Breathing

Breathing (ventilation) is the movement of air into and out of the lungs. It is needed because gas exchange in the alveoli relies on diffusion, and diffusion is only fast when there is a steep concentration gradient. Ventilation constantly brings in fresh air that is high in oxygen and low in carbon dioxide, and removes stale air, so the gradients across the alveolar walls are maintained.Air always moves from a region of higher pressure to a region of lower pressure. The lungs sit inside the airtight thoracic cavity (thorax), enclosed by the ribs, the intercostal muscles and the diaphragm, and lined by the pleural membranes. Changing the volume of the thorax changes the pressure inside it: at constant temperature, increasing the volume lowers the pressure and decreasing the volume raises it.Three sets of muscles do this work:• the diaphragm, a sheet of muscle below the lungs
• the external intercostal muscles, between the ribs
• the internal intercostal muscles, also between the ribs, which are antagonistic to the external intercostalsOne breath has two phases. Inhalation (inspiration) increases the volume of the thorax so that pressure inside falls below atmospheric pressure and air flows in. Exhalation (expiration) decreases the volume so that pressure rises above atmospheric pressure and air flows out. The exact muscle movements in each phase are covered in the next two lessons.Breathing is a mass flow of air, so it moves air far faster than diffusion alone could along the long airways.