In mammals the gas exchange surfaces are the lungs, which develop in the embryo from the gut wall. The larynx is formed from cartilage. It is connected to the trachea – a flexible tube held open by incomplete rings of cartilage. The trachea divides into the left and right bronchi which then enter the lungs and continue to divide forming the narrower bronchioles, which are surrounded by circular smooth muscle fibres. At the ends of the bronchioles are groups of alveoli or air-sacs. However, it is in the alveoli that gas exchange occurs.
The lungs are well suited to exchange gas efficiently:
• A large surface area – an average adult has about 600 million alveoli, giving a total surface area of about 100m².
• The alveoli consist of an epithelial layer and an extracellular matrix surrounded by small blood vessels called capillaries. In some alveolar walls there are pores between alveoli called Pores of Kohn. The alveoli contain some collagen fibres and elastic fibres. The elastic fibres allow the alveoli to stretch as they are filled with air during inhalation. They then spring back during exhalation to expel the carbon dioxide-rich air.
• A short diffusion pathway – the walls of the alveoli are composed of a single layer of flattened epithelial cells, as are the walls of the capillaries, so gases need to diffuse through just two thin cells which means that diffusion occurs very quickly.
The alveoli walls are also kept moist by water diffusing from the surrounding cells. Oxygen dissolves in this water before diffusing through the cells into the plasma, where it is taken up by haemoglobin in the red blood cells. The water also contains a soapy surfactant which reduces its surface tension and stops the alveoli collapsing. The alveoli also contain phagocytes to kill any bacteria that have not been trapped by the mucus.
• High concentration gradients – the steep concentration gradient across the alveoli wall is maintained in two ways: by blood flow on one side and by air flow on the other side. This means that oxygen can diffuse down its concentration gradient from the air to the blood, while at the same time carbon dioxide can diffuse down its concentration gradient from the blood to the air.
• A gas permeable surface – the moist surfaces of the alveoli allow gases to dissolve and then diffuse through the cells.