Mechanics of Breathing (Ventilation)

Human cells are primarily reliant on aerobic metabolism. Therefore, it is of vital importance to efficiently obtain oxygen from the environment and bring it to the tissues, while excreting the by-product of cellular respiration (carbon dioxide). Respiration involves both the respiratory and circulatory systems. There are 4 processes that supply the body with O2 and dispose of CO2. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration is represented by the O2 and CO2 exchange between the lungs and the blood.

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Anatomy of the Respiratory System Involved in Ventilation

Ventilation, or breathing, involves the action and movements of structures found in the neck and thoracic cavity belonging to the pulmonary, musculoskeletal, and cardiac systems. 

  • Conducting zone: 
    • Function:
      • Provides conduit for air to flow into lungs
      • Humidifies and warms incoming air
    • Structures:
      • Pharynx
      • Larynx
      • Trachea
      • Right and left mainstem bronchi
      • Bronchioles
  • Respiratory zone:
    • Function: location where gas exchange occurs
    • Structures:
      • Respiratory bronchioles
      • Alveoli
  • Musculoskeletal components:
    • Function: 
      • Provide sturdy framework for lungs
      • Generate mechanical forces necessary for breathing
    • Structures:
      • Rib cage
      • Respiratory muscles: diaphragm, external intercostals, portions of internal intercostals
      • Pleural membranes
      • Pleural cavity: space between pulmonary and thoracic pleura

Pressure Relationships in the Thoracic Cavity

  • Atmospheric pressure (Patm): 
    • Pressure exerted by air surrounding body 
    • At sea level, Patm is 760 mm Hg.
  • Respiratory pressure: 
    • Relative to Patm
    • Negative respiratory pressure is < Patm.
    • Positive respiratory pressure is > Patm.
    • Zero respiratory pressure = Patm.
  • Intrapulmonary pressure (intra-alveolar) pressure (Ppul):
    • Pressure in alveoli
    • Fluctuates with breathing
    • Always equalizes with Patm
  • Intrapleural pressure (Pip):
    • Pressure within pleural cavity
    • Fluctuates with breathing
    • Always a negative pressure
    • Pip is generated by opposing forces:
      • 2 inward forces promoting lung collapse (elastic recoil of lungs and surface tension in alveoli)
      • 1 outward force (elasticity of chest wall pulling thorax outward)
  • Transpulmonary pressure (Ppul – Pip):
    • Keeps airways open; lungs expand as transpulmonary pressure ↑
    • If Pip ≥ Ppul, lungs will collapse.

Inspiration and Expiration

Breathing consists of 2 phases:

  • Inspiration: flow of gases into lungs
  • Expiration: flow of gases out of lungs

Inspiration and expiration:

  • Mechanical processes caused by contraction of respiratory muscles
  • Cause volume changes in thoracic cavity 
  • Volume changes lead to gas movement according to Boyle’s law (pressure varies inversely with volume):
    • Volume changes cause pressure changes.
    • Pressure changes cause flow of gases to equalize pressure.
Changes in pressure relationships in the thoracic cavity during respiration

Changes in pressure relationships in the thoracic cavity during respiration:
During inspiration, muscles move to create negative intrapleural pressure (green line). This negative pressure is transferred to the lungs, making intrapulmonary pressure more negative (blue line) in relation to atmospheric pressure. Air flows into the lungs down this pressure gradient, increasing the breath volume (purple line). With exhalation, the process reverses, leading to airflow out of the lungs.

Image by Lecturio. License: CC BY-NC-SA 4.0


Inspiration is an active process:

  • Inspiratory muscles contract, pulling the rib cage out, decreasing the Pip, and increasing thoracic volume.
  • Adhesive forces pull on the pleural membrane, which in turn, pull on lung parenchyma.
  • Lungs are stretched and intrapulmonary volume ↑.
  • Ppul drops, becoming lower than Patm.
  • Air flows into lungs down pressure gradient until Ppul = Patm.


