Mechanisms of Breathing
The Mechanics of Breathing (aka Pulmonary Ventilation)
Breathing depends on the volume changes in the thoracic cavity.
Volume changes --> pressure changes --> affects flow of gases in and out of the lungs
Understanding pressure relationships in the thoracic cavity
Respiratory pressures are always described with reference to atmospheric pressure. At sea level, atmospheric pressure is 760 mmHg = 1 atm.
- A negative respiratory pressure suggests that the pressure in that region is lower than the atmospheric pressure. For example – 4 mm Hgindicates that the pressure in that region is lower than the atmospheric pressure by 4 mm Hg (760 – 4 = 756 mm Hg)
- A positive respiratory pressure suggests that the pressure in that region is higher than the atmospheric pressure.
Ppul = intrapulmonary pressure refers to the pressure within the alveoli
Pip = intrapleural pressure (normally lower than the intrapulmonary pressure) refers to the pressure in the pleural cavity
Ptp = transpulmonary pressure refers to the pressure difference between alveoli and the pleural cavity
To calculate the transpulmonary pressure,
Ptp = Palv - Pip
|Pneumothorax In a case of pneumothorax, the intrapleural pressure increases and becomes increasingly positive as air enters the pleural cavity from outside the chest via the puncture wound. This may eventually lead to a lung collapse when the intrapleural pressure becomes equivalent to the intrapulmonary pressure.|
Boyle’s Law explains the relationship between the pressure and volume of a gas.
For example, in a small container, the molecules of a given gas will be near each other and are more likely to collide with each other and the walls of the container. This results in the gas molecules in the smaller container exerting a higher pressure in comparison to a bigger container.
In short, pressure and volume of a gas are inversely related.
Table 1: Inspiration and Expiration
1. Inspiratory muscles (diaphragm and external intercostal muscles) contract. Ribs are elevated and diaphragm is pulled downwards.
2. Thoracic cavity volume increases.
3. Lungs are stretched so the intrapulmonary volume increases.
4. This leads to a decrease in intrapulmonary pressure.
5. The air flows into lungs down its pressure gradient until intrapulmonary pressure is equal to the atmospheric pressure.
(more of a passive process that depends on lung elasticity)
1. Inspiratory muscles relax. Ribs are depressed and diaphragm moves upwards as it becomes relaxed.
2. Thoracic cavity volume decreases.
3. Elastic lungs recoil as a result of the presence of collagen and elastin.
4. The intrapulmonary volume decreases causing the intrapulmonary pressure to rise.
5. Air flows out of lungs down its pressure gradient until intrapulmonary pressure is equal to the atmospheric pressure.
Forced inspiration and forced expiration requires the help of accessory muscles.
- Forced inspiration uses sternocleidomastoid and scalene muscles.
- Forced expiration uses the abdominal wall muscles, primarily the oblique and transversus muscle.
- Marieb EN, Hoehn K. Human Anatomy & Physiology, Ninth ed. : Pearson ; 2013.