blood and tissues. O₂ and CO₂ are exchanged at these sites by simple diffusion, based mainly on pressure (concentration) gradients. Solubility of the gases and the thickness of the membranes are also important factors.

• The pressure contributed by an individual gas in a mixture is called its partial pressure (pO₂ for oxygen, pCO₂ for carbon dioxide).
• The solubility of CO₂ is 20–25 times higher than that of O₂, so the amount of CO₂ that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher.
• The diffusion membrane is made of three major layers: the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance between them. Its total thickness is much less than a millimetre.

7. Transport of Gases

Blood is the medium of transport for O₂ and CO₂.

• About 97% of O₂ is transported by RBCs in the blood. The remaining 3% is carried in a dissolved state through the plasma.
• Nearly 20–25% of CO₂ is transported by RBCs (as carbamino-haemoglobin), about 70% is carried as bicarbonate, and about 7% is carried in a dissolved state through plasma.

Transport of oxygen

• Haemoglobin is a red, iron-containing pigment present in the RBCs. O₂ can bind with haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four molecules of O₂.
• Binding of oxygen with haemoglobin is primarily related to the partial pressure of O₂. Partial pressure of CO₂, hydrogen ion concentration and temperature also interfere with this binding.
• A sigmoid curve is obtained when percentage saturation of haemoglobin with O₂ is plotted against pO₂. This curve is called the oxygen dissociation curve, and it is highly useful in studying the effect of factors like pCO₂ and H⁺ concentration on binding of O₂ with haemoglobin.
• In the alveoli — high pO₂, low pCO₂, lower H⁺ concentration and lower temperature — conditions favour the formation of oxyhaemoglobin. In the tissues — low pO₂, high pCO₂, high H⁺ concentration and higher temperature — conditions favour the dissociation of oxygen from oxyhaemoglobin.
• Under normal physiological conditions, every 100 mL of oxygenated blood can deliver around 5 mL of O₂ to the tissues.

The effect of CO₂ and H⁺ on the curve is called the Bohr effect. High pCO₂ and H⁺ (low pH) and high temperature shift the curve to the right (Hb gives up O₂ more easily — good for tissues). Low pCO₂, high pH and low temperature shift it to the left (Hb holds O₂ more tightly — good for loading in lungs).

Transport of carbon dioxide

• CO₂ is carried by haemoglobin as carbamino-haemoglobin (about 20–25%). This binding depends on pCO₂; pO₂ is a major factor that affects it. When pCO₂ is high and pO₂ is low, as in tissues, more CO₂ binds; in the alveoli, where pCO₂ is low and pO₂ is high, CO₂ dissociates from carbamino-haemoglobin.
• RBCs contain a very high concentration of the enzyme carbonic anhydrase; a minute quantity is present in plasma too. It facilitates the reaction in both directions:

• At the tissue site, where pCO₂ is high due to catabolism, CO₂ diffuses into blood (RBCs and plasma) and forms HCO₃⁻ and H⁺. At the alveolar site, where pCO₂ is low, the reaction proceeds in the opposite direction, forming CO₂ and H₂O.
• CO₂ trapped as bicarbonate at the tissue level and transported to the alveoli is released as CO₂. Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to the alveoli.

8. Regulation of Respiration

• Human beings can moderate the respiratory rhythm to suit the demands of the body tissues. This is done by the neural system.
• A specialised centre in the medulla region of the brain called the respiratory rhythm centre is primarily responsible for this regulation.
• Another centre in the pons region, called the pneumotaxic centre, can moderate the functions of the respiratory rhythm centre. Neural signals from it can reduce the duration of inspiration and thereby alter the respiratory rate.
• A chemosensitive area situated adjacent to the rhythm centre is highly sensitive to CO₂ and hydrogen ions. An increase in these substances activates this centre, which signals the rhythm centre to make adjustments so that these substances can be eliminated.
• Receptors associated with the aortic arch and carotid artery also recognise changes in CO₂ and H⁺ concentration and send signals to the rhythm centre for remedial action.
• The role of oxygen in the regulation of respiratory rhythm is quite insignificant.

9. Disorders of the Respiratory System

10. Master Summary Table

11. Mind Map

12. Tips and Tricks

13. Important One-liners

• Insects respire through a network of tracheal tubes.
• The larynx is called the sound box.
• The epiglottis prevents entry of food into the larynx.
• A healthy human breathes 12–16 times a minute.
• The volume of air in breathing is measured with a spirometer.
• CO₂ is 20–25 times more soluble than O₂.
• Each haemoglobin molecule can carry four oxygen molecules.
• Every 100 mL of oxygenated blood delivers about 5 mL of O₂ to tissues.
• Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to alveoli.
• The respiratory rhythm centre is in the medulla oblongata.
• Emphysema reduces the respiratory surface of the lungs.

14. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

15. Quick Revision Summary