6.4: Gas exchange

Question 1

Distinguish between ventilation, gas exchange and cell respiration:

Answer 1

Ventilation: The exchange of air between the lungs and the atmosphere - it is achieved by the physical act of breathing.

Gas exchange: The exchange of oxygen and carbon dioxide in the alveoli and the bloodstream; it occurs passively via diffusion.

Cell Respiration: The release of ATP from organic molecules; it is greatly enhanced by the presence of oxygen (aerobic respiration).

Question 2

Explain the need for a ventilation system:

Answer 2

• Because gas exchange is a passive process, a ventilation system is needed to maintain a concentration gradient within the alveoli.
• Oxygen is needed by cells to make ATP via aerobic respiration, while carbon dioxide must diffuse from the blood into the lungs.
• This requires a high concentration of oxygen and a low concentration of carbon dioxide, in the lungs.
• A ventilation system maintains this concentration gradient by continually cycling the air in the lungs with the atmosphere.

Question 3

Describe the features of alveoli that adapt them to gas exchange:

Answer 3

Thin wall: Made of a single layer of flattened cells so that diffusion distance is small.

Rich capillary network: Alveoli are covered by a dense network of capillaries that help to maintain a concentration gradient.

Increased SA: Volume Ratio: High numbers of spherically-shaped alveoli optimise surface area for gas exchange.

Moist: Some cells in the lining secrete fluid to allow gases to dissolve and to prevent alveoli from collapsing.

Question 4

Explain the mechanism of ventilation of the lungs in terms of volume and pressure changed caused by the internal and external intercostal muscles, the diaphragm and abdominal muscles:

Answer 4

• Breathing is the active movement of respiratory muscles that enable the passage of air to and from the lungs.

Inhalation:

• The external intercostal muscles contract. This moves the ribcage up and out.
• The diaphragm contracts. As it does so it moves down and becomes relatively flat.
• Both of these muscle contractions result in an increase in the volume of the thorax which in turn results in a drop in pressure inside the thorax.
• Pressure eventually drops below atmospheric pressure.
• Air then flow into the lungs from outside the body, through the mouth or nose, trachea, bronchi and bronchioles.
• Air continues to enter the lungs until the pressure inside the lungs rises to the atmospheric pressure.

Exhalation:

• The internal intercostal muscles contract. This moves the ribcage down and in.
• The abdominal muscles contract. This pushes the diaphragm up, back into a dome shape.
• Both of these muscle contractions result in a decrease in the volume of the thorax.
• As a result of the decrease in volume, the pressure inside the thorax increases.
• Eventually the pressure rises above atmospheric pressure.
• Air then flows out of the lungs to outside of the body through the nose or mouth.
• Air continues to flow out of the lungs until the pressure in the lungs until the pressure in the lungs has fallen back to atmospheric pressure.