Physiology · MU Sofia

35. Mechanics of respiration. Pressures in the respiratory system (alveolar, pleural and transpulmonary). Elastic resistance to breathing. Surface tension; surfactant. Pressure‐volume relationship.

Explains how air moves between atmosphere and lungs through changes in alveolar, pleural and transpulmonary pressures, and how the chest dimensions change during inspiration. Covers elastic resistance as compliance and elastance, surface tension and the role of surfactant, including premature infants with respiratory distress syndrome, and ends with the pressure-volume loop and hysteresis. Closes with four recap questions.

Inside: 5 sections · 9 labelled pictures · 20 flashcards · 5 min

Topic 35 of 104 in the MU Sofia syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 5 sections, about 688 words (5 min of reading), with 20 flashcards.

In this note

  1. Mechanics of respiration. Pressures in the respiratory system (alveolar, pleural and transpulmonary). Elastic resistance to breathing. Surface tension; surfactant. Pressure‐volume relationship.
  2. Mechanics of Respiration
  3. Pressures in the Respiratory System
  4. Elastic Resistance to Breathing
  5. Surface Tension and Surfactant

Mechanics of Respiration

The exchange of air between the atmosphere and the lungs is called pulmonary ventilation, achieved through repeated changes in the pressure gradient between the atmosphere (Patm) and the alveolar space (Palv). When Palv is less than Patm, air flows from the atmosphere into the lungs. Volume changes of air during inspiration and expiration occur due to forces (muscle force, elastic recoil) acting on the chest and lungs.

The pleural cavity is a closed space containing 15-25ml of intrapleural fluid. This thin pleural layer holds the lungs to the chest wall due to the vacuum created within the closed space. Any disruption to this, such as the introduction of air, can lead to lung collapse (pneumothorax).

During inspiration:

  • The up-down dimension is increased because the diaphragm contracts.
  • The anterior-posterior dimension is increased both in upper and lower thorax.
  • The lateral dimension is increased only in lower thorax.

Pressures in the Respiratory System

Alveolar Pressure (Palv)

  • As the lungs expand during inspiration, the volume of the alveoli increases, resulting in a decrease in Palv.
  • This creates a pressure gradient where Patm becomes higher than Palv, causing air to flow into the lungs and alveoli.
  • During expiration, the opposite happens. The Palv increases, becoming higher than Patm, pushing air out of the alveoli.
  • These changes are described by Boyle’s Law, which states that at a constant temperature, the product of pressure and volume for a gas remains constant.

Pleural Pressure (Ppl)

  • Ppl depends on the balance of forces that expand or compress the lungs and thorax. It is usually opposite to Patm except during forced expiration.
  • During deep inspiration, Ppl becomes more negative, pulling on the lung and expanding it.
  • In forced expiration, Ppl becomes more positive than Patm.

Transpulmonary Pressure (Ptp)

  • Ptp is the difference between intra-alveolar pressure (Palv) and intrapleural pressure (Ppl).
  • Lung expansion depends on Ptp. If there is no pressure difference between Palv and Ppl, the lung collapses (pneumothorax).

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