Mammalian Gaseous Exchange

Question 1

What is the definition of diffusion?

Answer 1

The net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient until an equilibrium is reached.

Question 2

Why is diffusion alone enough to satisfy the needs of a unicellular organism?

Answer 2

• Metabolic activity is usually low
• Diffusion pathway is short
• Surface area : volume ratio is high

Question 3

What is the correlation between SA : V and organism size?

Answer 3

The larger the organism, the smaller the SA : V ratio.

Question 4

What is the issue with having a small SA : V ratio?

Answer 4

Diffusion distance is too great, meaning rate of exchange is not fast enough and quantities exchanged are not large enough to meet metabolic demands.

Question 5

What are the key features of efficient exchange systems?

Answer 5

• Increased surface area
• Thin layers
• Good blood supply
• Ventilation

Question 6

What is the benefit of an increased surface area?

Answer 6

It helps overcome the SA : V limitations of large, multicellular organisms. Examples include villi & root hair cells.

Question 7

What is the benefit of thin layers?

Answer 7

They provide a short diffusion pathway, making the process more efficient (e.g. alveoli).

Question 8

What is the benefit of a good blood supply?

Answer 8

A steeper concentration gradient = faster diffusion. Having a good blood supply means substances are constantly being exchanged, maintaining a steep concentration gradient.

Question 9

What is the benefit of ventilation?

Answer 9

Important for gaseous exchange as it helps maintain a steep concentration gradient. Examples include alveoli & fish gills.

Question 10

What is the role of the trachea?

Answer 10

It is the main airway which carries moist, clean, warm air to the lungs.

Question 11

How is the trachea supported?

Answer 11

It is supported by strong, flexible rings of cartilage to prevent collapse. The rings are incomplete so food can move down the oesophagus behind the trachea easily.

Question 12

How else is the trachea adapted?

Answer 12

It is lined with ciliated epithelial cells and goblet cells.

Question 13

What is the role of ciliated epithelial cells?

Answer 13

They beat to waft mucus (which traps dirt) away from the lungs.

Question 14

What is the role of goblet cells?

Answer 14

They secrete mucus onto the lining of the trachea.

Question 15

How is the naval cavity adapted to support gaseous exchange?

Answer 15

• Large surface area and good blood supply which warms air to room temperature.
• Hairy lining which secretes mucus, trapping dirt and microorganisms to avoid lung irritation / infection.
• Moist surfaces which increases air humidity and reduces evaporation from exchange surfaces.

Question 16

What is the thorax?

Answer 16

The chest cavity

Question 17

How are the bronchi structured?

Answer 17

They have the same structure and supporting cartilage rings as the trachea but are narrower.

Question 18

How are the bronchioles structured?

Answer 18

The bronchi divides into bronchioles which have no supporting rings of cartilage.

The walls of the bronchioles contain smooth muscle which can contract and relax, allowing the bronchioles to constrict (narrow) and dilate (widen).They are also lined with a thin layer of flattened epithelium, and there is some gaseous exchange.

Question 19

Where in the respiratory system is smooth muscle found?

Answer 19

Trachea, bronchi, bronchioles

Question 20

What are the alveoli?

Answer 20

Tiny, moist air sacs which are the main gas exchange surfaces. The diameter of an alveolus is around 200-300 micrometres.

They consist of a thin layer of flattened epithelial cells, known as alveolar epithelium, and some collagen and elastic fibres.

Question 21

What do the elastic fibres in the alveolus enable?

Answer 21

Elastic recoil - which is when the alveoli can stretch and return to a resting size to take in air and squeeze it out.

When the elastic fibres stretch, they prevent bursting. When they recoil, this helps exhalation.

Question 22

Why are alveoli best suitable for gas exchange?

Answer 22

• Good ventilation - delivery and removal of O2 / CO2 - steep diffusion gradient
• Good blood supply - lots of blood flow provides a steep diffusion gradient
• Thin layers - alveolar wall is one cell thick and flattened
• Moist lining - oxygen can dissolve into solution
• Large surface area

Question 23

What is the alveolar wall known as?

Answer 23

“Alveolar epithelium” which is one cell thick and flattened.

Question 24

How does the capillary wall support gas exchange?

Answer 24

It’s close proximity to the alveoli allows for a short diffusion pathway. The capillary wall provides a good blood supply, and is also one cell thick and flattened.

The lumen of the capillary is roughly the same diameter as that of the red blood cells.

Question 25

What are alveoli covered with?

Answer 25

A thin layer of solution consisting of water, salts and lung surfactant.

Lung surfactant lowers the surface tension to prevent alveoli collapse.

Question 26

What separates the lungs from the abdomen?

Answer 26

A broad, domed sheet of muscle called the diaphragm.

Question 27

What lines the thorax and what is their role?

Answer 27

The thorax is lined by “pleural membranes” which surround the lungs, and the space between them - the pleural cavity - is filled with lubricating fluid for the layers to slide over each other easily.

Question 28

What are the roles of the external and internal intercostal muscles?

Answer 28

They contract to raise and lower the ribcage.

Question 29

What are the 4 stages of ventilating the lungs?

Answer 29

• Physical / anatomical movement
• Volume
• Pressure
• Air movement

Question 30

How does inspiration / inhalation work?

Answer 30

1.The diaphragm contracts, flattening and lowering. The external intercostal muscles contract, moving the ribs upwards and outwards.

2.This increases the thorax’s volume.

3.The pressure inside the thorax decreases. The pressure is now LOWER than the atmospheric air pressure.

4. Air is drawn in by the nasal pathways and trachea , equalising pressure inside and outside of the chest.

Inspiration is an active process.

Question 31

How does expiration / exhalation work?

Answer 31

1. The muscles of the diaphragm relax and move up into a resting shape. The external intercostal muscles relax, and the ribs move down and inwards. The alveoli’s elastic fibres return to their resting size.
2. This results in a decrease of thorax volume.
3. Pressure inside the thorax increases and is now GREATER than the atmospheric air pressure.
4. Air moves out of the lungs until the pressure is equal again inside and out.

Expiration is a passive process.