Ventilation in Mammals

• 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
• Breathing


  Causes the air in the alveoli to change, which supplies fresh oxygen and takes away carbon dioxide
• 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


  1. The volume in the chest increases and the air pressure in the lungs decreases until it is slightly lower than the atmospheric pressure

  2. As a result, air moves down the pressure gradient and rushes into the lungs
• Mechanism of Breathing In at rest

  The diaphragm contracts and flattens, increasing chest volume
• Mechanism of Breathing In when exercising

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

  1. The volume in the chest decreases and pressure increases, causing air to be forced out

  2. External intercostal muscles relax

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

  4. Diaphragm becomes dome-shaped
• Mechanism of Breathing Out when exercising

  1. Internal intercostal muscles contract to pull the ribs down and back

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

  3. This causes forced exhalation