7.2- Mammalian Gaseous Exchange System

Question 1

What is the need for the human gaseous exchange system?

Answer 1

1. High metabolic rate due to being very active and having to maintain body temperature. There is therefore a greater demand for oxygen and the removal of co2.
2. Due to the small SA:V ratio and very large volume of cells.

Question 2

What are the 5 key structures of the human gaseous exchange system?

Answer 2

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

Question 3

Nasal cavity.

Answer 3

1. large surface area with good blood supply- warms incoming air to body temperature.
2. Hairy lining- secretes mucus to trap dust/bacteria, protecting irritation and infection.
3. Moist surfaces- increase humidity of incoming air, reducing evaporation from exchange surfaces.

Question 4

Trachea.

Answer 4

The trachea is the main airway carrying clean, warm and moist air from the nose down into chest.

• wide tube supported by incomplete rings of strong, flexible cartilage which stop it from collapsing.
• rings are incomplete to allow space for oesophagus.
• lined with ciliated epithelium, with goblet cells between and below the epithelial cells.
• goblet cells secrete mucus onto its lining, cilia brush mucus away into the throat. It is then swallowed and digested.

Question 5

Bronchus.

Answer 5

In the chest cavity, the trachea divides to for, the left and right bronchus.
Have a similar structure to trachea:
- the same supporting rings of cartilage, however, they are much smaller.

Question 6

Bronchioles.

Answer 6

In the lungs, the bronchi divide to form many small bronchioles.
- they have no cartilage rings.
- walls contain smooth muscle.
- smooth muscle contract= bronchioles constrict.
smooth muscle relaxes= bronchioles dilate.
- this changes the amount of air entering the lungs.
- lined with thin layer of flattened epithelium, making some gaseous exchange possible.

Question 7

Alveoli.

Answer 7

The alveoli are tiny air sacs, which are the main gas exchange surfaces in the body.
- diameter of 200-300um.
- consists of layer of thin flattened epithelial cells, along with some collagen and elastic fibres( made from elastin).
- elastic tissues allow alveoli to stretch as air is drawn in. When they return to resting size, they help squeeze air out. = known as elastic recoil of the lungs.
(Textbook)

Question 8

What are the main adaptations of the alveoli for effective gas exchange?

Answer 8

1. Large SA- there are 300-500 million alveoli per adult lung. Provides space needed for oxygen to diffuse.
2. Thin layers- alveoli and the capillaries surrounding them are one cell thick. Short diffusion distance.
3. Good blood supply- constant flow of blood through capillaries maintains steep conc gradient = faster diffusion.
4. Good ventilation- breathing moves air in and out maintaining steep conc gradient.
5. Inner surface covered in thin layer of solution os salt, water and lung surfactant.

Question 9

What is the role of the solution on the inner surface of the alveoli?

Answer 9

• lung surfactant makes it possible for alveoli to remain inflated. It does this by reducing surface tension.
• oxygen dissolves in the water before diffusing as diffusion can only occur once the gases are dissolved.

Question 10

What features play a role in ventilating the lungs?

Answer 10

• air is moved in/out due to pressure changes in the thorax (chest cavity).
• thorax is lined by the pleural membranes which surround the lungs.
  space between them (pleural cavity) filled with thin layer of lubricating fluid so membranes easily slide over each other during breathing.
• rib cage provides semi-rigid case within which pressure can be lowered.
• diaphragm is a broad, domed sheet of muscle which forms the floor of the thorax.
• external/internal intercostal muscles are found between the ribs.

Question 11

Explain what happens during inspiration.

Answer 11

An energy using process.

• diaphragm contracts, flattens and lowers.
• external intercostal muscles contract, moving ribs upwards and outwards.
• volume of thorax increases so pressure in thorax is reduced.
• pressure is lower than pressure of atmospheric air, so air is drawn into the lungs to equalise the pressures.

Question 12

Explain what happens during expiration.

Answer 12

Passive process.

• diaphragm relaxes, moves up into resting dome shape.
• external intercostal muscles relax, ribs move down and inwards.
• elastic fibres in alveoli return to their normal length.
• pressure in thorax increases as volume has increased.
• pressure is greater than atmospheric pressure so air moves out until equilibrium is reached.