Breathing and Exchange of Gases

Breathing is the process of exchanging oxygen from the atmosphere with carbon dioxide produced by cells. It first surveys respiratory organs: diffusion through the body surface in sponges and flatworms, tracheal tubes in insects, gills in fishes, and lungs in reptiles, birds and mammals. It then describes the human respiratory system from the external nostrils through the pharynx, larynx, trachea, bronchi and bronchioles to the alveoli, where gases are exchanged, and the mechanism of breathing. Inspiration is active, involving contraction of the diaphragm and external intercostal muscles, while normal expiration is passive. You will learn respiratory volumes and capacities, such as tidal volume (about 500 mL), inspiratory and expiratory reserve volumes, residual volume and vital capacity, measured with a spirometer. The chapter explains how gases are exchanged by simple diffusion along partial pressure gradients, how oxygen is carried by haemoglobin as oxyhaemoglobin with a sigmoid dissociation curve, and how carbon dioxide is transported as bicarbonate and carbamino-haemoglobin. It ends with the regulation of breathing by the medulla and pons, and disorders like asthma, emphysema and occupational diseases.

Detailed Worksheet: Breathing and Exchange of Gases Section A - Definitions (10 marks) 1. Define tidal volume and vital capacity with their normal values. (2 marks) 2. What is residual volume? Why can it not be measured by a spirometer? (2 marks) 3. What is the role of the pneumotaxic centre? (2 marks) 4. What is emphysema? What is its main cause? (2 marks) 5. Why is the oxygen dissociation curve sigmoid? (2 marks) Section B - Calculations and Applications (15 marks) 6. A person has a tidal volume of 500 mL, an inspiratory reserve volume of 2,500 mL, an expiratory reserve volume of 1,000 mL and a residual volume of 1,200 mL. Calculate the vital capacity, the inspiratory capacity, the functional residual capacity and the total lung capacity. (3 marks) 7. A healthy adult breathes 15 times per minute with a tidal volume of 500 mL. Calculate the volume of air breathed in per minute and per hour. If the person breathes 30 times per minute during exercise with a tidal volume of 1,500 mL, calculate the new minute volume. (3 marks) 8. The partial pressure of oxygen is 104 mm Hg in the alveoli and 40 mm Hg in the deoxygenated blood. The partial pressure of carbon dioxide is 40 mm Hg in the alveoli and 45 mm Hg in deoxygenated blood. Calculate the pressure gradients. Why does carbon dioxide diffuse as quickly as oxygen despite a smaller gradient? (3 marks) 9. Every 100 mL of oxygenated blood delivers about 5 mL of oxygen to the tissues. If the cardiac output is 5 litres per minute, calculate the oxygen delivered to the tissues per minute and per hour. (3 marks) 10. About 70% of carbon dioxide is carried as bicarbonate, 20-25% as carbamino-haemoglobin and about 7% dissolved in plasma. If 200 mL of carbon dioxide is transported per minute, calculate the volume carried in each form, taking 23% as carbamino-haemoglobin. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the human respiratory system. (4 marks) 12. Draw a labelled oxygen dissociation curve and show the effect of high carbon dioxide or low pH on it. (3 marks) 13. Draw a diagram showing the exchange of gases at the alveolus and at the tissues, with partial pressures. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain the mechanism of breathing. How do the diaphragm and intercostal muscles change the volume of the thoracic cavity? (5 marks) 15. Explain how oxygen and carbon dioxide are transported in blood. What factors favour the formation and dissociation of oxyhaemoglobin? (5 marks) 16. Explain the regulation of respiration. Why does a person breathe faster at high altitudes and during exercise? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw diagrams and graphs neatly with labels. Show calculations with units.