Question

1. Outline the physiology of inspiration vs. expiration. Consider these things: the muscles involved, if these muscles are contracting or relaxing, the way volumes and pressures are changing in the thoracic cavity, and the relationship between these pressures and volumes.

2. What does it mean if a system has negative pressure? What forces cause the negative intrapleural pressure between breaths?

Answer 1

Answer :

1. The process of inspiration (breathing in) and expiration (breathing out) are vital for providing oxygen to tissues and removing carbondioxide from the body. Inspiration occurs via active contraction of muscles- such as the diaphragm - whereas expiration tends to be passive, unless it is forced.

The Lungs and Breathing :

The space between the outer surface of the lungs and inner surface thoracic wall is known as the pleural space. This is usually filled with pleural fluid, forming a seal which holds the lungs against the thoracic wall by the force of surface tension. This seal ensures that when the thoracic cavity expands or reduces, the lungs undergo expansion or reduction in size accordingly.

During breathing, the contraction and relaxation of muscles acts to change the volume of the thoracic cavity. As the thoracic cavity and the lungs moves together, this changes the volume of the lungs, in turn changing the pressure inside the lungs.

Boyle's law states that the volume of gas is inversely proportional to pressure (when temparature is constant). Therefore :

• When the volume of the thoracic cavity increases - the volume of the lungs increases and the pressure within the lungs decreases.
• When the volume of the thoracic cavity decreases - the volume of the lungs decreases and the pressure within the lungs increases.

Process of Inspiration :

Inspiration is the phase of ventilationin which air enters in the lungs. It is initiated by contraction of the inspiratory muscles :

• Diaphragm : flattens, extending the superior / inferior dimension of the thoracic cavity.
• External intercostal muscles : elevates the ribs and sternum , extending the anterior / posterior demention of the thoracic cavity.

The relaxation of the inspiratory muscles results ina decrease in the volume of the thoracic cavity. The elastic recoilof the previously expanded lung tissue allows them to return to their original size.

As per Boyle's law , a decrease in lung volume results in an increase in the pressure within the lungs. The pressure inside the lungs is now greater than in the external environement, meaning air moves out of the lungs down the pressure gradient.

Forced Breathing :

Forced breathing is an active mode of breathing which utilises additional muscles to rapidly expand and contract the thoracic cavity volume. It most commonly occurs during exercise.

Active Inspiration :

Active inspiration involves the contraction of the accessory muscles of breathing (in addition to those of quiet inspiration, the diapragm and external intercostals). All of these muscles act to increase the volume of the thoracic cavity :

• Scalenes - elevates the upper ribs
• Sternocleidomastoid - elevates the sternum
• Pectoralis major and minor - pulls ribs outwards
• Serratus anterior - elevates the ribs (when the scalpulae are fixed)
• Latissimus dorsi - elevates the lower ribs

Active Expiration :

Active expiration utilises the contraction of sevaral thoracic and abdominal muscles. These muscles act to decrease the volume of the thoracic cavity :

• Anterio lateral abdominal wall - increases the intra-abdominal pressure, pushing the diaphragm further upwards into the thoracic caity.
• Internal intercostal - depresses the ribs
• Innermost intercostal - depresses the ribs  

2. Competing forces within the thorax cause the formation of the negative intrapleural pressure. One of these forces relates to the elasticity of the lungs themselves - elastic tissue pulls the lungs inward , away from the thoracic wall.

Fources cause the negative intrapleural pressure :

Intrapleural pressure is the pressuere of the air within the pleural cavity, between the visceral and parietal pleurae. Similar to intra-alveolar pressure, intrapleural pressure also changes during the differant phase of breathing. However due to certain charecteristics of the lungs , the intrapleural pressure is always lower than, or negative to, the intra alveolar pressure(and therefore also to atmospheric pressure). Although it fluctuates during inspiration and expiration, intrapleuarl pressure remains approximately -4 mm Hg throughout the breathing cycle.

Competing forces within the thorax cause the formation of the negative intrapleural pressure. One of these forces relates to the elasticity of the lungs themselves - elastic tissue pulls the lungs inward, away from the thoracic wall. Surface tension of alveolar fluid, which is mostly water, also creates an inward pull of the lung tissue. This inward tension from the lungs is countered by opposing forces from the pleural fluid and thoracic wall. Surface tension within the pleural cavity pulls the lungs outward.Too much or too little pleural fluid would hinder the creation of the negative intrapleural pressure; therefore, the level must be closely monitored by the mesothelial cells and drained by the lymphatic system. Since the parietal pleura is attached to the thoracic wall, the natural elsticity of the chest wall opposes the inward pull of the lungs. Ultimately, the outward pull is slightly greater than the inward pull , creating the -4 mmHg intrapleural pressure relative to the intra-alveolar pressure, Transpulmonary pressure is the differance between the intrapleuarl and intra-alveolar pressures, and it determines the size of the lungs. A higher transpulmonary pressure corresponds to a larger lung.