Exchange surfaces

Alveoli: tiny folds of the lung epithelium to increase the SA.

Bronchi and bronchioles: smaller airways leading into the lungs.

Diaphragm: a layer of muscle beneath the lungs.

Intercostal muscles: muscles between the ribs. Contraction of the external intercostal muscles raises the ribcage.

Trachea: the main airway leading from the back of the mouth to the lungs.

Ventilation: the refreshing of the air in the lungs, so that there is a higher oxygen concentration than in the blood, and a lower carbon dioxide concentation.

The gaseous exhange system in mammals consist of the lungs and associated airways that carry air into/out of the lungs. The lungs are a pair of inflatable sacs lying in the chest cavity. Air can pass into the lungs and along the trachea, bronchi and bronchioles. Finally, it reaches the alveoli. This is where the excahnge occurs. The lungs are protected by the ribcage and they are held together by intercostal muscles. The action of these muscles and diaphragm produce ventilation.

Gases pass by diffusion through the thin walls of the alveoli. Oxygen passes from the air in the alveoli to the blood in the capillaries. Crabon dioxide passes from the blood to the air in the alveoli. The lungs must maintain a steep concentartion gradient in each direction in order to ensure that diffusion can continue.

Individual alveoli are very small - 100-300um across. However becuase there are so many, the lungs surface area is so large, it is even bigger than the surface area of our skin. The total surface area of the exhange surface in humans is about 70m^2.

The alveoli are lined by a thin layer of moisture, which evaporates and is lost as we exhale. The lungs must produce a surfactant that coats the internal surface of the alveoli to reduce the choesive forces between the water molecules, as theses forces tend to make the alveoli collapse.

The barrier to exchange is comprised of the wall of the alveolus and the wall of the blood capillary. The cells and their plasma membranes readily allow the diffusion of oxygen and carbon dioxide, as the molecules are small and non-polar.

There are a number of adaptations to reduce the diffusion distance:

• The alveolus wall is one cell thick
• The capillary wall is one cell thick
• Both walls consist of squamous cells.
• The capillaries are in clos econtact with the alveolus walls.
• The capillaries are so narrow that the red blood cells are squeezed against the capillary wall - making them closer to the air in the alveoli reducing their rate of flow.

So, the total barrier to diffusion is only two flattened cells and is less than 1um thick.

The blood supply helps to maintain a steep concentration gradient, so that the gases continue to diffuse:

• The blood system transports carbon dioxide from the tissues to the lungs. This ensures that the concentration of carbon dioxide is higher in the blood than that of the air in the alveoli. So, carbon dioxide will diffuse into the alveoli.
• The blood also transports oxygen away from the lungs. This ensures that the concentration of oxygen in the blood is kept lower than that in the alveoli. So, oxygen diffuses into the blood.

The breathing movements ventilate the lungs. This replaces the used air with fresh air, bringing in more oxygen and removing carbon dioxide. Ventilation ensures that:

• The concentration of oxygen in the air of the alveolus remains higher than in the blood.
• The concentration of carbon dioxide in the alveoli reamins lower than that in the blood.

Therefore, the concentration gradient necessary for diffusion is maintained.

Inspiration (inhaling):

• The diaphragm contracts to move down and become flatter - this displaces the digestive organs downwards.
• The external intercostal muscles contract to raise the ribs.
• The volume of the chest cavity is increased.
• The pressure in the chest cavity drops below the atmospheric pressure.
• Air is moved into the lungs.

Expiration (exhalng):

• The diaphragm relaxes and is pushed up by the displaced organs underneath.
• The external intercostal muscles relax and the ribs fall; the internal intercostal muscles can contract to help push air out more forcefully - this usually only happens during exercise, coughing or sneezing.
• The volume of the chest cavity is decreased.
• The pressure in the lungs increases and rises above the atmospheric pressure.
• Air is moved out of the lungs.