Ventilation & Gas Exchange (AQA A Level Biology)

Ventilation & Gas Exchange

• Gas exchange in the lungs requires a concentration gradient

• Ventilation (mass flow of gases) in the lungs and the continuous flow of blood in the capillaries helps to ensure that there is always a higher concentration of oxygen in the alveoli than in the blood

• The movements involved in breathing causes the air in the alveoli to change, which supplies fresh oxygen and takes away carbon dioxide

• The oxygen in the alveoli diffuses into the red blood cells which are rapidly carried away in the blood and replaced by oxygen-depleted red blood cells

• Exercise causes oxygen demands to increase which can be facilitated by an increased rate of ventilation

Passage of Air

1. Nose / mouth

2. Trachea (windpipe)

3. Bronchi

4. Bronchioles

5. Alveoli

Breathing In

• The breathing-in process causes the volume in the chest to increase and the air pressure in the lungs to decrease until it is slightly lower than the atmospheric pressure

• As a result, air moves down the pressure gradient and rushes into the lungs

• Mechanism when at rest:

  • The diaphragm contracts and flattens, increasing chest volume

• Mechanism when exercising

  • In addition to the flattening of the diaphragm the external intercostal muscles contract, causing the ribcage to move upwards and outwards

The process of breathing in (inhalation)

Breathing Out

• When at rest breathing out occurs mostly due to the recoil of the lungs after they have been stretched

• Volume in the chest decreases and pressure increases, causing air to be forced out

• Mechanism when at rest:

  • External intercostal muscles relax

  • The recoil of elastic fibres surrounding alveoli causes the air to be forced out

  • Diaphragm becomes dome-shaped

• Mechanism when exercising:

  • Internal intercostal muscles contract to pull the ribs down and back

  • Abdominal muscles contract to push organs upwards against the diaphragm, increasing the internal pressure

  • This causes forced exhalation

The process of breathing out (exhalation)