Key words

• Alveoli - Minute air-sacs with a diameter between 100 and 300 micrometres at the end of the bronchioles. They are lined with epithelium. Between the alveoli, there are some collagen and elastic fibres
• Bronchi - Two divisions of the trachea each leading to one lung. Amount of cartilage reduces as they get smaller. Also produce mucus to trap dirt particles and cilia that move this towards the throat
• Bronchioles - A series of branching subdivisions of the bronchi whose walls are made up of muscle (which constricts to control the flow of air in and out of the alveoli) lined with epithelial cells
• Diaphragm - A sheet of muscle that separates the thorax from the abdomen
• Expiration - A largely passive process when external intercostal muscles relax, internal intercostal muscles contract, ribs move downwards and inwards and the diaphragm muscles relax, decreasing the volume of the thorax, which increases the pressure
• Intercostal muscles - Lie between the ribs

  • Two sets - internal whose contraction leads to expiration and external whose contraction leads to inspiration
• Inspiration - An active process when external intercostal muscles contract, internal intercostal muscles relax, ribs are pulled upwards and outwards and the diaphragm muscles contract causing it to flatten, increasing the volume of the thorax, which reduces the pressure
• Lungs - A pair of lobe structures made up of a series of highly branched tubules called bronchioles, which end in tiny air sacs called alveoli
• Ribcage - The bony frame formed by the ribs round the chest
• Trachea - A flexible airway that is supported by rings of cartilage which prevent it collapsing as the air pressure inside falls when breathing in. Its walls are made up of muscle, lined with ciliated epithelium and goblet cells
• Ventilation - The process in which air is constantly moved in and out of the lungs to maintain diffusion of gases across the alveolar epithelium (also known as breathing)