Ventilation: Lung Volumes and Capacities

• Ventilation is the process of moving air in and out of the lungs for respiration.
• Quiet breathing and forced ventilation are both covered for a comprehensive view of pulmonary mechanics.

Lung Volumes & Capacities

• IRV stands for Inspiratory Reserve Volume.
• TV stands for Tidal Volume.
• ERV stands for Expiratory Reserve Volume.
• RV stands for Residual Volume.
• IC stands for Inspiratory Capacity.
• VC stands for Vital Capacity.
• FRC stands for Functional Residual Capacity.
• TLC stands for Total Lung Capacity.

Expanding and Contracting the Lungs

• The lungs inflate and deflate through movement of the diaphragm.
• Diaphragm movement lengthens or shortens the chest cavity, which influences lung volume.
• Rib elevation and drepression increases and decreases the chest cavity diameters.
• Changes in lung volume occur from rib movement during breathing.

Mechanics of Inspiration

• During normal resting inspiration, the diaphragm contracts, forcing abdominal contents downward.
• Diaphragm contraction increases the chest cavity's vertical dimension.
• As the diaphragm contracts, the rib margins lift and move out, increasing the transverse diameter.
• External intercostal muscles contract, pulling the ribs upward like a bucket handle.
• Contraction of the external intercostal muscles increases the transverse diameter of the thorax.
• The sternum is pushed upward and forward with contraction of the intercostal muscles, increasing the anteroposterior diameter.

Forced Inspiration: Accessory Muscles

• Scalene muscles elevate the first two ribs, aiding in expanding the upper chest cavity.
• Scalene muscles are helpful during deep or labored breathing.
• Sternomastoid muscles raise the sternum, further increasing the anteroposterior diameter of the thorax during forced inhalation.
• Serratus anterior muscles lift many of the ribs.
• The serratus anterior muscles contribute to the overall expansion of the thoracic cage during intense breathing efforts.

Pressure values

• Atmospheric pressure is 760 mmHg.
• Intrapulmonary pressure is 760 mmHg, relative to atmospheric pressure (0 mm Hg).
• Intrapleural pressure is 756 mmHg, or -4 mmHg.
• Transpulmonary pressure is 4 mmHg, calculated as 760 mmHg - 756 mmHg.

Mechanics of Expiration

• Normal quiet expiration is passive, relying on the relaxation of inspiratory muscles.
• Elastic recoil of the lungs, chest wall, and abdominal structures compress the lungs, expelling air.
• Forced expiration is active, occurring during exercise or hyperventilation.
• Expiratory muscles contract, increasing intra-abdominal pressure and decreasing thoracic volume.

Muscles of Forced Expiration

• Abdominal wall muscles, including the rectus abdominus, internal and external oblique, and transversus abdominus, contract.
• Contraction of the abdominal wall muscles raises intra-abdominal pressure, pushing the diaphragm upward.
• Internal intercostals run downward and inward, pulling the ribs downward.
• The internal intercostals decrease thoracic volume during active exhalation.

Pressures Responsible for Air Movement

• Intrapleural Pressure (IPP) is the pressure in the pleural space between the lungs and chest wall.
• IPP is normally negative (subatmospheric) due to lung recoil and chest wall expansion.
• Intraalveolar Pressure (IAP) is the pressure inside the alveoli.
• IAP fluctuates with breathing, becoming slightly negative during inspiration and slightly positive during expiration.
• Transpulmonary Pressure is the difference between intraalveolar and intrapleural pressures.
• Transpulmonary pressure keeps the lungs inflated and prevents them from collapsing.

IPP (Intra-pleural pressure)

• IPP is the pressure in the "potential space" between the lungs and chest wall (pleural space).
• IPP is a negative pressure (subatmospheric).
• Lack of air in the pleural cavity also causes IPP
• Elastic recoil of the lung causes IPP, continuously pulling against the chest wall to expand.
• Elastic fibers of the lung tissue, which are continuously stretched and always tending to recoil, account for 1/3 of recoil.
• Surface tension of the fluid lining the alveoli accounts for 2/3 of recoil.
• Elastic properties of the chest wall tend to expand the thoracic cage.
• The chest wall pulls the parietal pleura outwards, contributing to negative IPP.

