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
- Nose / mouth
- Trachea (windpipe)
- Bronchi
- Bronchioles
- 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)
Examiner Tip
The intercostal muscles work in an antagonistic manner; as one contracts the other relaxes!You may see the terms inhalation OR inspiration (breathing in), and exhalation OR expiration (breathing out). Both sets of terms mean exactly the same thing, so don’t let them confuse you!This sequence of events is a common exam question and you should be able to explain in detail what is happening to the external and internal intercostal muscles, the rib cage, the diaphragm, the volume and the pressure of the lungs when breathing in and out.Remember, if you learn one, the other is almost exactly the opposite.
