Biology (0610) Gas Exchange in Humans Revision Note

This document explains gas exchange in humans for CAIE IGCSE Biology, focusing on the structure and function of the lungs and breathing processes. It outlines the properties of efficient gas exchange surfaces: thin walls for short diffusion distance, large surface area provided by alveoli, good ventilation, rich blood supply, and moisture to dissolve gases. The structure of the respiratory system is described, including the diaphragm, ribs, intercostal muscles, trachea, bronchi, bronchioles, and alveoli with capillaries. The role of cartilage in keeping the trachea open is emphasized. Differences between inspired and expired air are highlighted: inspired air has more oxygen and less carbon dioxide than expired air, with water vapour higher in exhaled air. Limewater is used to test for carbon dioxide. The document also explains how physical activity increases breathing rate and volume to meet higher oxygen demand and remove excess CO₂, with spirographs measuring these changes. Breathing mechanisms are compared: inspiration involves diaphragm contraction and ribcage expansion, while expiration involves relaxation of these muscles. Internal intercostals assist in forced breathing like coughing. The function of mucus and cilia in trapping dust and pathogens for removal is also covered.