3.1.1 Exchange surfaces - Mammalian gaseous exchange surfaces

Question 1

Flowchart of human gaseous exchange system

Answer 1

1. Nasal cavity
2. Trachea
3. Bronchus
4. Bronchioles
5. Alveoli

Question 2

What are the important features of a nasal cavity?

Answer 2

• a large surface area with a good blood supply (since there are so many capillaries) this warms up the air (causing more efficient exchange)
• a hairy lining, which secretes mucus to trap dust and bacteria, protecting delicate lung tissue from irritation and infection.
• moist surfaces facilitates a faster rate of exchange (and increase humidity of the incoming air, reducing evaporation from the exchange surfaces)

Question 3

What is the structure of the trachea?

Answer 3

Wide tube supported by incomplete rings (c-rings) of strong, flexible cartilage, which stop the trachea from collapsing.

Question 4

Why does the trachea have c-rings (incomplete rings) and not full rings?

Answer 4

The rings are incomplete so that food can move easily down the oesophagus behind the trachea.

Question 5

What is the role of the trachea?

Answer 5

The trachea and its branches are lined with ciliated epithelium, which goblet cells.
Goblet cells secrete mucus which traps dirt and bacteria, the cilia then swift the mucus away from the lungs.

Question 6

What is the structure of the Bronchus?

Answer 6

The trachea divides to form the left bronchus which leads to the left lung and the right bronchus leading to the right lung. The bronchus has a similar structure to the trachea as it contains the same c-rings of cartilage but they are smaller.

Question 7

What is the structure of the Bronchioles?

Answer 7

• In the lungs, the bronchi divide to form small bronchioles.
• The smaller bronchioles have a diameter of 1 mm or less and have no cartilage rings.
• The walls of the bronchioles contain smooth muscle.
• Bronchioles are lined with a thin layer of flattened epithelium, making gaseous exchange possible.

Question 8

Why do the walls of the bronchioles contain smooth muscle?

Answer 8

So that bronchioles can constrict and dilate.
When the smooth muscle contracts, the bronchioles constrict (close up), when it relaxes the bronchioles dilate (open up).

Question 9

What is the structure of alveoli?

Answer 9

Alveoli are tiny air sacs.
Each alveolus has a diameter of 200-300 μm.
They consist of a layer of thin, flattened epithelium cells, along with some collagen and elastic fibres.

Question 10

Define elastic recoil

Answer 10

Due to elastin, the object returns to its resting size after being stretched out.

Question 11

How does elastic recoil occur in the alveoli?

Answer 11

The elastic tissue allows the alveoli to stretch as air is drawn in. When they return to their resting size, they helo squeeze the air out.

Question 12

What is the inner surface of the alveoli covered in?

Answer 12

covered in a thin layer of a solution of water, salts, and lung surfactant.

Question 13

Why is surfactant important for alveoli?

Answer 13

Stops alveoli walls sticking together when oxygen leaves and makes it possible for the alveoli to remain inflated.
The solution coating the inner surface of the alveoli contains water which allows oxygen to be dissolved.

Question 14

What are the main adaptions of the alveoli for effective gaseous exchange?

Answer 14

• large surface area
• thin layers / short diffusion path
• good blood supply (maintains steep concentration gradient)
• good ventilation (maintains steep diffusion gradients)

Question 15

Describe how inspiration occurs

Answer 15

Inspiration is breathing in, and is an active process.

1. Diaphragm contracts causing it to flatten. This increases the space available for the lungs to expand.
2. Ribs move up and outwards. The external intercoastal muscles contract this increases the volume of the thorax = pressure decreases causing air to enter the lungs (passively)
3. Pressure equalises inside and outside the lungs.

Question 16

Describe how expiration occurs

Answer 16

Expiration is breathing out and is a passive process.

1. diaphragm relaxes (in its natural shape)
2. The external intercostal muscles relax so the ribs move down and inwards.
3. Elastic recoil is occurring in the lungs as the elastin within our lungs is recoiling to its original position.
4. The volume of the thorax decreases.
5. the pressure inside the thorax is greater than the pressure outside of the lungs (atmospheric air), so air moves out of the lungs until the pressure inside and out is equal again.

Question 17

How does coughing (exhaling forcefully, etc.) occur?

Answer 17

Active process.

1. The internal intercostal muscles contract, pulling the ribs down hard and fast.
2. The abdominal muscles contract
3. This forces the diaphragm up to increase the pressure in the lungs rapidly.

Question 18

Why do gas exchange systems need to be moist?

Answer 18

So that Oxygen can dissolve.

Question 19

How has the human exchange system evolved to minimise loss of water?

Answer 19

There is a rich blood supply in the nasal cavity (capillaries) which increases the temperature and humidity of incoming air.