Gas Exchange In Humans

Question 1

What structures does air move through as you breathe in?

Answer 1

1. The trachea which is supported by incomplete rings of cartilage which prevents it collapsing during the pressure the pressure changes which occur.
2. This divided into two bronchi that repeatedly divide into smaller tubes, bronchioles, (which can narrow and widen)
3. The alveoli at the ends of the bronchioles provide a large surface area where gas exchange occurs

Question 2

Why is ventilation in humans described as tidal?

Answer 2

Air flows in and out by the same route

Question 3

What are the two muscles that cause the thorax to move?

Answer 3

1. Internal and external intercostal muscles
2. Diaphragm muscles

Question 4

Describe the process of inspiration (breathing in) .

Answer 4

1. The diaphragm muscle contracts and flattens downwards
2. The external intercostal muscles contract, pulling the ribs up and out. The internal intercostal muscles remain relaxed.
3. This increases the volume of the thorax and the lungs, and stretches the elastic-walled alveoli
4. This decreases the pressure of the air in the alveoli below atmospheric .
5. Air flows in from high pressure to low pressure, down the pressure gradient.

Question 5

Describe the process of expiration (breathing out) :

Answer 5

1. The diaphragm muscle relaxes and curves upwards
2. The external intercostal muscles relax, allowing the ribs to fall, the internal intercostal muscles contract
3. This decreases the volume of the thorax and the lungs, and allows the alveoli to shrink by elastic recoil
4. This increases the pressure in the alveoli above atmospheric
5. Air flows out from high pressure to low pressure down the pressure gradient

Question 6

Describe the structural adaptations of the alveoli.

Answer 6

1. Large surface area
   - many, small alveoli which increases surface area
   - alveoli surrounded by many, small branching capillaries
   —> ensures faster diffusion of oxygen into the blood
2. Short diffusion pathway
   - alveoli made of a single layer of flattened epithelial cells
   - capillaries made up of a single layer of endothelial cells
   —> ensures faster diffusion of oxygen into the blood
3. Maintains concentration/ diffusion gradient
   - breathing ensures the alveoli always contains a high concentration of oxygen
   - blood flow at alveoli ensures the high concentration of oxygen is removed and replaced with blood containing low concentration of oxygen
   —> ensures concentration gradient for the diffusion of oxygen into the blood is always maintained
4. Red blood cells have to squeeze through narrow capillaries
   - slows rate of blood flow
   —> more time for gas exchange
5. Capillaries have a high total cross-sectional area
   - increases frictional resistance which slows blood flow
   —> gives more time for gas exchange