Ventilation & Gas Exchange (AQA AS Biology)

• 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)