Summary
Highlights
This lesson covers topic 11.1: Gas Exchange in Humans for Cambridge IGCSE Biology. Key topics include identifying respiratory structures, describing gas exchange surface features, investigating inspired vs. expired air differences, and the impact of physical activity on breathing. For extended study, it also covers the role of respiratory muscles, extended effects of physical activity, and how the respiratory system protects itself from pathogens.
The breathing system moves air in and out of the lungs for oxygen uptake and carbon dioxide release. The lungs are located in the thorax and are controlled by respiratory muscles like the diaphragm and intercostal muscles. Air enters via the mouth/nasal passages, larynx, trachea, then branches into bronchi, bronchioles, and finally reaches the alveoli where gas exchange occurs.
Alveoli are highly efficient for gas diffusion due to their vast number, maximizing surface area. They have an excellent blood supply from surrounding capillaries, maintaining a steep concentration gradient. Their walls are only one cell thick, minimizing diffusion distance. Good ventilation constantly replenishes oxygen and removes carbon dioxide, and a thin film of moisture allows oxygen to dissolve for efficient passage.
An experiment with limewater demonstrates that expired air contains more carbon dioxide. Inspired air has more oxygen and less carbon dioxide compared to expired air, as oxygen is used and carbon dioxide is produced during respiration. Expired air is also saturated with water vapor, unlike inspired air, which varies with atmospheric humidity.
A spirometer can be used to investigate how physical activity affects breathing rate and depth (tidal volume). Both breathing rate and tidal volume increase during exercise to meet the higher oxygen demand of muscle cells. The amplitude of the spirometer trace indicates tidal volume, and frequency indicates breathing rate.
The diaphragm and intercostal muscles are key respiratory muscles. During inhalation, the diaphragm contracts and flattens, and external intercostals contract, expanding the ribcage. This increases lung volume, lowering air pressure, causing air to rush in. During forceful exhalation, the diaphragm relaxes, and internal intercostals contract, decreasing lung volume and forcing air out. Air contains about 21% oxygen and 0.04% carbon dioxide upon inspiration, with expired air having less oxygen and more carbon dioxide, and also more water vapor.
During exercise, both breathing rate and tidal volume increase to satisfy the heightened oxygen requirements of working muscles. Increased metabolic activity leads to carbon dioxide accumulation, which is detected by the brain. This triggers more rapid and forceful contractions of breathing muscles, accelerating the expulsion of carbon dioxide and the intake of oxygen.
The delicate alveoli are protected by specialized goblet cells in the trachea, bronchi, and larger bronchioles that secrete sticky mucus. This mucus traps pathogens and dust. Ciliated cells then continuously move this mucus upwards towards the trachea, where it can be swallowed, preventing harmful substances from reaching the alveoli.