6.4 Gas Exchange

Question 1

Why do we need a ventilation system?

Answer 1

We are large organisms. Oxygen cannot diffuse into all our cells directly from the air, nor can waste products be directly ejected from the body. We have specialized organ systems, which are efficient but need delivery of nutrients and removal of waste. The ventilation system ensures the blood can be the medium for this.

We are land-borne. Gases need moist surfaces (membranes) in order to diffuse. Our lungs are moist membranes, allowing oxygen to diffuse into the blood and carbon dioxide to diffuse out.

The ventilation system maintains a large concentration gradient between the alveoli and the blood. The constant flow of past the alveoli brings blood with a high CO2 concentration and low O2 concentration. Breathing out keeps the CO2 concentration in the alveoli low, so it diffuses out of the blood. Breathing in keeps O2 concentration in the alveoli high, so it diffuses into the blood.

Question 2

Gas exchange

Answer 2

For gas exchange to be efficient at the lungs, high concentration gradients must be maintained in the alveoli

Breathing in increases the concentration gradient of O2 between the alveoli and the blood – so it diffuses into the blood

Breathing out increases the concentration gradient of CO2 between the blood and alveoli – so it diffuses out of the blood

If the alveoli were not ventilated, equilibrium would be reached, and no gas could be exchanged

Question 3

Alveoli purpose

Answer 3

Alveoli increase surface area for gas exchange.

Each with their own network of capillaries
Along with ventilation, a constant supply of fresh blood helps to maintain the concentration gradient of O2 and CO2

Question 4

Type I pneumocytes

Answer 4

Single layer of epithelium cells

Flattened cells, 0.15μm thick

Question 5

Type I pneumocytes
How do these features help facilitate gas exchange?

Answer 5

Capillaries also only one cell-thick

Very small distance for O2 and CO2 to travel during gas exchange

Question 6

Type II pneumocytes

Answer 6

Rounded cells that make up 5% of alveolar surface

Secrete fluid that coats inner surface of alveoli

Fluid contains surfactant

Surfactant reduces surface tension and prevents fluid from causing the sides of alveoli from adhering to each other

Helps prevent collapse of lung

Premature babies can suffer from insufficient pulmonary surfactant

Question 7

Type II pneumocytes
How does this feature help facilitate gas exchange?

Answer 7

Moisture aids in diffusion of gases

Question 8

Trachea

Answer 8

rings of cartilage

Keep open even in low pressure

Question 9

Bronchi

Answer 9

rings of cartilage

Question 10

Bronchioles

Answer 10

smooth muscle fibres in walls

Allows width of airways to change

Question 11

Diaphragm

Answer 11

controls volume of chest cavity

Question 12

Alveoli

Answer 12

gas exchange

Question 13

Contracting muscles do work –

Answer 13

get shorter

Question 14

Relaxing muscles do no work –

Answer 14

lengthen passively

Question 15

Muscles can only move in one direction –

Answer 15

so moving in two directions requires two muscles that alternate between relaxing/contracting – antagonistic pairs

Question 16

Inspiration/exhalation require opposite movements, so…

Answer 16

different muscles are required

Question 17

inspiration

Answer 17

pressure change: decrease in pressure (draws air inwards)
volume change: increase
ribcage movement: up and outward
external intercostal muscles: contract
internal intercostal muscles: relax
diaphragm: contract (flatten, moves down)
abdominal muscles: relax

Question 18

expiration

Answer 18

pressure change: decrease in pressure (draws air inwards)
volume change:
ribcage movement:
external intercostal muscles:
diaphragm:
abdominal muscles: