A breathing cycle by the lungs of the human body is guided by three pressures. They are intra-pleural pressure, intra-alveolar pressure, and transmural pressure. We will discuss these three pressures in the later paragraphs.
Structure of Lungs
As shown in the figure above, lungs are invaginated into the sac of the pleura forming double layers. The inner layer is known as the visceral pleura which is connected to the lungs. Similarly, the outer parietal pleura connect to the walls of the thoracic cavity.
Pressures exerted in the lungs
Three pressures exert in the lungs. They are as follows:
1. Intra-alveolar Pressure
An increase in the volume of an alveolar sac decreases the pressure and vice versa. So, it can range from +1 cm H2O to -1 cm H2O.
2. Intra-pleural Pressure
It is exerted due to the elastic recoil of the lung tissues. It can range from -5 cm H2O to -7.5 cm H2O. Since the direction of intra-pleural pressure is opposite to the transmural pressure it is always negative.
3. Transmural Pressure
It is due to the surface tension exerted by the parietal pleura to the walls of the chest. It is always positive.
Breathing Cycle in Human Body
During the process of inspiration, the chest walls and diaphragm expand outward increasing the volume of alveoli. Intra-alveolar pressure becomes -1 cm H2O at mid inspiration. Thus the atmospheric air (0 cm H2O) enters the alveolar sac. After mid inspiration, the intra-alveolar pressure increases due to pressure exerted by the air in the alveolar sac. So, it becomes 0 cm H2O (= to atmospheric pressure) at the end of inspiration.
Expansion of lungs causes an increase in intra-pleural pressure from – 5 cm H2O to -7.5 cm H2O.
Inspiration is an active process. So, it will require more energy for muscle contraction. Normally, inspiration lasts for 2 seconds.
During the process of expiration, muscles contract. The volume of an alveolar sac decreases. So, the intra-alveolar pressure becomes positive (+1 cm H2O) at mid- expiration due to which air goes back to the atmosphere. After mid-expiration, the intra-alveolar pressure decreases due to a lack of air to exert pressure in the alveolar sac. Thus, at the end of the expiration, intra-alveolar pressure becomes 0 cm H2O (= atmospheric pressure).
Also, intra-pleural pressure comes back to -5 cm H2O.
Expiration is a passive process. Thus it does not need much energy. Expiration lasts for about 3 seconds.
After expiration lungs undergo a resting stage before the next cycle begins.