Expiration (at rest) is a passive process:

  • Inspiratory muscles relax.
  • Thoracic cavity volume ↓ owing to elastic recoil.
  • Intrapulmonary volume ↓
  • Ppul rises above Patm.
  • Air flows out of lungs down pressure gradient until Ppul = Patm.
  • Forced expiration: 
    • Active process 
    • Expiratory muscles used to ↓ thoracic volumes
    • ↑ Airflow speed out of lungs

Lung Volumes and Capacities

Lung volumes

Lung volumes are specific volumes of air contained by different portions of lungs at specific points in the respiratory cycle.

  • Tidal volume (TV): volume of air inhaled or exhaled with each breath under resting conditions
  • Residual volume (RV): volume of air left in lungs after forced exhalation
  • Expiratory reserve volume (ERV): volume of air that can be forcefully exhaled after normal tidal volume exhalation
  • Inspiratory reserve volume (IRV): volume of air that can be forcefully inhaled after normal tidal volume inhalation
Respiratory physiology_lung volumes and capacities

Lung volumes and capacities

Image by Lecturio. License: CC BY-NC-SA 4.0

Lung capacities

Lung capacities are a combination of 2 or more volumes.

  • Total lung capacity (TLC): 
    • Maximum volume of air contained in lungs after maximum inspiratory effort
    • TLC = TV + RV + ERV + IRV
  • Vital capacity (VC): 
    • Maximum volume of air that a person can move in or out of lungs
    • VC = TV + IRV + ERV
  • Functional residual capacity (FRC): 
    • Volume of air remaining in lungs after tidal expiration
    • FRC = ERV + RV
  • Inspiratory capacity (IC): 
    • Maximum volume of air that can be inspired after normal expiration 
    • IC = TV + IRV

Dead space

Dead space is air that enters and exits lungs but does not make it to areas where gas exchange can occur.

  • Anatomical dead space: air in airways that does not reach alveoli or respiratory bronchioles
  • Alveolar dead space: air in alveoli that cannot be absorbed into bloodstream due to lung disease or blood flow issues
  • Total dead space = alveolar dead space + anatomical dead space

Ventilation and Work of Breathing


Ventilation is the process of moving air in and out.

  • Minute ventilation (VE):
    • Volume of air moved in and out of lungs per minute
    • VE =  breathing frequency expressed in breaths/minute (Bf) × tidal volume (TV)
  • Alveolar ventilation (VA):
    • Volume of air reaching alveoli per minute and is available for gas exchange
    • VA = Bf × (TV – total dead space)

“Work of breathing”

The work of breathing is the amount of energy a person needs to breathe.

  • Elastic work: done to overcome elastic recoil of chest wall and pulmonary parenchyma and surface tension of alveoli
  • Resistive work: done to overcome resistance of airways and tissues

Factors Influencing Pulmonary Ventilation

Aside from the pressures that the thoracic musculature is capable of creating, ventilation is limited by the physical properties of the structures of the lungs. The most important physical properties to be taken into account are:

  • The resistance of the airways
  • The compliance of the lung tissue
  • The surface tension of the alveoli


  • Definition:
    • Resistance: force that opposes flow (in this case of air)
    • Poiseuille’s law: Flow of air is inversely proportional to resistance.
  • Effect on the pulmonary system:
    • Resistance impedes airflow into lungs.
    • ↑ Resistance = ↑ energy needed to breathe in.
    • Airways generate 80% of resistance.
    • Diameter of airway is inversely proportional to the resistance it produces.
  • Physiologic implications:
    • In healthy patients, resistance is insignificant, as total diameter of airways is large:
      • Large airway diameters in beginning of conducting zone
      • Smaller airways maintain high total cross-sectional area because there are so many.
      • Resistance disappears in terminal bronchiole → diffusion drives gas exchange
    • In certain ailments, ↓ total airway diameter, ↑ resistance to overcome to breathe:
      • Smooth muscle of airway constricts (e.g., asthma)
      • Mucous plugging of airways (e.g., chronic obstructive pulmonary disease (COPD), bronchitis)
      • Obstruction of bronchioles and alveoli by infectious material (e.g., pneumonia)

