Gas Exchange

In humans the gas exchange organ system is the respiratory or breathing system. The actual respiratory surface is on the alveoli inside the lungs. An average adult has about 600 million alveoli, giving a total surface area of about 100mē, so the area is huge. 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. Water diffuses from the alveoli cells into the alveoli so that they are constantly moist. Oxygen dissolves in this water before diffusing through the cells into the blood, 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 phagocyte cells to kill any bacteria that have not been trapped by the mucus.  The steep concentration gradient across the respiratory surface is maintained in two ways: by blood flow on one side and by air flow on the other side. This means oxygen can always 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. The flow of air in and out of the alveoli is called ventilation and has two stages: inspiration (or inhalation) and expiration (or exhalation). Lungs are not muscular and cannot ventilate themselves, but instead the whole thorax moves and changes size, due to the action of two sets of muscles: the intercostal muscles and the diaphragm.

Inspiration

• The diaphragm contracts and flattens downwards

• The external intercostal muscles contract, pulling the ribs up and out

• this increases the volume of the thorax

• this increases the lung and alveoli volume

• this decreases the pressure of air in the alveoli below atmospheric (Boyle's law)

• air flows in to equalise the pressure

Normal Expiration

• The diaphragm relaxes and curves upwards

• The external intercostal muscles relax, allowing the ribs to fall

• this decreases the volume of the thorax

• this decreases the lung and alveoli volume

• this increases the pressure of air in the alveoli above atmospheric (Boyle's law)

• air flows out to equalise the pressure

Forced Expiration

• The abdominal muscles contract, pushing the diaphragm upwards

• The internal intercostal muscles contract, pulling the ribs downward

• This gives a larger and faster expiration, used in exercise

 These movements are transmitted to the lungs via the pleural sac surrounding each lung. The outer membrane is attached to the thorax and the inner membrane is attached to the lungs. Between the membranes is the pleural fluid, which is incompressible, so if the thorax moves, the lungs move too. The alveoli are elastic and collapse if not held stretched by the thorax (as happens in stab wounds or deliberately to rest a lung).