How Do Pressure Changes Control Breathing?

TL;DR

Breathing mechanics depend on pressure relationships among the alveoli, pleural cavity, and atmosphere. At the stated sea-level reference, intrapulmonary and atmospheric pressures are both about 760 mmHg, while intrapleural pressure is about 756 mmHg, or 4 mmHg below atmospheric pressure. Pleural fluid keeps the visceral and parietal pleura closely coupled while reducing friction during inhalation and expiration.

Transcript

all right ninja nerds in this video we're going to talk about the mechanics of breathing so this can be a tough topic for certain people to understand especially with the pressures so we're going to do our best here at Ninja Science to make sense of that so let's go ahead and dig right in so before we do that we need to look at a little bit of anat... Read More

Key Insights

• The visceral pleura is a thin layer covering the lung, while the parietal pleura is the outer pleural layer associated with the chest wall. These two layers lie extremely close together and are functionally connected across the pleural cavity.
• The pleural cavity is a potential space containing a small amount of serous-like pleural fluid. Although diagrams may depict a visible gap, the visceral and parietal pleura are normally almost completely tethered together rather than separated by a large open space.
• Pleural fluid is a lubricant that minimizes friction as the visceral and parietal pleura move against one another during inhalation and expiration. Insufficient fluid and increased rubbing can irritate these layers, producing inflammation and a condition identified in the lecture as pleurisy.
• Intrapulmonary pressure is the pressure within the lungs and is more precisely described as intra-alveolar pressure. The trachea branches through the bronchi and bronchioles until airways reach the small, grape-like alveoli where this pressure is considered.
• Intrapleural pressure is the pressure inside the pleural cavity. It is presented as consistently lower than intrapulmonary pressure, with an approximate value of 756 mmHg compared with an intrapulmonary pressure of about 760 mmHg.
• Atmospheric pressure is also called barometric pressure. At sea level and one atmosphere, it is given as approximately 760 mmHg, providing the reference against which zero, negative, and positive respiratory pressures are described.
• Zero relative pressure means that a measured pressure equals atmospheric pressure. Because intrapulmonary and atmospheric pressures are each approximately 760 mmHg in the stated resting comparison, intrapulmonary pressure can also be expressed as zero relative to the atmosphere.
• Negative intrapleural pressure is attributed to lung elasticity, surface tension, and chest-wall elasticity. Its approximate 756 mmHg absolute value becomes negative 4 mmHg when compared with atmospheric pressure, while gravity is said to contribute to pressure differences across locations in the pleural cavity.

Questions & Answers

Q: What are the three main pressures involved in breathing mechanics?

The three main pressures are intrapulmonary pressure, intrapleural pressure, and atmospheric pressure. Intrapulmonary pressure, also called intra-alveolar pressure, is measured within the alveoli. Intrapleural pressure exists inside the pleural cavity between the pleural layers. Atmospheric pressure, also called barometric pressure, surrounds the body and serves as the reference for describing respiratory pressures as zero, negative, or positive.

Q: What is the difference between intrapulmonary and intra-alveolar pressure?

In the lecture, intrapulmonary pressure and intra-alveolar pressure refer to the same functional pressure. The more precise term is intra-alveolar pressure because the pressure being discussed exists inside the alveoli, the small grape-like structures reached after the trachea branches into bronchi and progressively smaller bronchioles. The broader term intrapulmonary simply describes this as pressure within the lungs.

Q: Why is intrapleural pressure described as negative?

Intrapleural pressure is described as negative because it is lower than atmospheric pressure, not because its absolute value is below zero. The lecture gives an intrapleural pressure of approximately 756 mmHg and an atmospheric pressure of approximately 760 mmHg. Subtracting atmospheric pressure from intrapleural pressure produces a relative value of negative 4 mmHg. Lung elasticity, surface tension, and chest-wall elasticity contribute to this relationship.

Q: What are the pressure values used to explain resting breathing mechanics?

The lecture uses approximately 760 mmHg for intrapulmonary pressure and approximately 760 mmHg for atmospheric pressure at sea level and one atmosphere. Intrapleural pressure is given as approximately 756 mmHg, which is about 4 mmHg lower. When expressed relative to atmospheric pressure, intrapulmonary pressure is zero and intrapleural pressure is negative 4 mmHg. The presentation uses millimeters of mercury rather than centimeters of water.

Q: What is the pleural cavity and why is it called a potential space?

The pleural cavity is the region between the visceral pleura covering the lung and the parietal pleura forming the outer pleural layer. It is called a potential space because the two pleural surfaces are normally extremely close and almost completely tethered together. A diagram may show a visible gap for clarity, but the body does not normally contain a comparably large open space there.

Q: What does pleural fluid do during inhalation and expiration?

Pleural fluid occupies the pleural cavity and allows the visceral and parietal pleura to move against one another with very little friction during inhalation and expiration. Without sufficient lubrication, repeated rubbing could cause irritation, inflammation, and tissue damage. The fluid therefore supports smooth movement while helping the closely positioned pleural layers remain functionally connected as the lungs and chest wall move.

Q: How can reduced pleural lubrication affect the pleural layers?

Reduced pleural lubrication can allow the parietal and visceral pleura to rub against one another with greater friction. The lecture explains that this agitation may produce inflammation and identifies pleurisy as a possible resulting condition. Pleural fluid normally limits such friction during breathing, so an inadequate amount can interfere with the smooth interaction of the layers during inhalation and expiration.

Q: How are zero and negative respiratory pressures calculated?

Respiratory pressures are classified by comparing them with atmospheric pressure. A pressure equal to the stated atmospheric value of 760 mmHg has a relative pressure of zero. Thus, an intrapulmonary pressure of 760 mmHg is zero relative to the atmosphere. An intrapleural pressure of 756 mmHg is 4 mmHg lower than atmospheric pressure, so its relative value is negative 4 mmHg.

Summary & Key Takeaways

• The lungs are covered by the visceral pleura, while the parietal pleura forms the outer pleural layer. Between them is the pleural cavity, described as a potential rather than open space. A small quantity of serous-like pleural fluid connects the closely positioned layers and permits low-friction movement during breathing.

• Three pressures establish the framework for understanding breathing mechanics. Intrapulmonary, or intra-alveolar, pressure exists inside the alveoli. Intrapleural pressure exists within the pleural cavity. Atmospheric pressure surrounds the body and may also be called barometric pressure. Comparing these pressures clarifies the meanings of zero, positive, and negative pressure.

• At sea level and one atmosphere, atmospheric and intrapulmonary pressures are presented as approximately 760 mmHg. Their relative difference is therefore zero. Intrapleural pressure is approximately 756 mmHg, making it negative 4 mmHg relative to atmospheric pressure. Lung elasticity, surface tension, and chest-wall elasticity are identified as reasons for this negative pressure.