The alveoli

What are the alveoli

The respiratory surface in humans

What characteristics make the alveoli very efficient at gas exchange?

1. Huge surface area: There are about 350 million alveoli in your lungs, and together they create a massive surface area of 90m2. This large area allows a lot of O2 and CO2 to be exchanged quickly

2. Thin walls: the alveoli have a thin, elastic epithelium (lining). This means that the gases only have to travel a short distance to pass between the alveoli and the blood → making diffusion super fast

3. Moist lining: a thin layer of water (moisture) covers the lining of each alveolus which helps gases dissolve before diffusing (gases move more easily in solution/ liquid state)

4. Good blood supply: A dense network of capillaries surrounds each alveolus, allowing O2 to quickly move from the alveoli to the blood and CO2 to move quickly from the blood into the alveoli to be exhaled

5. Maintains a high concentration gradient: The body keeps a steady flow of blood and air moving in and out of the lungs. This ensures O2 levels stay high and CO2 levels stay low (high conc. gradient) making diffusion happen faster.

What are three brief steps of gas exchange?

1. Oxygen enters the blood

2. Oxygen is delivered to the body

3. Carbon Dioxide is removed

Oxygen enters the blood - explain.

1. When you breath in, oxygen fills the alveoli (tiny air sacs in the lungs)

2. There is more O2 in the alveoli than in the blood, so O2 diffuses from the alveoli to the blood capillaries (high concentration of oxygen in the alveoli to a low concentration oxygen in the blood capillaries, down a concentration gradient)

3. Inside the red blood cells, O2 binds to a protein called haemoglobin

Oxygen is delivered to the body - Explain

1. The pulmonary vein transports oxygenated (oxygen-rich) blood from the alveoli to the heart

2. The heart then pumps it to the rest of the body

3. Since in body tissues there is a lower O2 concentration, oxygen splits away from haemoglobin and diffuses into the cells.

Carbon dioxide is removed - explain

1. After delivering O2, the blood picks up CO2 (a waste product of cells)

2. The deoxygenated (rich in CO2) blood travels back to the heart, which it (the heart) pumps to the lungs via the pulmonary artery

3. Since there is more CO2 in the blood than in the alveoli, CO2 diffuses from a higher concentration in the blood capillaries to a lower concentration in the alveolus, down a concentration gradient to be removed with exhaled air.