Physiology · MU Sofia

37. Non‐elastic resistance to breathing. Flow‐volume relationship. Work of breathing.

Explains how breathing resistance divides into elastic and non-elastic parts, with airway resistance dominating the non-elastic share and the radius of the airways its key determinant under Poiseuille's law. It then covers how lung volume and bronchial factors shape airflow during inspiration and expiration, and the elastic and resistive components of breathing work. The note closes with oxygen cost and how the optimal respiratory rate shifts in restrictive and obstructive lung disease, followed by recap questions.

Inside: 5 sections · 8 labelled pictures · 15 flashcards · 5 min

Topic 37 of 104 in the MU Sofia syllabus for Physiology (Dental Medicine, Year 2). The free AceMed note covers it in 5 sections, about 755 words (5 min of reading), with 15 flashcards.

In this note

  1. Non‐elastic resistance to breathing. Flow‐volume relationship. Work of breathing.
  2. Non-Elastic Resistance to Breathing
  3. Flow-Volume Relationship
  4. Work of Breathing.
  5. Oxygen Cost and Optimal Breathing Rate

Non-Elastic Resistance to Breathing

Breathing requires overcoming resistance. This resistance can be categorized into elastic and non-elastic resistance.

  • Elastic Resistance: Elastic resistance arises from the elastic properties of the lungs and chest wall. It accounts for about 65% of the total resistance to breathing.
  • Non-Elastic Resistance: This type constitutes the remaining 35% of resistance and encompasses two primary components:
  • Airway Resistance: Accounts for 80-90% of the non-elastic resistance. It refers to the resistance encountered by airflow through the airways.
  • Tissue Resistance: Contributes 10-20% of the non-elastic resistance. This resistance is generated by the movement of tissues during breathing, including the lungs themselves and the chest wall.

Factors Affecting Airway Resistance

Several factors influence airway resistance. Of these, the radius of the airways is the most critical. The relationship between airway resistance and radius is governed by Poiseuille's law, which highlights an inverse fourth-power relationship.

  • Poiseuille's Law and Airway Resistance: Poiseuille's law states that resistance (R) is directly proportional to the viscosity of the fluid (air in this case) and the length of the tube (airway), and inversely proportional to the fourth power of the radius (r). This means even a slight decrease in airway radius can dramatically increase resistance.
  • Parallel Arrangement of Smaller Airways: While smaller airways individually possess higher resistance, their parallel arrangement significantly reduces the total resistance in these regions. This parallel configuration ensures that airflow is not severely compromised in the smaller airways.
  • Location of Highest Airway Resistance: The trachea and larger bronchi contribute the most to airway resistance. This is because, despite their larger individual diameters, they are fewer in number compared to the smaller airways.

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