Alveolar surface tension

  • Definition:
    • Property derived from adhesion of water molecules
    • Water molecules stick to other water molecules.
    • Resists any force that tends to ↑ surface area of liquid
  • Effect on the pulmonary system:
    • Occurs in alveoli owing to the water molecules inside them
    • If left unchecked, keeps alveoli from stretching during inspiration
  • Physiologic implications:
    • Surfactant ↓ alveolar surface tension by separating water molecules from each other.
      • Type II pneumocytes synthesize pulmonary surfactant.
      • Pulmonary surfactant is 85%–90% phospholipids and amphipathic molecules
      • Polar ends stand in contact with water and ↓ surface tension.
    • Allows easier expansion during inspiration and prevents alveoli from collapsing during expiration
    • Anything that destroys surfactant can cause pulmonary disease:
      • The lack of surfactant in premature infants leads to infant respiratory distress syndrome.
      • Chronic smokers produce less surfactant, leading to issues seen in COPD.


  • Definition:
    • Measure of volume change with given change in transpulmonary pressure
    • Determined by tensile properties of lung tissue
    • Simply: how stiff the lungs are
  • Effect on the pulmonary system:
    • ↑ Lungs stiffness, ↑ energy to stretch them during inspiration:
      • Ventilation is more efficient in areas of lung with ↑ compliance.
      • Areas with ↓ compliance expand less.
    • Compliance of lungs also influenced by gravity:
      • Apex is ↓ compliant.
      • Base is ↑ compliant.
  • Physiologic implications:
    • In healthy lungs, ↑ compliance because of:
      • Distensibility of lung tissue
      • ↓ Alveolar surface tension 
    • ↓ Lung compliance appears in:
      • Fibrosis (e.g., acute respiratory surfactant distress syndrome, scarring after chemotherapy)
      • ↓ Production of surfactant (e.g., premature lungs, COPD)
      • ↓ Flexibility of thoracic cage (e.g., scoliosis, cerebral palsy)

Clinical Relevance

  • Obstructive lung disease: group of conditions that increase resistance of airways, either small or large: Obstructive lung disease causes narrowing of the airways. The increased resistance creates more work that needs to be overcome, so patients often present with increased respiratory rates and work of breathing. Patients often are unable to completely expel air during exhalation. Unless obstruction is widespread, these patients can continue to have normal or near-normal oxygen saturation. Pulmonary exam findings depend on where the obstruction is along the airway. Examples include asthma, chronic bronchitis, emphysema, obstructive sleep apnea, and airway foreign bodies
  • Restrictive lung disease: group of conditions that causes decrease in compliance of lungs, increasing amount of work needed to expand and contract lungs: Patients are unable to completely stretch the lungs to breathe in sufficient air. Restrictive lung disease usually affects the entire lung parenchyma and presents with increased respiratory rate and work of breathing, as well as decreased oxygen saturations. Examples include neonatal respiratory distress syndrome, pulmonary fibrosis, and sarcoidosis.
  • Mixed lung disease: Lung diseases often have elements of both restrictive and obstructive pathologies. The most common example of mixed lung disease is cystic fibrosis, where airways are narrowed and the lung parenchyma is stiffened.


  1. Hall, J. E. (2015). Guyton and hall textbook of medical physiology, 13th ed.. W. B. Saunders.
  2. OpenStax College. (2013). Anatomy and physiology. OpenStax. 
  3. Levitzky, M. G. (2017). Mechanics of breathing. Chapter 2 in Pulmonary Physiology, 9th ed. New York: McGraw-Hill Education.
  4. Levitzky, M. G. (2017). Alveolar ventilation. Chapter 3 in Pulmonary Physiology, 9th ed. New York, NY: McGraw-Hill Education.

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