Significance of Negative Intrapleural Pressure

• Negative intrapleural pressure helps in expanding the lungs by maintaining a pressure gradient.
• It aids venous return to the heart by creating a negative intrathoracic pressure.
• Negative intrapleural pressure facilitates lymphatic flow in the thoracic duct.

Intrapleural Pressure (IPP) During Normal Breathing

• IPP starts at -5 cm H2O at the beginning of inspiration.
• IPP decreases to -7.5 cm H2O during inspiration.

Alveolar Pressure (Intrapulmonary Pressure)

• Alveolar pressure refers to the pressure within the alveoli.
• The pressure in the alveoli in equilibrium is equal to atmospheric pressure when no air is flowing.
• During inspiration, alveolar pressure decreases to -1 mmHg, which draws in 500 ml of air (Tidal volume).
• During expiration, alveolar pressure rises to +1 mmHg, forcing air out of the lungs.

Transpulmonary Pressure

• Transpulmonary pressure is defined as the pressure difference between alveolar and pleural pressure.
• Transpulmonary pressure reflects the elastic recoil pressure of the lungs.

Normal Lung Compliance

• Lung compliance is the extent to which the lungs expand for each unit increase in transpulmonary pressure.
• Total compliance, measured for lungs and thoracic cage together is 130 ml/cm H2O.
• Lung compliance alone is 200 ml/cm H2O.
• Lungs alone have higher compliance because the chest wall limits total compliance.

Compliance vs elasticity

• When the lungs are remarkably distensible, this means compliance is high, while their elasticity is low.
• Compliance reflects distensibility.
• The reciprocal of compliance is elastance, i.e. elastic recoil pressure.

Reduced vs. Increased Compliance

• Rigid lung, caused by fibrosis, congestion, and pulmonary edema, will reduce lung compliance.
• Rigid thorax, caused by vertebral deformities and extensive burns, will reduce lung compliance.
• Both old age and COPD (emphysema and chronic bronchitis) increase lung compliance.
• In Old Age lungs loses their elastic recoil.

Understanding Surfactant and Lung Mechanics

• Surfactant is essential for lung function because it reduces surface tension in the alveoli.
• Surfactant prevents lung collapse and eases breathing.

What is Surfactant?

• Surfactant is a substance that lowers surface tension in the lungs.
• Type II alveolar cells produce surfactant.
• Surfactant is a mix of phospholipids, lipids, and proteins.

How Surfactant Works

• Surfactant spreads as a monomolecular layer.
• Surfactant molecules have a hydrophilic head that dissolves in water lining the alveoli.
• Surfactant molecules have a hydrophobic tail that orients towards alveolar air

Physiological Significance of Surfactant

• Surfactant decreases surface tension, preventing lung collapse.
• Surfactant maintains alveoli size and equalizes pressure.
• It keeps alveoli dry by preventing fluid transudation.
• It prevents alveoli from collapse after expansion at birth.

Infant Respiratory Distress Syndrome (IRDS)

• Type II cells appear around 20 weeks.
• Surfactant secretion is established around 30 Weeks.
• Premature birth will effect Surfactant deficiency, which causes IRDS.

Factors Affecting Surfactant Formation

• Stimulate: Thyroid and glucocorticoid hormones
• Decrease: Insulin, pure O2, cessation of circulation.

Review Questions and Answers

• The primary muscle responsible for inhalation is the diaphragm (B).
• During normal expiration, relaxation of the diaphragm occurs (B).
• The role of surfactant in the lungs is to reduce surface tension in the alveoli (C).
• The epiglottis acts as a barrier to prevent air from entering the esophagus during swallowing (A).
• During inspiration, intrathoracic pressure decreases (B).
• Passive expiration results from elastic recoil of the lungs (C).
• Increased airway resistance increases the work of breathing (C).
• Pulmonary fibrosis is a condition in which lung compliance is typically decreased (B).
• Thickness of the alveolar membrane is the primary determinant of the rate of gas exchange in the alveoli (A).