Mechanics of Breathing: Inspiration & Expiration

Mechanics of Breathing

• Breathing consists of two parts: inspiration (breathing in) and expiration (breathing out).

Inspiration

• Occurs primarily through contraction of the diaphragm.
• The diaphragm pushes abdominal organs down, expanding the lungs.
• Heavy exercise or deep breaths also engage external intercostal muscles and axillary muscles to expand the chest wall.
• The expansion of the chest wall allows air to rush into the lungs.

Expiration

• Generally a passive process due to the elastic tissue in the lungs.
• Expansion of the lungs during inspiration creates an elastic recoil, ready to expel air.
• Muscles relax, leading to passive elastic recoil of the lungs, which expires air.
• In cases requiring forced or faster exhalation (e.g., exercise, expiratory diseases like asthma), abdominal muscles squeeze the abdomen.
• Internal intercostal muscles squeeze down on the chest wall, forcing air out of the lungs.

Compliance

• A measure of how volume changes in relation to pressure changes.
• High compliance: significant volume increase with a small pressure increase.
• Low compliance: small volume increase results in a large pressure increase.
• Compliance is inversely correlated with elasticity.
• High elasticity means lower compliance (harder to stretch tissue).
• Example: A thick rubber band has lower compliance and greater elastic recoil.
• Transmural pressure: the pressure difference between the alveoli and the intrapleural space.

Compliance of the Lung

• Inspiratory and expiratory curves differ in compliance.
• Greater compliance shifts the curve upwards (greater volume increase with lower pressure increase).
• Lower compliance flattens the curve (small volume increase leads to a dramatic pressure increase).
• At the start of inspiration, lung compliance is low due to high surface tension in the alveoli.
• As lungs expand and alveoli enlarge, surface tension decreases, increasing compliance.
• Expiration has higher compliance because it starts with large alveoli and reduced surface tension.
• Hysteresis: the difference between the inspiratory and expiratory curves.
• The expiratory curve is more indicative of overall lung compliance because it negates the influence of surface tension and surfactant.
• When lungs are filled with saline, surface tension is eliminated, and the inspiration/expiration curves become identical.

Chest Wall and Lung Interaction

• Lung’s elastic properties promote shrinking.
• Chest wall's elastic properties promote expansion.
• These opposing forces create a negative intrapleural pressure (vacuum).
• Lymphatics and microcirculation maintain the negative pressure by absorbing fluid.
• A pneumothorax (puncturing the pleural space) allows air to enter, causing the chest wall to expand and the lungs to collapse slightly.

Functional Residual Capacity

• The remaining air in the lungs after expiration.
• At functional residual capacity (FRC), the chest wall has an expanding force, while the lung has a collapsing force.
• The airway pressure is zero at FRC, with no air movement.
• Increasing lung air increases forces encouraging expiration.
• Reducing lung air increases forces that want to expand the lung and chest wall.

Disease States and Lung Compliance

• Diseases mainly affect the lung, not the chest wall.
• Emphysema: loss of elastic fibers increases lung compliance, leading to higher lung volume and increased FRC.
• Fibrosis: stiffening of lung tissues reduces lung compliance, leading to lower lung volume and decreased FRC.

Surface Tension of Alveoli

• Describes the attraction between liquid molecules within the alveoli.
• In small alveoli, liquid molecules are close, creating strong forces that collapse the lungs.
• Increasing alveoli size reduces surface tension but makes gas exchange less efficient.
• Smaller alveoli are better for gas exchange, but surface tension can cause them to collapse.
• Laplace's law: Pressure = (2 x Tension) / Radius; increasing radius reduces pressure within alveoli.

Surfactant

• Produced by type II pneumocytes (alveolar cells).
• Surfactant is a mix of phospholipids (mainly dipalmitoylphosphatidylcholine) with hydrophilic and hydrophobic ends.
• Surfactant breaks up attractive forces between liquid molecules, reducing surface tension.
• Increases lung compliance by reducing collapsing pressure.
• Neonatal respiratory distress syndrome: premature babies lack surfactant, leading to high surface tension, collapsed alveoli (atelectasis), and poor gas exchange.

Airflow

• Governed by Ohm's law: Airflow = (Pressure difference) / Resistance.
• Air moves into lungs when environmental pressure is higher than lung pressure.
• Lung pressure decreases during inhalation due to increased volume (Boyle's law).
• Airway resistance is described by Poiseuille's law.
• Resistance = (8 x Viscosity of inspired air x Length of airway) / (π x Radius^4)
• Radius has the biggest influence on resistance due to the fourth power.
• Smaller airways have greater resistance, however the greatest resistance lies in the medium-sized bronchi due to relatively smaller numbers of them.

Factors Affecting Airway Resistance

• Autonomic nervous system:
• Parasympathetic stimulation constricts bronchial smooth muscles via muscarinic receptors.
• Sympathetic stimulation relaxes bronchiole smooth muscle through beta-2 receptors.
• Beta-2 agonists are used to relax bronchial smooth muscle in asthma.
• Lung volume: greater volume reduces resistance, lower volume increases resistance.
• Viscosity of inspired air becomes a factor for deep sea divers where it can increase.

Breathing Cycle

• Inspiration: decreasing pressure within the lungs allows air to flow in.
• Expiration: contracting muscles increases pressure, forcing air out.
• Alveoli remain open due to negative intrapleural pressure.
• Forced expiration: abdominal muscles squeeze organs, increasing alveolar pressure.
• Alveoli do not collapse during forced expiration due to the intrapleural pressure.
• Emphysema: reduced lung compliance can cause airway collapse during expiration due to reduced intrapleural pressure.
• Slow expiration is better for those with emphysema to prevent airway collapse.