The lungs as an organ of exchange

The lungs

these are a large pair of inflatable structures lying in the chest cavity. Air can pass through the nose and along the trachea, Bronchi and Bronchioles. Each part of this airway is adapted to its function of allowing the passage of air. Finally the air reaches the surface of the  Alveoli. The walls of the Alveoli are the surface where exchanges of gases take place.

The lungs are protected by ribs. Movement of both the ribs and the diaphragm helps to produce breathing movements known as ventilation

Gaseous exchanges/how the lungs are adapted

Gases pass both ways through the thin walls of the Alveoli. Oxygen passes from the air in the Alveoli to the blood in the capillaries  Carbon dioxide pass from the blood to the air in the Alveoli.

• the large surface area provides more space for molecules to pass through. The individual Alveoli are very small (about 100-300Micrometers across). They are so numerous that the total surface area is larger than our skin. The total surface area of the lung exchange surface is about 70m(squared)
• A barrier permeable to oxygen and carbon dioxide means that plasma membranes that surround the thin cytoplasm form the barrier for exchange. These allow the diffusion of co2 and o2
• The thin barrier to reduce the diffusion distance has a number of adaptations to reduce the distance that gases have to diffuse

1. The alveolus wall is one cell thick
2. the capillary wall is one cell thick
3. both walls consist of squamous cells- this means flattened or very thin cells
4. The capillaries are in close contact with the Alveolus walls
5. The capillaries are so narrow that the blood cells are squeezed against the capillary wall, making them closer to the air in the alveoli, reducing the rate that it flows past in the blood
6. the barrier of diffusion is only 2 flattened cells thick, and is less than 1 micrometer thick.

A thin layer of moisture surround the alveoli. This moisture passes through the cell membranes from the cytoplasm of the alveolus cells. As we breathe out, it evaporates and is lost. The lungs must produce a substance called a surfactant, that reduces the cohesion between the water molecules. Without the surfactant, the alveolus would collapse due to the cohesive forces between the water molecules lining the air sac.

 

Maintaining a diffusion gradient

For diffusion to be rapid, a steep diffusion gradient is needed. This means having a high concentration of molecules on the supply side of the exchange surface, and a low concentration on the demand side.

To maintain the diffusion gradient, a fresh supply of molecules on one side is needed to keep the concentration high, and a way of removing the molecules from the other side is needed to keep the concentration low

This is achieved by the action of the blood transport system and the ventilation movements

The blood brings carbon dioxide from the tissue to the lungs. This ensures that the concentration of carbon dioxide in the blood is higher than in the alveoli air space. It also carries oxygen away from the lungs. This ensures that the concentration of oxygen in the blood is kept lower than the concentration than the air inside the alveoli.

The heart pumps blood along the pulmonary artery to the lungs. In the lungs the artery divides up to form finer and finer vessels. These eventually carry blood in tiny capillaries that are only just big enough for a red blood cell to squeeze through. These capillaries lie over the surface of the alveoli.

The breathing movements of the lungs helps to ventilate the lungs. They replace the used air with fresh air. This brings more oxygen in the lungs and ensures that the concentration of oxygen in the air of the alveoli remains higher than the concentration of oxygen in the blood.

The constant supply of gas to one side of the exchange surface and the removal from the other side ensures that diffusion, and therefore exchange can continue.

 

Inhaling

1. The Diaphragm contracts and moves upwards
2. The rib cage external intercostal muscles contract to move ribs upwards and outwards
3. The volume of the thorax increases
4. The pressure in the lungs falls below atmospheric pressure so air moves in

Exhaling

1. The Diaphragm relaxes and moves upwards
   
2. the ribcage external intercostal muscles relax so that the ribs move down and inwards
3. the volume of the thorax decreases
4. the pressure in the lungs rises above that of atmospheric pressure so air moves out