A3. Gas Exchange in Humans

Question 1

Figure 1: A diagram of the human gas exchange system with the alveoli enlarged.

Answer 1

Question 2

Gas exchange in 4 steps

Answer 2

1) As you breathe in…, air enters the trachea (windpipe).
2) The trachea splits into two bronchi.
3) Each bronchus then branches off into bronchioles.
4) The bronchioles end in alveoli where gas exchange occurs

Question 3

Ventilation - Inspiration (6 things)

Answer 3

1) During inspiration the external intercostal and diaphragm muscles contract.

2) This causes the ribcage to move upwards and outwards and the diaphragm to flatten

3) increasing the volume of the thoracic cavity

4) as the volume of the thoracic cavity increases, the lung pressure decreases to below atmospheric pressure

5) Air moves down the trachea into the lungs as it flows down a pressure gradient

6) Inspiration is an active process - it requires energy.

Question 4

Ventilation - Expiration (6 things)

Answer 4

1) During expiration the external intercostal and diaphragm muscles relax.

2) The ribcage moves downwards and inwards, and the diaphragm curves upwards again.

3)The volume of the thoracic cavity decreases, causing the air pressure to increase to above atmospheric pressure.

4) Air is forced down the pressure gradient and out of the lungs.

5) When expiration is forced - internal intercostal muscles contract as well, pulling the ribcage further down and in.

Question 5

Alveoli and gas exchange - Alveoli structure (2 things)

Figure 7: A capillary next to an alveolus.

Answer 5

• The wall of each alveolus is made from a single layer of thin, flat cells called alveolar epithelium. The walls of the capillaries are made from capillary endothelium
• The walls of the alveoli contain a protein called elastin. Elastin is elastic-it helps the alveoli to return (recoil) to their normal shape after inhaling and exhaling air.

Question 6

Movement of oxygen and carbon dioxide through the gas exchange system

What moves in, where and how?
What happens next?
Where does carbon dioxide go and how?

Answer 6

Air (containing oxygen) moves down the trachea, bronchi and bronchioles into the alveoli. This movement happens down a pressure gradient. Oxygen then moves into the blood where it can be transported round the body - this movement happens down a diffusion gradient.

Carbon dioxide moves down its own diffusion and pressure gradients, but in the opposite direction to oxygen, so that it can be breathed out.

Question 7

Gas exchange in the alveoli (2+2 things)

Answer 7

• Oxygen diffuses out of the alveoli, across the alveolar epithelium
• through the capillary endothelium
• and into haemoglobin in the blood.
• Carbon dioxide diffuses into the alveoli from the blood

Question 8

Figure 9: How diagram showing how oxygen moves through the gas exchange system.

Answer 8

Question 9

Factors affecting the rate of diffusion
Alveoli have features that speed up the rate of diffusion so gases can be exchanged quickly:…

Answer 9

• A thin exchange surface - the alveolar epithelium is only one cell thick - means there’s a short diffusion pathway (which speeds up diffusion).
• A large surface area - there are millions of alveoli - large surface area for gas exchange.
• steep concentration gradient of oxygen and carbon dioxide between the alveoli and the capillaries, which increases the rate of diffusion. This is constantly maintained by the flow of blood and ventilation

Question 10

Figure 10: Diagram showing how blood flow and ventilation maintain high concentration gradients of O2 and CO2

Answer 10