Mechanics of Breathing

Mechanics of Breathing

• Inspiration is an active process.
• The space between the lungs and thoracic wall is the pleural space.
• The pleural space is filled with a few milliliters of fluid, which forms a pleural seal holding the outer surface of the lungs to the inner surface of the thoracic wall.
• Lung volume will change if the volume of the thorax cage changes.
• Each lung is surrounded by two layers of serous membrane known as the pleurae, specifically the visceral and parietal pleurae which are a continuation of the same membrane.
• Pleural fluid, produced by pleural membranes, acts as a lubricant and helps hold parietal and visceral pleural membranes together.
• The inward force of the elastic recoil of the lungs is balanced by the outward recoil of the chest wall. This balance is known as the resting expiratory level.

Pleural Pressure Variations

• Pleural pressure varies during inspiration and expiration cycles.

Muscles of Inspiration

• Diaphragm and external intercostal muscles in quiet breathing.
• Sternocleidomastoid and scalene muscles of the neck, serratus anterior, and pectoralis major muscles in forced inspiration.
• More diaphragm contraction (7 cm descent) and external intercostal activity occurs during deep inspiration.
• Accessory muscles of inspiration, e.g. sternocleidomastoid, serratus anterior, and scaleni muscles, also contract during deep inspiration.
• The lungs passively follow chest wall movement due to fluid between parietal and visceral pleurae.
• The diaphragm accounts for 75% of inspiration.
• The external intercostal muscles alone can produce inspiration needed for moderate activity if the diaphragm gets paralyzed.

Expiration

• Quiet breathing: Expiration is passive due to elastic recoil.
• Forced expiration: Accessory muscles are used
• Internal intercostal muscles and abdominal wall muscles, including external and internal oblique muscles and the rectus abdominis muscles.
• Abdominal muscles that push the diaphragm upward.
• Internal intercostal muscles that “lower" the ribs, reduce transverse & A-P chest diameters.

Respiratory Rate and Cycle

• Normal rate is about 12-16 cycles/minute.
• Respiratory cycle composition consists of: A short inspiration (I), a longer expiration (E), and an expiratory pause(glottis closed).

Compliance of the Lungs

• Lung stretchiness is known as compliance.
• Compliance is volume change per unit pressure change.
• Compliance = Volume change per unit pressure change/ Starting volume of lung
• Lung compliance curves showcase variations, like increase with emphysema or decrease with lung fibrosis.
• Lung compliance curves also illustrate inspiratory/expiratory compliance.
• Compliance is seen at low volumes because of difficulty with initial lung inflation, and at high volumes because of limit of chest wall expansion.
• Total breathing work is the area contained in the compliance diagram loop.

Factors Affecting Lung Compliance

• Elastic tissues in the lungs and surface tension forces of alveolar fluid affect lungs’ elastic properties.
• Compliance depends on the lungs elastic forces:
• Lung tissue, e.g., elastin + collagen fibers.
• Fluid lining of the alveoli, substance present inside walls of alveoli called surfactant.
• Compliance decreases when pulmonary venous pressure increases, causing lung engorgement with blood.
• Compliance decreases with alveolar edema due to alveolar inflation insufficiency and when a lung remains unventilated
• Compliance decreases with diseases causing lung fibrosis.
• Compliance increases in COPD, e.g., emphysema and the alveolar walls progressively degenerate.
• Asthma (smooth muscle hyper activity) often has normal lung compliance.

Surface Tension

• Airways and alveoli are lined with fluid, area increases as lungs expand.
• Alveolar fluid surface tension varies with surface area, making surfactant molecules.
• An expanding alveolus increases its surface area, thus spreading surfactant molecules further, and its efficiency is reduced in reducing the surface tension.
• A shrinking alveolus brings surfactant molecules closer, amplifying their concentration which reduces surface tension.
• Surfactant effect: reduces surface tension forces greatly as area of the alveolus decreases.
• Small alveoli will expand with less force than large ones.
• Surfactant also keeps the lungs stable by stopping alveoli collapsing.
• Alveoli are an interconnected series of bubbles, pressure is governed by Law of Laplace (P = 2T/r)
• P = pressure in the alveolus, T = surface tension r= radius of alveolus if 'T' is constant, smaller bubbles (with a smaller radius 'r') would have higher pressures (P)

Surfactant

• Present in alveoli, stabilizes lungs by preventing collapse.
• Absent in fetuses younger than about 25 weeks/full-term babies, cause of 'Respiratory Distress Syndrome'.
• Increases lung compliance by reducing surface tension.
• Stabilizes the lungs by preventing small alveoli collapsing into big ones
• Prevents surface tension in alveoli creating suction force, which stops transudation fluid from pulmonary capillaries.
• Phospholipid, produced from alveolar type II cells.
• Decreases attractive H bonding by moving between H2O molecules.
• As alveoli radius shrinks, its ability to lower surface tension increases.

Lung Surfactant Composition

• DPPC makes up 36%
• Unsaturated PC makes up 33%
• PG makes up 10%
• Proteins make up 10%
• PI, PE, Lyso-bis-PA, SPM, DG, and Chol make up the rest in varying percentages
• Surface tension of alveolar fluid varies with surface area, so when the alveolus expands the surface tension increases.

Airway Resistance

• Determined by Poiseulles Law
• Resistance = Pressure/ Rate of flow = Viscosity of air x Length of tube/ P x Radius⁴
• Single tube resistance rises sharply with falling radius; combined resistance is typically low with parallel connections.
• Breathing resistance mainly resides in upper respiratory tract, unless small airways are compressed from forced expiration.
• Work is done against elastic recoil of lungs/thorax.
• Major work done is against the lungs elastic properties and alveoli surface tension.
• Airway flow resistance has is of little overall significant in healthy subjects, but is often affected by disease.